The Biophysical Foundations of Human Movement - 2nd

SECOND EDITION

Vaughan Kippers Tf-"G UniversiTyi o f Oueens/ono' Ti-;e

Laurel T. Mackinnon of Q ueenslarlo'

Un iversi~1

Marcus G. Pandy

HUMA N KINETICS

Lib ra r y of Congress Ca ta loglng-In-Pub lica tiun Da ta The biophy~il:al fuunuations ufhu lIl,m IllQvemen! I Bruee Abemethy .. [et al. ].--2nd ed. p. ;cm. Includesbi bliogmphicalreferencesandindex ISBN 0-7360-4276-8 (softl:ovcr) I. Human mochanics. 2. Biophysics. 3. Ki nesiology. !DNLM: 1. Movemem--physiology. 2. Biomocbanics. 3. Biuphysics. 4. E.>.crcisc-physiology. 5. Sports--psychology. WE 103 B61592005] 1. Abe rnethy, Bruce.195SQ1'3 03.8 586 200S

6 12.7'6--dc22

2004008595

IS BN: 0-7360-4276-8 Copyright «) 2005 by B. Abernethy, S. Hanrahan. V. Kippers, L. T. Mackinnon, and M.G. Pandy Copyright (.) 1996, 1997 by A.B. Abt:mcthy, L.T. Mackinnon. RJ. Neal , V. Kippers. and SJ . Hanrahan FirM puh lished in Australia in I ~6 by Macmillan Education AustmJia. Second Austral ian edi tion published by Pnlgrave (ISBN 0 -7329 -9758 -5).

~laemillan

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To all those scholars, past and present, who have contributed to the academic credibi li ty of the study of human movement.

Preface Ix Acknowledgements CHAPTEI? 1

xi

Introduction Human Movement Studies As a Discipline and a Profession Human Movement Studie s: Definit ion and Importance Disc ipli nes und ProfessiurlS Is Huma n M o vement S1udies a Discipline? Structuri ng a Discipline of Human Movement St udies Naming the Discipline of Humon Movement Studies Professions Based on Human Movement Studies Relationships Between the Discipline and the Professions Summory Further Reading ond Reteren ces Some Relevant Web Sites

PART I

ANATOMICAL BASES OF HUMAN MOVEMENT The Subdiscipline of Functional Anatomy Typical Questions Posed and Problems Add ressed levels of Analys is Historical Perspective s Professional Trainin g a nd Organizatior'lS Further Reading and References Some Relevant Web Sites

Basic Concepts of the Musculoskeletal System Tools for Measurement Th e Ske le tal System , Th e Articular System The M-Iscu lar System Summary Fu rther Readng and Refere nces

ID 10 10

11 11 11 12 14 14 lb

16 16 16

21 25 31

32

Basic Concepts ot Anthropometry

33

De finition of Arthropometry Tools for Measurement BodySlz9 Determination of Body Shape The Tissues Composing the Body Somatotyplng As a Descrplion of Body Buile Human Variatio~ SUmmary Further Reading and References

33 33 34 . . . . 34 , , 34 36 37

'400

Contents

Musculoskeletal Changes Across the Life Span

41

Definitions of Aw(oiogy and Gerontology IDOls for Meas... rement Physicol G rowth, Moturation, and Aging . Ag e-Related Changes in the Skeletal a nd Articular Systems Ag e-Related Changes in the Muscular System Changes in Body Dimensions Across the I ite Span Methods of Determining Age SumMary Further Reading and Refe rer.ces

41 · 41

· 42 43 47 · 48 · b2

53 63

Musculoskeletal Adaptations to Training

54

Effects of Physicol Activity on Bone . . Effects of Physical Acti vity en ~Ioint Struduce and Ranges of Mo~io ' l Effects of Physical Activity en Muscle-Tendon Units . Effects o f Physical ActMty en Body Size. Shape. and Compositio n Summary Furher Reading a nd Refe rences

~

·iiARt II

- - - - - - -- -MECHANICAL BASES OF HUMAN MOVEMENT The Subdiscipline of Biomechanics Typical Questio ns Posed and Levels of Anol'{sis Histor1cal Perspectives Profess io nal Organizations . Further Read ing and References Some Relevant Web sass

Basic Concepts of Kinematics and Kinetics Veclors . Motion Degrws of Freedom Forc e Moment of Force Equilibrium Computer Modelling of Movement . Summary Furthe r Rea ding and References ' C HAP1ER 7

Basic Concepts of Energetics Energy and Power Metabolic energy Co nsumption Effic iency of Movement

8

Biomechanics Across the Ufe Span Biomechanics of Normal Walking Gait Deve lopment in Glildren

54 66 · . 58 · . 59 · 62 · . 62

-

63 . 63 64 · . 64 · . 65 · . 65

66 66 · . 68 · . 72

· . 73 · . 74 · . 74 78

80 80

81 81 9()

91 . . . 92 92

93 93 100

102 . . . 104

Summary Further Reading and References

9

.105 . . . 105

Biomechanical Adaptations to Training

106

Muscular Adaptations to Tra ining Neuromuscular Adapta tions to Training

. 106 .107

Contents

810mechanical Adaptations to Inju!)' Dependence of Motor Performance on C hanges in Muscle Properties Insights Into the Effects of Training Provided by Computer Models Summa!), Further Reading and References

PARTIII

CHAPTER 10

CH APTER 11

PHYSIOLOGICAL BASES OF HUMAN MOVEMENT The Subdiscipline of Exercise Physiology

CHA PTER 13

119

Applications of Exercise Physiology Typical Questions Posed a nd Levels of Analysis Historical Perspectives Professional Organizations a nd Tro ln:ng Further Reading and References Some Relevant Web Sites

. 119 120

121 121 122 122

Basic Concepts of Exercise Metabolism

123

Production of Em~rgy for Exercise Oxygen Supply During Sustained Exercise Measurement of Exercise Capacity The Cardiorespiralory Syslem a nd Oxygan Supply During Exercise Human Skeletal Muscle Cells Energy Cost of Activity Importance of Diet to Energy Metabolism a nd Exercise Performance Summary Further Reading and References

. 123 . 128 . 130 132 134 137 137

Physiological Adaptations to Training

143

Training-Induced Metabolic Adaplalions Endurance Training-Induced Changes in the Cardiorespiratory System Muscular System Changes After Strength Training BaSic Principles of Training Training fo r Cardiovascular Endurance Metho ds of Streng th Trcin;ng Causes of Muscle Soreness Exercise for Health-Related Fitness Summary Further Reading and References C HAPTE R 12

.1 11 .115 .116 .118 .118

141

142 143

147 150 152 156 156 . .. 159 159 164 164

Physiological Capacity ond Performance Across the Life Span

165

Responses to Exercise in Children Adapta tions to Exerc ise Training In Child re n E)(ercise Capacity During Aging Exercise Prescription fOf Older Adults ute Span Sex Oifferences in Physiolog ~cal Responses and Adaptations to ExerCise Summary Further Reading and Re ferences

165 168 170 174 175 . 178 .1 78

Applications of Exercise Physiology to Health

179

Physical Activity and Public Heolth PhysIcal Activi ty, Cardiovascular Disease. and Metabolic Syndrome Physical Activity and Other Major Diseases and Conditions Summa!)' Further Reading and References

. 179 . 183

. 192 . . . . . 195 .. 196

Contents

vii

197 . . 197

198 198 19)

200 ?OI . .. 201

202 Components o' th e NelVou~ System Neurons and Synapses . Sensory Receptor Systems for Movement Effector Systems for Movement Motor Control C'une;Hons o f the Spinal Cord Motor Control ::uncHons of 11 18 Brui 'l InteG rat ive Brain Mecha:-.Isms for Movement Summary Further Reeding and Refe rences 15

16

203 203 . . .206 21 3 2 1:1 2 17 221 ??1 . . .222

Basic Concepts of Motor Control: Cognitive Science Perspectives

223

JsJ r g Models t o Study Motar Contro l nformoNor Proc essing Models of Metor Control Some Alternctive Models Of Motor Ce nt rol Summary Further Read ing and References

223 225 233 .238 .238

Motor Control Changes Across the Life Span

239

Changes in O bs e ~vcble Mot or Performance Changes at th e Neurophysiolog 'cal Level Changes In Information-Pre cessing Capabilit ies Summa !), Furth er Reading and References

240 251 255 .258

Motor Control Adaptations to Training

259

Changes In O bservable Motor Peefor ma nee Changes at the Neurophysiolog ical Level Changes in Infr.rmotion-Processing Capabilities Factors Affect ing the Learning of Motor Skills Summary Further Reading cn d Refere nc es

253 260 26 1 264 270 274 27 t1

PSYCHOLOGICAL BASES OF HUMAN MOVEMENT

PARTY

The Subdiscipline of Sport and Exercise Psychology Typical Questions Posed and Prob lems Addressed Levels of Analysis Hislorical Perspectives Professiona l Organizations . Further Reading a nd Reference Some Releva nt W9b Sites 18

Basic Concepts in Sport Psychology Personality . Motlvotlon in Sport

275 276 . .. 277 277

2i7

279 279 . . . . 28 1

viii

Contents

CHAPTER 19

Arousal, Anxiety, and Sport Performonce The PractiCe of Applied Sport PsyChOlogy Imagery: An Exampie of a Psychological Skill Summary Further Reading and References

.287 . 289 .290

Basic Concepts in Exercise Psychology

291

Effects of Psychological Factors on Exerc ise Effects 01 Exe rCise on Psychological Factors Summary Furfhef Reading and References CHAPTER 20

Physical AcHvlty and Psychological Factors Across the Life Span Changes in Personality Psychosocial Development Through Sport ParticipaHon Exercise in Older Adults TerminatiOn of Athletic Careers Summary Further Reading and References

CH APTER 21

CHA PTER 22

.283

.286

. 291 . 298 . 299 .. 300

301 . . 301 . 302 .. 303 . . 305 . . 307 . 308

Psychological Adaptations to Training

309

Aerobic Fitness and the Response to Psychologicol Stress Changes in Personality Changes in Motivation: staleness, Overtraining. and Burnout Changes In Mental 3+ of external forces (e.g., contact of the foot with the ground d uring walking). Free-body diagrams and the concept of equilibrium aTe u sed to analyse the resultant effect of force systems acting on body parts. Analyses based on the principle of static equilibrium are often used to estimate the magnitudes and din.>ctions uf mm;cle, li ~:,.ament, and joint-reaction forces present during activity.

Further Reading and References Bedford, A., and W. Fowler. 1995. Engineering meclumics: Dynnmics. Reading, MA: Addbon-Wesley. IJedford, A., and w. Fowler. 1995. EJ1giurering mechonics' Statics. Reading. MA: Addison-Wesley.

Lieber, R.L. 1992. Skeletal muscle structure and fonction' Implications for rehabilitation and sports medicine. Baltimore: Williams &; Wilkins. Nord in, M., and Y.H. Frankel. 1989. Basic biomechanics of the musculoskeletal system. 2nd ed. Philadelphia' Lea & Febiger.

Pandy, M.e. 2001. Computer modeling and simulation

ofhumanmovement.AnnulIl8c-viewoJBinmediml Engineering 3: 245-273 Winter, D.A. 1990. Biomechanics and 1I1010r cOlliroJ of human movement. New York: Wiley.

CHAPTER 7

O

ne of the fu nd amental Jaws of mech anics, as elemen tary

as Newton's Laws of Motion, is

The major learning concepts in this chapter relate to • calculating the kinetic energy of a body segment-the amount of mechanical energy a body segment possesses due to its motion; • calculating the potential energy of a body segment-the amount of mechanical energy a body segment possesses by virtue of its height above the ground ; • total mechanical energy of a body segment-the sum of kinetic energy and gravitational potential energy; • calculating the instantaneous power of a body segment-the rate at which work Is done on the body segment; • calculating elastic shain energy-the amount of mechanical energy stored in the elastic tissues of muscle and tendon during movement; • describing qualitatively and quantitatively how metabolic energy Is consumed during movement; and • defining the ertlclency of movement-the ratio of mechanical energy output to metabolic energy input.

the principle of work and energy. Indeed, this p rinciple, w hich states that the w ork done on an object is equal to the change in its kine tic energy, can be derived from Newton's famous Second Law (F =m a). The work done on an object is equal to the net force acting on the object m ultip lied by its d isp lacement (change in position) as a result of the force. The mechanical energy of the object, its capacity to do w ork, may be visible as kine tic energy, gravitationa l potential energy, and (unless the object is rigid) elastic stra in energy, whid l is stored as a result of the deformation created bv the applied force. ' This chapter describes the roles that kinetic energy, potential energy, and elastic energy play in human movement. While kinetic and potential e nergy are related to the mass of the body, elastic strain energy is associated with the p roperties of muscle and tendon tissue. VolWltary movement is made possible by the development of muscle force, which is fueled by chemical ene rgy made available through the oxidation of foodshtffs. The last hvo sections of this chapter describe the energetics of muscle contraction and s how how the metabolic cost of movement is governed by the interplay of kinetic energy, gravitational potential energy, a nd strain energy stored in the clastic tissues of muscle and tend on .

Energy and Power In this section we discuss three common forms of mecha nical energy- kinetic energy, p otential

energy, a nd elastic strain energy-and show how to calculate each quantity. We also describe the concept of power, w hich is the rate of change of mechan ical energy over time.

Kinetic Energy The kinetic energy of a body is the a mount of mechanical energy the body p ossesses due to its motion . Conside r a rigid body translating and rotating in three-dimensional space (figure 7.1). Let tile mass of the body be represented by m and the moment of inertia of the body abou t its centre of mass be given by fe. (The moment of inertia of a body is a meusurc of the ability of the body to resist changes in its angular velodty. More simply, it may be thought of as a measure of a body's resistance to rotation. The moment of inertia depend s on the point of the body about which it is calculated. It is minimal about the centre of mass of the body. The

81

82

The Biophysical Foundations o f Human Movement

5ysteme International (51) unit of moment of inertia is kg·m 1 [lbff:lJ.) At some time t during the body's motion, 1! represents the veloci ty of the centre of mass of the body and lQ is its angular velocity. The kinetic energy of the body is made up of two parts: the kinetic energy due to translation of the cen tre of mass, and the kinetic energy due to rotation about the centre of mass. The translational kinetic energy is given by

~==~m£2,

7.1

and the rotational kinetic energy is

Tr=~lcQl.

7.2

Note that T j and 1', are both scalars because any vector multiplied by itself is a scalar. That i.", u" = 1! . l' = v 2, and v 2 is a scalar. Thus, the total kinetic energy of the body is

T = ~m £2 +~I< ~~,

7.3

POlenllal Energy Potential energy is the amount of mechanical energy a body possesses by virtue of its height above the ground. Consider the rigid body of figure 7.1 moving from position 1 to position 2 (see figure 7.2). The change in gravitational potential energy of the body is given by

Ug = mg(Y2-Y)'

75

where Yj is the vertical position (height) of the centre of mass of the body in position 1, Y1 is its height in position 2, and g is the gravitational acceleration constant near the surface of the earth (equal to 9.81 m/sl or 32.2 It/s2). Potential energy is a scalar, and its 51 unit is the joule (newton·metre Ur./b]).

folol Mechonlcal Energy The total mechanical energy of a rigid body is equa l to the sum of its kinetic energy and gravitational potential energy,

and because J!.2 = :!l . 1' = v 1 and !£l = f!! 'f!! =w1, equation 7.3 can be rewritten more simply as

ET=~mv2+~Ic Q/+mg(Y2 - YI)'

T=%mv2+ ~[cui.

When the total mech.:.mical energy of the syslem remains constant (i.e., the value of Erdoes not change over time), energy is conserved, and the system is said to be conservative. In the real world, no system

7.4

The 51 unit of kinetic energy is th e joule (which is l"<J.uivalent to newton-metre [ff.1bJ) .

7.6

" -, 'C7 "M )-. )-~------- ~ )\.:

Figure 7.1 Rigid body rotating with an angular velocity f!! and translating with a linear velocity of its centre of mass of 1;.

Figure 7.2 Displacement of a rig id body between two positions. The change In potential energy of t he body is proportional to the displacement of the centre of mass from position 1 to posit ion 2.

Basic Concepts of Energetics is completely conservative; some energy is always lost by virtue of the system's interaction \vith the environment. Take the pendulum commonly found in a gra ndfather dock. In order for (he clock to keep perfect time, the pendulum bob must swing with exactly the same amplitude from cycle tocyele. The small amount of friction in the pendulum's bearings will act to decrease (by a very small am ount) the ampl iLude of the pendulum's swing over time. To ensure that the pendulum swings with precisely the same amplitude from cycle to cycle, a small mument is exerted on the pendulum to compensate for the slowing effect of friction. It is worthwhile looking at the fluctuations in kinelic and potential energy of theidea.l pendulum (in the absence of friction) during each cycle of its swing, because this model illustrates many of the same biomeclwn ical fea tures eviden t in human walkulg. Figure 7.3 :-:;hows a single pendulum of length I and mass m. At any instant during its motion, the kinetic energy of the pendulum can be written as 7.7

where v, the velocity of the tip of the pendulum, is given by v == wl, (lnd w is the (lngu.1«r velocity of the pendulum. Note that the pendulum has no rotational kinetic energy here, because its moment of inertia is asswned to be zero. The gravitational potential energy is given b)' l[== mg(l-!cos (J),

I

~ o I

Figure 7.3 Diagram of on Invorted single pendulum. An the moss of the pendulum is concentrate d ct its t ip, and the moment o ~ in e rtia of the pendulum 's neg1ccted . The hinge about which the pendulum p ivots is assumed to be frictionless. 400

35DJ-,,----~:-,- - - . , , -

~250

_%mv 2 +mg(l _ ICOS8).

I

'.

;: 200

\:

/'

100

,

\

\:~

/

!150

~.:-:~~

\:'

/\

,,

.

-

0.2

\

0.4

0.6

ET

\

50

·50

ET

,

30D

7.8

where the zero position is (arbitrarily) taken to be the lowest point reached by the pendulum du.r:iJlg its motion. Thus, the total mechanical energy of the pendulum at any instant is

83

,,

0.8

Tim e(s)

Figure 7.4 Kinetic energy (KE), gravitational potential energy (PE). and tota l mechan ical energy eEl) calcu lated for the inverted single pend ulum mode l of figure 7.3

7.9

As shown in figure 7.4, kinetic energy is max;mal when the pendulum reaches iLs lowest poin l because its velocity is then maximal. Conversely, potential energy is minimal a t the lowest point. Thus, ki netic energy is maximal w hen potential energy is minimal, imd vice versa. The Auctuations of kinetic and gravitational potential energy ilre exacLly equ'll and opposite i.n phase during eilch cycle becoltlse energy losses due lo frict ion are neglected here. Thus, the total mechanical energy, E" of this ideal pendulum remains constant for all time (see fi gure 7.4)

The hu man body is not a conservotivc system; if it were, the total mech anical energy of the body would remain constant and no additional energy wuuld be n eeded fru m the muscles to sustain movement. This is not the case, as muscles consume metabolic energy during contraction and do mechanical work to move the joints. Fluctuations in kinetic and potential energy of the whole body usually do not cancel eilch other during each cycle of movem ent, and so additional energy is needt>1.i to keep the body moving---energy that must be supplit'd by the muscles (see "Is Mechanical Energy Conserved in Walking?" on p. 84).

IN FOCUS: IS MECHANICAL ENERGY CONSERVED IN WALKING? The changes in kinetic and potential energy of the centre of moss thot occur during locomotion can be calculated from the measured force exerted on the ground. Force-measuring devices called force plates ore used for this purpose. These devices can measure force very accurately to within one or two newtons, depending on whether the force Is applied statically (constant In time) or dynamically (changing In time). One con find the position and velocity of the centre of moss of the body by integrating the ground force recorded from the force plate. For example, on equation that relates the vertical component of the ground force to the vertical acceleration of the centre of moss can be written as follows:

Fgy=mY•.m+mg,

Body center of gravity

484

7.9. 1

where For Is the measured vertical ground force, Vem Is the vertical acceleration of the centre of mass, m Is the mass of the body, and 9 is the value of gravitational acceleration at the surface of the earth , Solving equation 7.9.1 for the vertical acceleration of the moss centre and then integrating gives 7.9.2

and

Ycm =

f if,," dt,

7.9.3

where jj"" .. (F&), -mg j/ m from equation 7.9.1: Yem and Vern are the vertical velocity and position of

i the centre of moss, respectively; and tf defines the inlerval of time over which the integralion Is caflied oul. Equations 7.9.2 and 7.9.3 g ive the change in vertical veloCity and vertical position of the centre of moss, respectively. A similar analysis can be done to find the horizontal veloCity and position of the centre of moss. Subsfltuting these quantilies into equation 7.6 then gives the change in total mechanical energy of the centre of mass over one cycle, where w in equation 7.6 is laken 10 be zero because the centre of moss is a point (i.e., only rigid bodies can rotale and have angular velocities; points on bodies can only t ranslate). If the preceding analysis is carried out for walkIng at a normal speed, the fluctuations in kinetic and gravitational potential energy are found to be neady equal in magnitude and oppOSite in

84

1.0

1.2

Time(s)

Figure 7.5 Kinetic energy (KE). gravitational potential energy (PE), and total mechanical energy (EI ) calculated for normal walking on level ground. RHC = right heel contael: RTO = right toe-off: LHC '" left heel contact; LTO = left toe-off. Reprinted from Winter 1979.

phose (see figure 7.5 and c ompare with results shown in figure 7.4; in figure 7.5. all energies were calculated from force plate dota recorded from humans wolking ot their self-selected speeds n.e.. at speeds of approximately 1.35 m/s)). In walking. Ihe centre of mass reaches its highest point at mldstance, and gravitational potential energy is therefore maximal at this point. Almost all of the kinetic energy in walking is due to the forward velOcity of the body; Ihe changes In vertical velocity are much smaller, so the fluctuations In vertical kinetic energy ore negligible. The forward velocily of the body is maximal when the body Is at its lo west paint when both legs ore In contact WITh the ground; and it is leas! when the body Is at

Basic ConcepTS of EnergeTics

85

its highest point at mldstance. Thus, kinetic energy Is maximal during double support and minimal in mldstance. precisely opposite to the pattern of changing gravitational potential energy. When the curves representing kinetic and pot entia l energies In figure 7.5 a re ad ded numerically (w hich Ls possib le because energy is a scalar quantity), the fluctuatio ns in total mecha nic al energy o f the centre of moss are seen to be relatively small . This result indicates that the mechanical energy of the body is nearly conserved when people are free 10 choose Ihe speed at which they walk. That is, at the speed at which metabolic energy consumption is mini-

mized, there is almost a complete exchange of kinetic and gravitational potential energy, similar to what the ideal single pendulum model of walking predic ts.

Power

body segments in various tasks, including jumping, pedaling, and walking (see "Which Muscles Are Most Importan t to Vertical Jumping Performance?" on p . 87).

Power is the rate of doing work. The work done by any external force E acting on a mass during a very small (infinitesimal) displacement dr is E· dr. To find the power P, which is a scalar quantity, we d ivid e the work done by the interval of time, dt, during which the displa.cement occurs. Thus, 7.10

where l' is the velocity of the point on the rigid body at which the force is applied. The analogous relation for rotational motion is

p= M~:fl=M· f!2.'

7.1]

where M is the net moment applied to the body and ~ is the angular velocity of the oody. In 51 units, power is expressed in watts (which is equivalent to newton·metres/s lft·ib/s]). Power can b€ positive or negative, depending on whether it is being transferred to or taken from the mass. A muscle produces energy when it contracts concentrically (i .e., shortens) against an external load; in this case the va lue of muscle power is taken to be positive. Conversely, muscles that contract eccentrically (Le., lengthen) absorb energy, and the "alueof muscle power is taken to be negative here (see also figure 2.14). If muscle force and the rate of muscle shortening (or lengthening) are known, equation 7.10 can be used toestimale the power produced (or absorbed) by the muscle during contra(:tion (see "Modelling the Mechanics of Muscle Contraction" on p. 86). lndeed, equations 7.10 and 7.11 are the basis on wh ich detai led analyses have been performed to determine how muscles contribute power to the

SOURCES • Cavagna,G.A.. H.Thys,andA .Zamboni. 1976 The sources of external work in level walking and running. Journal of Physiology 262: 639657. • Farley c.T., and D.P. Ferris. 1998. Biomechanics of walking and running: Center of moss movements to muscle action . Exercise and Sport Science Reviews 26: 253-285.

Elastic Strain Energy Potential energy may also be stored in an elastic rather than a gravitational form. In explosive movements like running and jumping, considerable amoun ts of strain energy may be stored in the elastic tissues of muscle and tendon. If some of this stored el astic energy can be retum ed to the skeleton, less metabolic energy will be needed to keep the body moving. Storage and utilization of clastic strain energy may therefore reduce the amount of metabolic energy consu med by the muscles. As explained in "Is Mechanical Energy Conserved in Walking?" on page 84, calculations based on force-pia tt:! measurements of human walking show that changes in forward kinetic energy are nearly perfe ctly out of phase with changes in gravitational potential energy so tha t the tota l mechanical energy of the centre of mass is kept nearly constant during a step. The reason is tb., t the cent re o( mass is highest in midstance, when the forward kinetic energy is least, and lowest in double support, when the fonvard kinetic energy is greatest. The opposite is true for running, where changes in forward kinetic and gravitational potentia l energy are substantially in phase, leading to large changes in the total mechanical energy of the mass centre during each slep. TIlUS, in rwming, the centre of mass is lowest in mid stance when the forward kinetic energy is least, and it is highest during the flight phase when the forward kinetic energy is greatest.

IN FOCUS: MODELLING THE MECHANICS OF MUSCLE CONTRACTION In the la te 1930s, muscle physiologist A.V. Hill pos-

tulated a conceptual model of muscle controction. This theoret ical mod el integrates three of the most important force-producing p roperties of muscle: the force-Ieng!'l a nd force-velocity p ro perties p lus muscle's active state. In the late 19505, Sir Andrew Hu>dey proposed the first

complete mechanistic model to explain how the adin and myosin filaments interact with each other inlhe d evelopm e nt of muscle force . This theory has come to be known as the sliding filament theory of muscle contraction, Althoug h Hill' s model c annot explain the mechanisms by w hich force and energy are produc ed during a contrac tion. it yields much insight into t he m echano physiological re lat ionShips b etween le ngth, velocity, activ ation, and force Figu re 7.6 is a model of a rnuscu:otendo n actu ato r that is often used in theoretical studies of movement. The musculotendon actuator is represente d as a t hree-e lement m uscle in se ries w ith te ndon . The m echanical b e haviour of muscle is d escribed by a Hill-type c ontrac tile e le me nt (CE), which models the muscle's forcele ng th a nd fo rce- velocity p roperties; a series elastic e leme nt (SEE), which models m uscle ' s active stiffness; and a parallel elaslie e lement (PEE), whie h models muscle' s pcssive st iffness. Th e active stiffness is thoug ht to arise from the c ross-bridges form ed by c c to myo sin bind ing . so this property is present only when the m uscle is octivated . The passive stiffness resides In the materia l properties of the collagen molecules that are the building blocks of each muscle fibre. Because tendon (and . for t hat matte r. liga ment)

/\

\

is composed of collagen as w e ll, its force-length property is q ua litatively similar to that of passive muscle. Passive muscle develops a force that is pro port:ona l to its stretch. This fo llows from the fact that the behaviour of the muscle is fully described by its force'- Ienqth CUNe in the passive state . However. when muscle is activated, the force it develops depends on Ihe instantaneous values of its length. velocity. a nd a c tivation level. In the model shown in fi g ure 7.6, a ll muscle fibres ore assu med to insert at an a ngle, (t, on tendon. Thus, pennation angle, ( t.,. is assumed to be constant . FW is t he force developed by the whole muscuiotendon actuator, and this is the force transmitted to tendon. I-our muscle-spe c ific parameters ore needed to specify the behaviour of the model: Ine p eak isometric forc e deve :oped by the muscle and the correspond ing fibre lengln and pennation angle. p lus the resting length of the tendon. A first-orderdifferential equctioncan be derived to describe t he dynamics of the musculote ndon actuator show n :n fi g ure 7.6 (Zajac 1989):

O:::; a(t) :::; 1. 7.11.1 Equa tion 7.11 . 1 indicates that t he time rate of change of musculolendon force, J F Mr , depends

"

on musculolendon le ngth, {'Ai; m usculotendon velocity, vW; m uscle activation level, a: and musc ulotendon fo rce, p.1!. If we know the values of musculotendon length, m usculotendon velocity, a ctivation level, and musculotendon force at one time instant. we c a n integrate eq uation 7.11.1 to find the value of musculotendon force at the next time instant. Thus, if the traje ctaies of musculotendon length, velocity, and muscle activation are known for a ll times and the va lue of musculotendon force is give n at the initia l sta te (i.e ., at time t = 0), w e can t'i nd the time history of musculotendon lace by integrating equation 7.11.1 fo r the dura:ion of the motor task. The p roblem of integra-:-ing a differential equation g iven a forcing function and a set of initial conditions is referred to as the fOl\Nord-dynamics problem in b iomechanics.

SOURCES Tendon "

Actuator

,1

Figure 7,6 Sch e matic represent atio n o f a m odel o f mu scu lo te ndon octuation. Three compo nents are used to mode l the mecho nical behaviour o f muscle: a c ontractile element (CE). a series elastic eleme nt (SEE). and a parallel elastic element (PEE). Tendon Is repre sen led as 0 nonline ar spring Reprinted from Zojac 1989 86

• Pa ndy, M.G. 2001. Computer mode ling and simulation of hu ma n movement. Annual Review of Biomedical Engineering 3: 245-2 73. • Zajac, F.E. 1989. Muscie end tendon: Prop erties, models, scaling, and a pplication to bIornechonics and motor control. eRC Critical Reviews in Biomedical Engineering 19: 359-4 11. • Zajac, F.E., and M.E. G ordon. 1989. Determining muscle 's forc a and action in m ulti-articulor movement . Exercise and Sport Science r~evle w$

17: 187-230.

IN FOCUS: WHICH MUSCLES ARE MOST IMPORTANT TO VERTICAL JUMPING PERFORMANCE? Because muscle force cannot be measured noninvQsively In people. computer models are often used to est imate muscle force in tasks such as walking, running, Jumping, and cycling. The model o f figure 7.7 was used to simulate maximum-height jumping in humans. The skeleton was represented as a four-segment four degree-of-freedom, planar linkage. Joined to the ground at the toes and a rticulated at the ankle, knee, and hip by frictionless hinge joints. The head, a rms, and torso were lumped together and represented as one rigid body. Thus, the effect of arm swing was neglected in the model simulations. The model skele ton was actuated by eight lower extremity musculotendinous units, each unit modelled as a three-element muscle in series with tendon (see • Modelling the Mechanics of Muscle Contraction" on p. 86). Muscles included in the model were gluteus maximus (GMAX), reclus femoris (RF). hamstrings (HAMS),

vasti (VAS), gastrocnemius (GAS), soleus (SOL), other uniarticular plantar flexors (OPF), and tibialis anterior (fA). A mat hemat ical theory cal led optimisation was used to simulate the biomechanics of maximum-height squat jumping. In this task, the body begins from a static squatting position with the hlp, knee, and ankle all flexed to 9CY. Comparison of the model results wi th kinematic, force plate, and muscle electromyographic data obtained trom experiments showed tha t the model reproduced the major features of the ground contact phase of the jump. The simulation results were then analysed to determine how muscles accelera te and contribute power to the body segments d uring jump ing. Equations similar to equations 7.10 and 7.11 were used to calculate the power developed by the muscles a nd the amount of energy subsequently transferred to the skeleton during the g round contact phase of a maximum-height squat jump. Specifically, power was calculated by multiplying musculotendon force by mU5Culotendon contraction (shortening o r lengthening) velocity (see equaNon 7. 10). In figure 7.8, UPF represents the combined power of the uniarticular

1500

• ~

1000

£ OPF

~

8.

j

500

~

~ AF -500

0

20

40

60

80

100

Groundconlacllime(%)

Figure 7.7 Diagram showing a musculoskeletal model of the body thaI was used to simulate a maximum-height squat Jump. Reprinted from Pandy, Zajac, Sim, and Levine 1990.

Figure 1.8 Mechanical power generated by the musculotendon actuators in the model of figure 7.7 during the ground contact phose o f a maximum-height squat Jump Reprinted from Pandy and Zajac 1991. (conffnued)

87

I

IN FOCUS: WHICH MUSCLES ARE MOST IMPORTANT TO VERTICAL JUMPING PERFORMANCE? (contInued) ankle p lantar flexors in the model (Le" OPF and SOL) . The area under eoch c urve is equal to the tetol energy d eveloped or absorbed by each actuator. Vasti and gluteus rnoxlmus muscles were found

to be the major energy producers, the prime movers, of the lower limb in vertical jumping . These muscles contributed most significantly to the total energy made available for propulsion (note the area under e ach curve in figure 7.8). However. in the final 20% of ground contact time. just before liftoff, the ankle plantar flexors (soleus

2000

~ 1000

1

[ ~

~ - 1000

Inertial

and gastrocnemius) olso contributed significant1y

to the total energy delivered to the skeleton. Figure 7.9 is a plot of the total instantaneous power delivered to each body segment during the jump. The total area under each c urve is equal to the total mechanical energy (kinetic plus gravitational potential energy) of each body segment at the instant the body leaves the ground. A large proportion of me total energy developed by the muscles was delivered to the trunk segment. In fact, the combined energy of the thigh, shank. and foot amounted to only 3m; of the total energy available at liftoff. The remainder (approximately 70%) of the input muscle energy was transferred to the trunk. This is not a surprising result given that the trunk segment represents approximate ly 7a>!o of the total body moss.

20

40

60

80

100

Ground contact tim e (%)

Figure 7.9 Instantaneous power delivered to each body segment In the mod el during the ground contact phose of t he simuloled squal jump I Reprinted from Pandy and Zajac 1991. ~

88

-2000 - ' - - - - - - - - - - - _

Figure 7.10 Contributions of all t he muscles (Muscle), gravitational forces (Gravity), and Inertial forces ( Inertial) to the instantaneous power o f th e t runk for the ground c ontact phase of the jump. Reprinted from Pandy and Zajac 1991.

The model simulation results a lso showed that muscles dominate the instanta neous power of the trunk segment for most of the ground contact phase of the jump (figure 7. 10). (In figure 7.10. the total power delivered to the trunk segment (head. arms, and trunk lumped together) is Indicated by the shaded region.) Only near liftoff do c entrifugal forces (I.e .. forces arising from motion of the jOints) become so important that they dominate the power delivered to the trunk . Centrifugal forces are large only near liftoff because the velocities o f the joints increase g reatly just before the bOdy leaves the ground. According to the model calculations, gravitational (orces contribute little to lrunk energy in maximum-height jumping. Figure 7.1 J shows the relative contributions of individual leg m uscles to trunk power during the jump. The shaded region is the toto! power delivered to the trunk by all the muscles in the model. The dashed line is the contribu tion of all the biorliculor muscles in the model (hamstrings and rectus femoris). The a rea under each c UJVe represents the t ota l energy delivered by that muscle (or group of muscles) to the trunk segment at the instant the body leaves the ground,

- -- -- - - -- -- - - -- -- -- -- - -

Basic Concepts 01 Energetics

Total muscle

20 40 60 80 Ground conlacl lime (%)

100

Figure 7.11 Contribution of each muscle to the total power delivered to the trunk segment during the ground contact phase of the simulated squat jump. VAS is the contribution of vasti; GMAX is the contribution of gluteus moximus; and PF is the contribution of all the ankle plantar flexors (SOL OPE and GAS) in the model (see figure 7.7).

89

Of all the muscles, vasti and gluteus maxlmus contributed most significantly to the power delivered to the trunk (figure 7. 11). The energy delivered by these muscles amounted to nearly 90% of the total energy delivered to the trunk during the jump. Thus, vastl and gluteus maxlmus appear to be the most Important muscles for maximum-height jumping. The ankle plantar flexors, soleus and gastrocnemius, are also important. but only in the last 2fR, of ground contact. Near liftoff, the ankle plantar flexors (PF) deliver as much power to the trunk as erther vasti or gluteus maximus. Finally, the model simulation results also suggest that t he biarticular muscles are relat ively unimportant to overall jumping performance.

SOU/ICES

Reprinted from Pandy and Zajac 1991.

• Pandy. M.G .. and FE Zajac. 1991 . Optimal muscula r coordination strategies for jumping. Journal of Biomechanics 24: 1-10. • ZoJac, F.E. 2002. Understanding muscle coordination of the human leg with dynamical si m ulations. Journal of BiomechanIcs 35: 1011-1018.

Two very different mechanisms explain this difference in mechanical energy betv.reen walking and running. The almosl romplele lransfer of potential and kinetic energies in walking means that not all the energy required to lift and acceJerate the centre of mass has to come from the muscles-storage and utilization of gravitational potential energy allow the muscles to do less work, as is the case in the pendulum. In nmning, however, an exchange of gravitational and forward kinetic energy is not possible because the centre of mass goes through an increase in height and speed at the same time durin g a step. The lilrge fluctuiltions in totil l mechanical energy during each cycle suggest that the pendulum model of walking does not apply to running. Nonetheless, metabolic energy is still sdved, not by the exchange of gravitational potential and kinetic energy, but instead by storage and utilization of elastic strilin energy. Although we cannot calculate the amount of elastic energy stored in muscle and tendon precisely hom force-plate measurements, we can estimate it if we know the efficiency with which muscles convert chem.ical energy into mechanical work. The kangaroo perhaps best exemplifies an animal that is able to exploit the benefits of elastic

energy storage. A 4O-kg (88-1b) kangaroo has an Achilles tendon measuring approximately 1.5 Col (0.6 in.) in diameter and 35 cm (14 in.) in length. It is likely, then, that substantial quan tities of elastic energy are stored in the tendons of these animals following impact with the ground. Indeed, at a speed ot 30 km/ h (18.6 miles/h), it has been estimated that the contractile machinery supplies only 1/30f the energy required to lift and accelerate the centre of mass while the legs are on the ground. The remaining 2/3 of the energy required for each hop is thought to come from strain energy stored in the ClniOlal's tendons. Al though this fra ction is likely to be much smaller in a person who is running, some estimates put it as high as 1/2; that is, one-half of the mechanical energy needed to lift and accelerate the centre of mass during ground contact, plus the energy needed to move the limbs, is supplied by strain energy stored in the elastic tissues of muscle and tendon. More quantitative estimates of elastic energy storage require knowledge of the mechanical properties of muscle and tendon and of the forces they eXl:!rt. Although all biological tissues exhibit nonlinear force-extension curves, there is always a region in which extension varies linearly with

90

The BiophySical Foundations of Human Movement

force. Consider the elastic behaviour of tendon as represented by a simple linear spring. Let the stiifness of the tendon (spring) be given by kr and the amount of stretch be L![T. The elastic strain energy stored in the tendon spring is then

U.= ~e(6IT )2.

7.12

This equation has been used to estimate the amount of strain energy stored in the elastic tissU('''S of muscle and tendun when humans jump to their maximum height.

Metabolic Energy Consumption When a m uscle is stimula ted, heat is liberated, and the amount of heat produced can be measured by the temperature change within the muscle. If there is also a change in length during a contraction, then mechanical work is done by the muscle. According to the First law of Thennodynamics, the total rate of e.nergy production during muscle contraction, E, is equal to the ra te at which heat is liberated,H, p lus the rate at which work is done to move the extemal loild, W. Thus, 7.13

The rate at which mechanical work is done is equal 10 the power developed by the muscle. Power, in turn, is given by the force exerted by the muscle multiplied by its shortening (or lengthening) velocity (see equation 7.10). The rate of heat production is more difficult to estimate. This quantity depends on a number of factors related to the mechanics and physiology of muscle contraclion, including muscle length, mass, contraction velocity, activation level, musdl! fibre recruitment rate, and muscle fibre type. At least qualitatively, though, the rate at which heat is produced during a contraction can be estimated from four quantities: activation heat, maintenance heat, shortening heat, and resting heat. When muscle is excited, a re latively large amount of heat is p roduced early in the con traction. This activation heat reflects the energetics of calcium relea:;;e and re-accumulation, and it can account for as much as 25% to 30% of the total energy consumed during a contraction. The continued heat production observed once steady state has been reached is called the maintenance heat. This portion of the heat rate is thought

to represent the chemomechanica! events associated with muscle contraction and relaxation (see chapter 2). The maintenance heat accounts for the lnrgcst fraction of the total heat liberated during contraction,and its magn itude i" a strong fu nction of muscle's force-length property (see "How Much Force Can the Quadriceps Develop in a Maximal Contract jon" on p. 75 in chapter 6). Shortening: heat was fin;t discovered by A.v. HiU in the mid- to lale 19305. Hill defined this qunntity os the difference between the heat liberated by a muscle when it shortens and the heat liberated by the same muscle when it contracts isome tricall}'. The shortening muscle l.i berates more energy beGIUSe it does external work to move a load and also because it liberates more heat. From his earl}' experiments on frog muscle, Hill concluded that the amount of shortening heat was directly p roportionol to the distance the muscle shortened. Shortening heat represents only a small (often insignificant) fraction of the total energy consumed during controction All muscles produce a little heat as a consequence of being alive. Resting heat is the heat released as chemical energy, derived from oxidation of foodstuffs, consumed in the R'Sting state. The rate of heat proouction in the resting state (i.e., sitting quietly or lying down) is about 1.04 1/(kg ·s).lnterestingly, the cost associated with quiet standing is some 30% higher, npproximately 1.5 J/(kg· s). As soon as a person begins to wa lk, a greot increase inenergy expenditure occurs, reflecting the metabolic cost to Ule muscles of moving the body against gravity and of accelerating and decelerating the limbs. A large number of experimental s tudies have shown thnt metabolic energy expenditure increases with walking speed in the manner indicated (see fi gure 7.13). The relatiOnship is expressed fairly well by a parabolic equation of the form £=:\2 .0 +0.005(/,

7.14

where Eis the ra te of metabolic energy consumption expressed in J/(kg ·s) and v is walking speed. The constants in equation 7.!4 were determined on the basis of data obtained from nearly 100 men and women who walked on level ground at a frequency and step length of their choosing. Dividing equation 7.14 by wa lking speed, v, gives the rate at which mf'tabolic energy is consumed per unit distance traveled, or

1:, " 3~O + 0005 v,

7. 15

Basic Concepts of Energetics

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0.0 50 100 150 200 Walking speed (mfmin)

0.0 250

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Figure 7.12 Metabolic energy expend iture ploHed as a function of walking speed. The rate of metabolic energy consumption increases parabolically with walking speed (solid line). When the rate of metabolic energy consumption is normalized by the distance traveled, an optimal walking speed is predicted at roughly 80 m/min or 1.35 m/s (dashed line) . Reprinted from Ralston 1976.

where ~~ is expressed in J/(kg ·m). importantly, while E. increa ses proportionally with walking speed, EJ has a well-defined minimum at about 1.3 m /s (figure 7.12). As shown in figure 7.12, the minimum in metabolic energy expenditure predicted by equation 7.15 coincides with that obtairu.xl from oxygen consumption measurements made on people. Energy consumption rate increases as a curvilinear function of speed for wa lking, but it becomes a stra ight-line function of speed for rurming (figure 7.13). At a given speed, the rate of energy consumption increases as the slope of thl! ground increases, and it decreases as the slope decreases, reaching a minimum at a gradient of -10% (figure 7.13). For gradients steeper than - 10%, energy consumption increases again because of the postural changes necessary for continued locomotion and the significant braking action needed from Ule muscles.

Efficiency of Movement Muscles crea te and absorb mechanical energy by shortening and lengthen ing during Oexion and

91

if

R

g 10 § "E

"8

5

f 5 10 Speed(kmh - I )

15

Figure 7.13 Rate of metabol ic energy consumption plotted against speed for walking and running on level ground (0%), on a 5% incline (5%). and on a 5% decline (-5%). ReprintEKI from Morgana 1976.

extension of the joints. When mechanical energy is absorbed, muscles are said to do negative work; this is the work done by a muscle when it isdeveloping an active force at the same time that it is being lengthened by some externally applied force. [f the muscle is shortening as it develops a force, it is sa id to do positive work. The effi ciency with which a muscle operates under these conditions can be defined as

effidency

mech~/icaf work dOlle , 7.16 metabolic mergy cOl/SUllied

where the mechanical work done 0 11 the muscle i5 regarded as negative work, while that donl:' hy the muscle is positive work. The efficiencies of walking and running on negatively and positively sloped surfaces have been calculated and are plotted in figure 7.14. The lines emanating from the zero point on the graph represent constant efficiency of tran5port. Walking up steeper and steeper inclines requires more and more energy for the same distance traveled. Notice that the curve for walking up incl ines approaches the limit of 25% efficiency. TIllS represen ls Ule maximum efficiency with w hich a m uscle can convert chemical energy into the mechanical work needed to move the centre of mass. For walking

92

The Biophysical Foundations of Human Movement

1.4

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lum model. Although some amount of elastic strain energy is stored and reutilized in each cycle of rWlning, the results of figu re 7.14 suggest that this mechanism is less efficient than the pendulum·like mechanism present in walking. That is, in relation to the cost of transport, storage and u tilization of elastic energy in numing are not as efficient as the conversion of gravitationa l potential energy into fo rward kinetic energy in walking.

-

O~

- eXl'rcisc physiologist may study the mechanisms responsible for muscular hyperlrophy following strength training. Finally, work at the molecular level mighl focus on the extent to which elite sport performance is determined by genetics.

Historical Perspectives Just as the applications and levels of analysis of exercise physiology are multifaceted, so too are its historical origins. The ancient Greeks and Romans recob'Tlized ma ny exercise phYSiology principles-for example, that muscles hypertrophy (grow llevant to exercise physiology arc the International Federation of Sports Medicine (FIMS)! lhe International COlmcil of Sport Science and Physical Education (ICSSPE) and the American College of Sports Medicine (ACSM). Deta ils of these organizations are provided toward the end of chapter 1. In addition to these international organizations, there are many regional and national associations .representing the professional interests of sport and exercise physiology practitioners. Web sites of some associations are listed at the end of this intnxiuction. Excrcise physiologists are often members of other professional associations with specialized interest in various aspects of basic physiology, physical activity, or health. These might include associations devoted to research and applications in physiology (e.g., American Physiological Society), rehabilitation (e.g., American Association of Cardiovascular Pulmonary Rehabilitation), resistance training (e.g., Na tion(11 Strength and Conditioning Association), or health promotion (e.g., American Public Health Associahon).lt is not W1usua I to belong to several different professional associations at the s..1.me time, depending: on one's training, interests, and occupation.

122

The Biophysical Foundations of Human Movement

Further Reading and References Tipton, CM. 1998. Contemporary exercise physiology: Fifty years after the dosurcof the Harvard Fatigue

Laboratory. Exercisealld Sport Science Reviews 26: 315-340.

U.s. Department

of Health and Human Services 1996. Physical activity and health: A report of tile Surgeon Gencral (pp.1l-57).Atlanta: U.s. Depart-

ment of Health and Human Services, Centers for DiseaseControl and Prevention, National Center for Chronic Disease Prevention "_._ . _

Basic Concepts of Exercise Met abolism

a Energy for muscular wo rk

b PC splitting to resynthesise ATP

'f1igh energy' phosphate bond

'high energy' phosphate bond

ATP

ADP + PI

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energy to resynthesise ATP

energy for muscular work

c

125

At onset of axercise

!~

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+

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0 ~~ ~

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ATP fo r muscular wo rk Figure 10.1 Schematic representa tion of ~ high -energyN phosphagens. (a)Aden~ine triphosphate (ATP) is split into adenosine diphosphate (ADP) + Inorganic phosphate (P). (b) Phosphocreatine (Per) is split into c reatine + PI. (c) At the start of exercise and during brief intense exerc ise, per provides most of the energy for ATP produ cti on.

storage form of glucose, consisting of a p olymer of many glucose molecules. Lactic acid is a byproduct of anaerobic gl}'colysifi, and this system is sometimes called the lactic acid system One molecule of glucose, a six-carbon sugar, is degraded via a series o f 13 steps to two molecu les of pyruvic acid (figure 10.3), Pyruvic acid can then either be converted to lactic acid or enter the Krebs cycle (a]so known as the tricarboxylic acid cycle and the citric acid cycle). Two ATP molecules arc produced a naerobically for each molecule of glucose converted Lo two pyruvic acid m olecules

The Oxidative System Oxidative production of ATP occurs in the rnitochondrin, membrane-bound subcellular organelles found in most cells. Pyruvic acid, fatty acids, and amino acids are furthe r degraded \'ia the Krebs cycle, producing carbon dioxide, electrons, and hydrogen ions (H~) (figure lU .3). The carbon dioxide diffuses out of muscle cells into the blood and is transported to the lungs where it is exhaled. The electrons and hyd rogen ions enter the electron transport chain, a series of enzymes that eventually combine the electrons, hydrogen ions, and oxygen

126

The Blophyslcol Foundol1ons of Human Movement

to produce water. This transfer of electrons and hyd rogen ions provides chemical energy to resynthesize ATP from ADP and Pi. Continued production of ATP via the electron transport chain requires a constant supply of oxygen to Ule m uscle cell.

.t

The Three Energy Systems As a Continuum The three energy systems operate as a continuum; each system is always functioning, even a t rest. What varies is the relative contribution each system makes to total ATP production at any given time (figure 10.4). 3 min Smln Even in the extremes of activity, such as maraStart exercise Stopexcrcisc thon rUlming or brief sprinting, all three systems _ Exercise~ Recovery _ are used. For example, in a marathon, the immediate system p rovides ATP at the start of the race; Figure 10.2 Phosphocreatine (Per) depletion the anaerobic system provides m uch of the needed and replenishment. Up to 6 to 7 min are required ATP for the first few minutes until the oxidative to fully replenish muscle PCr stores after deplesystem reaches steady state. The an.. erobic system tion by intense exercise. also provides a significant amount of All' d uring the racc, for examp le, for uphill running and for sprinting at Triglyceride in Proteins in Glycogen in the end . Over the course of the muscle and liver adipose tissue muscle and liver entire marathon, the oxidative system p rovides most o f the ATP needed .

J

J

J

>~~T~~~ AcetylCoA

Lactic acid

J

J

Krebscyclo

La-+ H+

J

/ \

~f~~:Sed ---C02+ H+ +e·

J

o)(idisedby heart. liver andsk.eletal muscle

bufferadin blood and muscle

Electron Iransportchain

A~~P}----! ATP+ H20 Figure 10.3 Simplified scheme of energy metabolism , Fats (fatty acids). proteins (amino acids), and carbohydrate (glycogen or glucose) can all be metabolised to produce adenosine triphosphate (AlP). Ca·r bohydrates con be metabolised aerobically or anaerobically, but the other 1\'10 req uire oxygen . The end products are corban dioxide, lactic acid, AlP, and water.

The Fueling of ATP Production by Fats, Proteins, and Carbohydrates As just discu ssed, ATP can be synthesized via met abolism of fats, proteins, and carbohydra tes. It is important to recognize that these nutrients are not transformed into ATP. Rather, the body brea ks d own these nutrients to release energy from their chemical bonds, which is then used to synthesize ATP (figure 10.3). Carbohydrate (Le., glucose) can be used to p roduce ATP either anaerobically or aerobically. In contrast, fa ts in the form of fa tty acids a nd proteins in the form of amino adds can be used to p rod uce ATP only via the aerobic pathway. At any given timc, th c body metabolises a mixtu re of these n utrients to

BasiC Concepts of Exercise Metabolism

127

IN FOCUS: CREATINE SUPPLEMENTATION: TO WHAT EXTENT DOES THE IMMEDIATE ENERGY SYSTEM LIMIT EXERCISE PERFORMANCE?

l

The immediate energy system provides AlP for brief. intense exercise such as sprinting. Muscle stores of PCr can be depleted by all-out exercise lasting 30 s; this is thought to limit performance in such events. Increasing the amount of muscle PCr may enhance performance in power events. Creatine occurs naturally in food, mainly meat and fish, and is a lso synthe~zed by the liver. If we could increase muscle creatine stores, and if dOing so enhanced performance in power events, we would know the extent to which performance is limited by PCr levels Muscle tokes up ingested c reatine from the blood and uses il to synthesize PCr. Muscle PCr levels can increase by only 10% to 20%, however, and any excess is excreled . The effectiveness of supplementation depends on the subject. amou nt ingested, and type of exercise. Creatine supplemenlalion does not affect endurance exercise or maximum strength, because neither is limited by PCr deplet ion. Creatine supplementation works best for repeated highintensity exercise such as resislance training and multiple sprints. In one recent study, leam handball players were randomly as~gned 10 a creatine-supplemented (20 g creatine per day for five days) or placebo group (Izquierdo et 01. 2002) . A variety of muscular fitness measures were made before and after the five days, Athletes in the supplemented group showed increases in body mass (0.6 kg (1.3 Ib)). muscular endurance, overage power output in an anaerobic test, and performance in repeated sprints or jumping. There were no changes in aerobic capacity or 1-repetition maximum (1 RM) strength. The authors concluded that the main effects of creatine supplementation were enhanced muscular endurance and overage power output in repeated rather than single efforts. The increase in body moss most likely resu~ed from water retention by the muscles.

In another recent study, active but untrained males performed short sprints (6 s) on a cycle ergometer repeatedly over 80 min (Preen et 01. 2001). One group ingested 20 glday creatine and another group ingested a plccebo for five days. When the cycling test was repeated after five days. the supplemented group showed higher totol work and power output over the 80 min compared with the placebo group. This was true for a range of recovery t imes. from 24 to 84 s, between repeat sprints. These studies suggest that short·term creatine supplementation enhances performance of repeated bouts of intense exercise. Creatine supplementotion may not help performance in a single event. but it allows the athlete to train harder by enhancing PCr resynthesis between intervals. Although creatine is not currently banned in sport, there is some concern about long-term health consequences of supplementation, which are not completely known.

prnd uce ATP. However, the relative contdbution of each nutrient to ATP production varies with exercise intensity and, thus, the metabolic rate At rest and during low-intensity exercise, fatty acids and glucose are used in approximately equal amolU1ts as substrates for ATP production. As exercise intensity increases, ATP production progressively relies more on glucose and less on

fatty acid s. During maximal exercise, the muscles metabolisc primarily glucose, derived from muscle glycogen. Amino acids usua lly contribute little to ATP resynthesis «5%) during moderate exercise. Metabolism of amino adds may provide up to 20% of energy production after several hours of prolonged exercise in which glucose supply to the muscic is severely limited.

SOUI?CES • Izquierdo. M" J. Ibanez. J.J. Gonzalez-Badillo. and E.M, Gorostiogo. 2002. Effects ofcreotine supplementation on muscle power, endurance, and sprint performance. Medicine and Science in Sports and Exercise 34: 332-343. • Preen, D" B. Dawson, C, Goodman, S, Lawrence, J. Beilby, and S. Ching. 2001 . Effect of creatine loading on fong-term sprint exercise performance and metabolism. Medicine and Science in Sports and Exercise 33; 814-821. • Terjung, R.L, P. Clarkson. E.R. Eichner, P.L Greenhaff, P.J. Hespel, R.G . Israel. WJ. Kraemer, RA Meyer, L Spriet MA Tarnopolsky, A.J .M. Wagenmakers, and M,H. WiUiams. 2000. ACSM roundtable: The physiological and health effects of oral creatine supplementation. Medicine and Science in Sports and Exercise 23: 706-717.

128

The Biophysical Foundations of Human Movement

System

Immediate ~

Anaerobic glycolytic

..

Oxidative

---------------~

~------.--------

Time

Os

lOs

20s 30s

120s

180 s

10 min

30 min :> 1 hour

Activity

Power

shotput high jump power lift

Running

60 m 100m 200 m

Swimming Games

SOm

400 m 800 m

1500m

200m

400 m

100m

10,000 m

marathon

1500m

+__---- soccer, basketball, fi eld or ice hockey,football (all typas)_

figure 10.4 The energy system continuum, The relative cont ribution of each system to ATP resynthesis depends on exercise duration and intensity.

Lactic Acid-Friend or Foe? Lactic add is produced as a by-product of anaerobic glycolysis. Excess lactic add in the form of the lactate ion (explained next) is transported across the muscle cell membrane into the blood and circulated throughout the body. During maximal exerciSt!, lactic add concentration Illay increase 15fold, from resting levels of 1 to 2 mmol /L up to 30 mmollL in muscle and 15 mmollL in blood. Excess lactic acid is associated with muscular fatigue. Lactic add rapidly dissociates into a lactate anion and free hydrogen ion (H+) (figure 10.3). An increase in H' concentration increases the acidity (lowers the p H) of muscle and blood. TIssues and b lood contain substances that pa rtially, but not full y, buffer the increased acidity. The anaerobic glycolytic system is sensitive to changes in addity, and the decrease in pH slows the anaerobic pathway. Thus, excess lactic acid accumulation resulting from anaerobic glycolysis inhibits fu rther ATP production. This is perceived as fatigue, or the inability to maintain exercise pace. This inhibitory effec t is a protective response, since excess acidity can lead to cell death. During and after exercise, excess lactic acid diffuses from the working muscles and is circulated via the blood to tisf>ues f>uch as the heart, liver, and other muscles. Lactic add can be converted back to pyruvic acid and degraded via oxidative metabolism to produce ATP in Ulese tissues (figure 10.3). Thus, excess lactic acid produced via anaerobic glycolysis can become a fuel for further ATP production in skeletal muscle.

After the end of exercise, excess lactate is also reconverted in the liver back to glucose, which can then be used to resynthesize glycogen depleted during exercise. It takes approximately 20 to 40 min to fully remove lactic acid p roduced during maximal exercise. The rate of lactic acid removal is fa ster during active compa red with passive recovery. The best form of active recovery is light activity, such as slow jogging at approximately 30% to 60% maximum pace. During active recovery, the working muscles use the excess lactic (lcid as a fuel for ATP production. It is important that the pace of recovery be low enough so that more lactic add is not produced . As discussed in the next chapter, interval training programs should consider the rates of lactic acid accu mulation and removal during exercise.

Oxygen Supply During Sustained Exercise The aerobic energy system provides most of the ATP fo r sustained exercise lasting longer than 3 min, and about 20"1" to 30% of ATP for all-out exercise lasting 30 to 60 s. If oxygen supply is insuffici ent to sustain ATPproduction, the muscles must increasingly rely on the anaerobic system with consequent buildup of lactic acid and inhibition of further ATP production Oxygen consumption (VO) is an important measure of energy expenditure during exercise. A standard curve of oxygen consumption during exercise has several components (fi gure 10.5). During

Basic Concepts of Exercise Metabolism

1

1

Slop exercise

Start

exercise Time

RestingV0 2

1

Start

Stop

exercise

exercise

Time

Figure 10.5 Oxygen consumption during exercise . (0) During submoximal steadY-fote

exercise. V0 2 reaches a plateau. (b) During maximol exercise, oxygen consumption continues to increase unfllV02max is achieved. During supramaxlma l exercise, adenosine t riphospha te above that produced by oxidative metabolism Is generated via anaerobic glycolysis.

the initial few minutes of exercise, oxygen uptake is not sufficient to provide all the energy needed. and the bod}' is said to go in to oxygen deficit. During this time, ATP is supplied primarily by the two anaerobic systems--stored phosphagen (PCr)

and anaerobic glycolysis. The oxygen deficit occurs bcs (kD, that can be accomplished in a specified time, usually 30 to 60 s An
.n

o

50

100

150

200

250

Glycogen in muscle (mmollkg)

Figure 10. 14 Dietary carbohydrate and exercise performance. Increasing dietary carbohydrate enhances muscle glycogen stores Higher muscle glycogen content increases exerc ise time to exhaustion by delaying the point of glycogen depletion and, thus. the onset of fatigue. Reprinted from Wilmore a nd Costill 2004.

Table 70.3

Energy Released per Litre of Oxygen Consumed SUbstrate E.nergy per litre of oxygen, kca·1(kJ) Carbohydrate 5.05 (21.2)

Fat

4.70 (19.7)

Protein

4.82 (gO:.2) _

depletion is associated with an inability to maintain the rate of exercise and with the perception of fat igue; in marathon running, this point is called "hitting the wall." Muscle glycogen stores can also be depleted by prolonged periods of interval exercise in which anaerobic glycolysis is the predominant ATPproducing system. For example, 30 ~ of all -out sprinting may deplete 25% of m uscle glycogen, and ten I-min sprints ma y deplete 50% of muscle glycogen. Sports in whichglycogcn depiction may occur include soccer, football, basketball, and any activity requiring repeated high-intensity sprinting over a prolonged period. Swimmers, rowers, and weightlifters training over extended periods of time may also experience fatigue due to glycogen depletion. Thus, dietary carbohydrate is important to many athleles, not just those parlicipating in continuous, prolonged events. Once g lycogen is depleted from muscle, it takes 24 t0 48 h 10 fully restore g lycogen levels. Athletes who train intensely each day may thus be chronically glycogen depleted and have d ifficulty maintaining training and competiti ve performance. Fortunately, one adaptation to intense tra ining is tha t muscle stores more glycogen . Moreover, m uscle glycogen sto res can be further mereused by consumption of a high-carbohydrate d iet. M uscle glycogen replenishment depends on the typc of diet type of carbohydrate, and how soon a meal is consumed after exercise. Glycogen repletion is generally faster if high-carbohydrate food is consumed as soon as possible after exercise (see "Replacing Muscle Glycogen After Exerdse: Does the Type of Food Matter?" on p. 139) . Athletes who train for several hours each day should regularly consume a high-carbohydrale diet. For most athletes, a high-ca rbohydrate diet consists of 60% to 80% of daily energy intake as carbohydrate, equivalent to 6 to 8 g carbohydrate per kilogram of body weight per day o r 420 g (15 oz) to 560 g (20 oz) carboh ydrate per day for a 70kg (l54-lb) person. During times of very intense

Basic Concepts of exercise Metabolism

139

IN FOCUS: REPLACING MUSCLE GLYCOGEN AFTER EXERCISE: DOES THE TYPE OF FOOD MATTER? Intense endurance exercise or repeated highintensity exerc ise can deple te muscle glycog en stores, lead ing to fati gue, To help replace muscle glycogen after exercise, many athletes consume a high-corbohydrate diet. Carbohydrates are contained in many foods, and athletes need sp ecific information about whether certain types of carbohydrate foods are better than others for replenishing muscle g lycogen Glyce m ic Index (GI) is a measure o f the impact of a food on blood glucose level: GI is determined through measurement of ttle b lood g lucose response in a fasted individual a fter ingestion of a specific am.ou nt o f c arbonydra te food. Foods with a high G I, suc h as bread or mashed potatoes, elicit a higher b lood g lucose and insulin response after ingestion. In contrast. of foo ds with a low GI. sue, as rolle d or legumes. elicit~ a lower blood glucose and insu lin response. Hlgh-G I foods co use a la rger increase in b lood insuli n an d glucose levels, which is thought to enhance glucose uptake ond glycogen synthesis by muscle. In one study (Burke, Collier, and Hargreaves 1993), we ll-tra ined m ale cyc lists pe rform e d exercise to deple Ie muscle glycogen on f'w'o occasions. For 24 h after each exercise bout. the cyclists consumed either high-GI or low-GI foods Blood giucose a nd irsulin levels, as well as muscle glycogen storage, w ere higher w ith consumption of the high-GI foods. These results suggesl that m lJ~c l e g lycogen stores are replenished faster when high-GI foods are eaten in the 24 h after intense endurance exerc ise. T!"', us. a ttlletes needing to rep lace muscle glycogen should consume moderate- to high-GI foods during the day a fter intense exerc ise Add ing prote in t o carbohydrate may enhance glycogen replacement after deple tion (Zawadzki, Yaspelkis, and Ivy 1992) . Roy end Tarnopolsky ( 1998) measured the rate of glyco-

training for more than 2 h per day, or during the tap er before major eom pl:'titiulI! this may increase to 9 tol Dg carbohydraLe per kilogram per day. One recommendalion is that, w h en an athlete plans further exercise within th e nexl 6 Lo 12 h, he or she ~hould con'mme 1 g of carboh ydrate per k ilogra m body w eight within the first 3U min after

gen synthesis in 10 healthy men who performed resistance training to exhaustion, Immedia tely and 1 h after exercise, one group drank a hlghcarbohydrate d rink a nd a nother g roup drank a comb ined carbo hydrate-protein~fat drink; the two drinks conto ined the some total amount of energy. The ra te of muscle glycogen rosynthesis after exercise was similar in t he two groups. It was concluded t ha t the rote of muscle glycogen synthesis is sensitive to both the amount of carbohydrate and the total amount of energy contained in foods consumed Imme diately after exerc ise that deplet es muscle g lycogen. Taken together. these studies indicate tha t the rate of m usc le g lycogen replenishment otter exercise depends on the types of food s ingested in the first few hours of re covery. Glycogen replenishment is enhanced by high-energy, hign-GJ carbohydrate foods w ith or without high protein. Both the total amount of carbohydrate and total amount of energy Ingested me important fectors. These studies have great ly helped athletes and d iet itians determine the b est foo ds to eo t during recovery after muscle glycogendeple ting exercise SOURCES • Burke, L.M" G.R. Collier, and M. Hargreaves. 1993. Musc le glvcogen storage after p rolong ed exercise : Effect of t he g lycemic index of c arbohydrate feedings. Journ a,l o f Applied Physiology 75: 1019-1023.

• Roy, BD ., and MA Tarnopolsky. 1998. Influence o f diffe ring macronutrient intakes on muscle glycogen resynthesis after resistance exe rcise . Journal of Applied Physiology 84: 890-896. • Zawadzki. KM" B. B. Yaspelkis, andJ.L. Ivy. 1992. Carbohydrate-protein complex increases the ro le of muscle glycogen storage after exercise. Journal of Applied Physiology 72: 1854-1859

Ih e first buut of exeocise, and then again every 2 h u p to tile next exercise session. Th is practice is especially im portant in m ulti-evenl competitions in wh ich athlctes compete in several events within a few d ays. Ea~i1y d igested foods such as sports drinks or soft drinks, fnJ its, breads, w heat cereals, and glu cose confectionery are effective.

140

The Biophysical Foundations of Human Movement

Do Athletes Need Extra Protein? Many athletes, especially sprinters and weightlift~ ers, consume a high-protein diet and supplement their diets with protein or amino acid powders or drinks, believing that the extra protein w ill increase muscle growth or hypertrophy. It is true thai athletes need more protein in the diet than the flverflge nonflthlete. However, a well-balanced diet is more than adequate to meet the protein needs of virtually all athletes. For the average healthy nonathlete, the recommended daily inta ke of protein is 12% to 15% of daily energy consumption, or 0.8 g protein per kilogram per day. For a 70-kg (154-lb) person, this is about 56 g (2 oz) of protein per day. ('The protein content of certain foods is listed in table 10.4.) On anabsolute protein basis, both power /strength and endurance athletes need about 50% to 100% more protein per day lhan the nonathlete. The general recommendation is thai s trength/power/speed athletes should consume 15 to 2.0 g protein per kilogram body weight per day, and end urance athletes should consume 15 to 1.6 g protein per kilogram per day. However, because ath.letes expe nd la rge amounts of energy during training, th ey also consume at least 50% more energy per day than the nonathlete. Athletes can easily meet their additional protein needs through a well-balanced diet that contains 12% to 15% of daily energy as protein, simply by increasing the total amount of food consumed. These protein needs of athletes

are depicted in table 10.5. To provide the recommended 1.5 to 2.0 g protein per kilogram per day, the 70-kg (154-lb) athlete would require approximately 105 to 140 g (3.7 to 4.9 oz) p rotein per day. Adaily diet of 3,750 kcal (15,750 kJ) containing 15% of total energy as protein would provide sufficient carbohydrate and protein to meet the energy and protein needs of any athlete. Excess dietary protein is not incorporated into muscle but is excreted by the kidney or used to synthesize fat. Neither is desirable, since excess protein excretion places an extra burden on the kidneys and excess fat impairs performance in athletes. High-protein diets and protein supplements are also costly. In addition, a high-protein diet usually does not have sufficient carbohydmte to fuel extended training sessions. By emphasizing protein at the expense of carbohydrate in the diet, the athlete may be unable to train at a high intensity for long. Moreover, muscular hypertrophy may be compromised because inadequate energy is available to fuel protein synthesis in the muscle cell.

Imponance 01 Replacing Water Lost During Exercise Although water is rarely thought of asa nutrient, it is an essential part of the diet. Because the human body is relatively inefficient, most (70-80%) of the energy produced during physical activity is not used to perfo rm work but appears as heat. The body can w ithstand only a relatively narrow range of core temperatme. To prevent core temperatu re

Table 10.4

Prolein Conloined in Some Foods F,od

Pro~ln

Serving size

(grams)

250 ml{8 oz)

Milk

30g(10z)

Cheese

5-7

Steak

26-34

Fish

18-26

100 g

Chicken

30-34

100 9

Wholemeal bread (whole-grain)

3,5

,.

100 9 (3.5 oz)

' 1 slice

Cereal

2-6

30g

Nuts

4

25g

Egg

6

,1 egg

Fruit

'"

Vegetabl~s

1-2

1 piece ,: 30g

Complied from BUlkeand Deakin 1994, Inge and Brukner 198a. McArdle, Katch. and Katch 1996. Stanton 1988

-,' ,-:,' ,to

Basic Concepts of

ExercL~e

Metabolism

141

*1 g protein yields 4.2 kcal when metabolised by the body:

from rising dangerously during exercise, heat produced during exercise is transff'rred from muscles to blood, then through the circula tion to the blood vessels in the skin. The primary mechanism for heat loss during exercise is evaporation of sweat. Depending on the individuaL exercise intenSity and duration, and environmental conditions, sweat rates may vary between 0.5 and 3.0 L/h (between 1 pint and 6.3 pints/h). Even ,vith evaporative cooling, an athlete's core temperature may increase from the norma137"C (98.6"F) up to 39.5"C (103.1 ° F) during intense prolonged exercise. During prolonged exercise, redistribution of water within the body and loss of body water via sweating may reduce blood volwne. Losing body water equivalent to 4% to 5% of body mass may adversely affect thermoregulation and. exercise capacity. The cardiovascular system adjusts to this loss of blood volume by increasing heart rate to offsct the concomitant decline in strokevolume and cardiac output. Thus, a given exercise will cause greater stress on the cardiovascular system when performed in a hot versus a morc moderate environment. Ifprolonged exercise in the heat continues without replacement of body water, blood volume may drop significantly and the body may be unable to lose excess heat. Body temperature may increase dangerously, above 42° C (107.6" F). Heat illness or heatstroke, in which the athlete's cardiovascular and thermoregulatory systems are severely impaired, may occur. Heatstroke is Iift:. threatening if not treated promptly. A thletes s hould consume water at regular intervals during prolDnged exercise, especially

when exercising in the heat. TIle general recommendation is to drink approximately 500 to 1,000 ml (I to 2 pints) plain water 1 h before exercise, an additional 250 to 500 ml (1/2 to 1 pint) 20 min before exercise, and then 250 ml every 15 min during exercise. Plain water is sufficient for exercise up to 6O-min duration. Addition of glucose and electrolytes (e.g., sports drinks) is recommended for intense exercise lasting longer than 60 min, when muscles may become depleted of glycogen. A modest amount of glucose (about 6%) in solution will help improve performance without compromising water replacement in very long cvents. A low concentration of electrolytes promotes faster absorption of \vater. Rehydration after exercil:le is especially important, and it may take several hours to completely replace waler lost during \:!xercise. Thirst is a poor indica lor of the need lor fluids or amount of fluid lost via sweating, and the athlete mu..nmalcxercl5e

Increased blood and Q!(ygen delivery to ml!sclj3S

XnC~ed blood vOIUmO:e,=,e~d"b"'o=od'-G-e~II'-"u-m~be-'-,.-n-c ,n= cre' ased oxygen deliVery to muscles d +-'

~h~~W~ln CQ~rtt _'_ __. .'..:

jnGr-Elasoo ~gen extraction from .~ .

:~.ased ·bl00d vi~coSity

l'

~ _ _ _,,_:... ; .~. ::: ••.••• ,,:;,:: : : ,;J::,~

Increa!:ied Qxygen delivery

to mitoc.hory8f!!lLi:hm::i,i;; : •.

Easier rriOV~ment ofbtood ..th rm.;ghoiJt :b:~~~:-; : · " ., increased .cardiac output guring maxi~?:~~xeicise air must be inspired by the lu ng~ to pmvide the same amount of oxygen to the muscles. The reason is the higher respiratory rate and shallower brea thing in children. The respiratory muscles of children must work harder during exercise; and rcspiratorymuscle fatih'11.Ccontributes to the higher metabolic cost, feelings of discomfort, and early fatigue during intense exercise. These difference::. in the cardiovascular and respiratory system responses to exercise limit oxygen delivery to working muscles, resll1tingin a lower endurance exercise capacity i.n children. This does not mean that children cannot perform end urance exercise or improve endurance exercise capaci ty; rather, it

168

The Biophysical FoundaHons of Human Movement

means that children carmot be expected to perfonn endurance exercise, or train for endurance events, at the level expected of adults.

Thermoregulatory Response 10 Exercise in Children Children produce more metabolic hea l during exercise than adults and must lose more body heat to avoid an increase in core temperature during exercise. During exercise, the body loses heat mainly by sweuting. Computed withudults, children begin to sweat at a higher relati ve work rate and also sweat less during exercise. The lower rate of heat loss via sweating is partially compensated for by a higher rate of heat loss via circulatory adjustments and conduction and convection from the skin's surface. This is so because children have a higher surface area-ta-mass ratio. However, the combination of higher metabolic heat prod uction and poorer ability to remove heat via sweating means that children are less tolerant of prolonged exercise, especia lly in a warm envirorunent. Child ren also require a longer time to acclimatize (adjust) to exercise in wann environments. In addition , children have a less responsive thirst mechanism; that is, they are less likely to voluntarily replace wutcr lost during exercise. Coaches and teachers who supervise children exercising in the heat should be aware of the lower heat tolerance in children and should plan sport training and classes accordingly. For example, sport training or physical education classes should schedule different sports according to the season s and should include interval-type exercise with longer rest intervals and freq uen t water breaks on hot days.

Muscular Strength In Children As described in earlier chapters, muscular strength is related to muscle size. As eXpeCle tissue that consumes oxygen to produce energy for exercise. About 50"1" of the decrease in Vn,max is duc to a decrease in maximum. heart ratc, ;Csulting from changes in neural input to the heartj this is not altered by training at any age. Since cardiac output is a function of heart rate and stroke volume, the decrease in maximum hea rt ra te resu lts in a decrea se i.n maximum cardiac output. A~ discussed in chapters 10 and 11, cardiac output is an important determinant of\fO~ax, In contrast to cardiac output and heart rate, maximum stroke volume Illay not change much during aging. I'he ability of skeletal muscle to extract and use oxygen during exercise also decreases with age in a sedent~l ry population. This decrea,'>e is due to a red uction in the number of capillaries within skc1cLe.1 musdcand in the Cilpacity to redirect blood flow to muscle, which compromises oxygen delivery to working muscles during exercise. The oxid200k over recommended weight

10·20% over recommended weight for

~~__~~__~~h~ ei~ g h~t~~ Body composition

>30

Adult male 18-20% tal

=-,-----c~_:_--~-'A:;:: d ull female 27-30% fat

Waist-la-hip

ra~o

to;"

________+h~.~ i g~ht~~~~---------Adult male >20% fat Adult female >30% fat

Adult male > 1 .0 Adult female >0.85

+ ___________________

~sureme nt .~+:, Ad ",u", lt ;; m~ al~ e~ >t!-,O:":O",,cm= (3:c 9 ," in,:,:.)______

Adult female >90 cm (35 in.)

Overweight and obesity can be defin~d in several ways (table 13.6). With use of the Quetelet index (Qn or body mass index (BMI) (sec chapter 3), overweight is defined as a Ql between 25 and 30 kg/ml and obesity as a QJ over 30. Th e recommended QI range for good health is 20 to 25. lf standard height-weight charts are used, overweight is usually defined as 10%, and obesity as 20%, over recommended weight for height. Because of the simpl icity of measu rement and calcu lati on, these two defin itions are appropriate for the general population. However, athletes often carry additional body mass as muscle and many athletes, especially those in strength and power sports, would be classified as overweight using these simple definitions. For physically active people, including athletes, body composition-although it is a more complex measure-is a better method to define obesity because it distinguishes fat from lean bod y mass. After all, it is the excess fat, not total body mass, Ulat is associated with disease. For men, overweight is defined as body fat between 18% and 20%, and obesity is above 20% fat. For women, the values are 27% to 30% fat for overweight and above 30'''/0 fat for obesity. Obesity is usually caused by an imbalance between energy intake and expenditure, or a higher amount of energy consumed as food than expended. Obesity is rarely caused by hormonal factors, but it is now widely believed that there is a strong genetic influence. The pattern of fat distribution around th l! bod y is important. Central or abdominal obesity is defined as an excess of fat in the abdominal region. This pattern of fat deposition primarily

in the uppe r body (ches t and waist) occurs more often in men and carries a higher ris k of heart disease, stroke, and diabetes than the female pa ttern of fat deposition primarily around the hips and thighs. This relationship holds true even when one is comparing fat deposition within a given sex. For exa mp le, women w ith fat deposition primarily in the upper body are a t higher ris k of cardiovascuJa.Tdisease than women w ith fat deposition primarily in the lower body. The two patterns of fat deposition and obesity have been termed the "apple" and "pear," or and roid and gynoid, shapes for male and fema le patterns, respectively (fi gure 13.1). The pattern of fa t distribution maybe estimated using the waist· to-hip ratio (WHR), calculated by dividing the circumference of the waist by thilt ufthe hips. The recuJ1ullended WHR is less than 0.85 for women and less than 1.0 for men . A simple measure of only the waist circumference can aLso be used to indicate excess fat; recommended values are less than 90 cm (35 in.) for women and less than 100 em (39 in.) for men. Exercise and Obesity

Obesity is caused by an imbalance bctWt..-'C1l energy intake and expenditure, so body weight shouJd be controlled through decreasing energy intake, increasing energy expend iture, or some com bination of the two. Weight reduction programs that rely solely on dieting are generally ineffective for permanent weight loss. Regular physicol activity is important to long-lasting control of body weight for many reasons (see "Benefits of Exercise in Weight Loss," p. 188).

188

The Biophysical Foundations of Human Movement

Figure 13.1 Pattern of fat d istribution in the body, (a) Android or male paHern Capple" shape): (b)gynoid or female p a ttern ("pear" shape) Reprinted from WHmore and Costill 2004

BENEFITS OF EXERCISE IN WEIGHT LOSS

weight and obese). Exercise to reduce bodyweighL helps maintain lean booy mass, whereas diel withalit exercise may result in loss of muscle mass. It

Permanent weight loss requires either exercise alone (for the moderately overweight) or a combination of exerci'ie and diet (for the seriously o\"er-

is important to maintain muscle mass because skeletal mURde is a major site of fat metabolism; losing muscle decreaRefi the body's capacity to bum fa t. Exercise is also important to maintaining the body's rcstingmetubolic rate (RlvlR), the minimum amount of energy needed to maintain bodily functions. In most people except athletes inheavy training, RMR accounts for between 60% and 75'}-;' of total daily energy expenditure. Resting metabolic rate is strongly related to age, gender, body size, and Jean body mass. R!l.1R may decrease by 10% to 20% after significant weight loss through diet alone. This reduces the amount of energy needed by the bod y (i.~., the amoun t that can be consumed without gaining weigh!) and is one reason people find it so difficult to maintain weighllossafterdieting. In contrast, wejght loss through exercise helps prevent the decline in RMR, thus ensuring permanent loss of body mass and fat. Moreover, exercise

Applications of Exercise Physiology to Health

increases muscle mass, enhancing lean body mass and leading to a more favourable body composition. In addition, in moderately obese individuals, low- to moderate-intensity exercise helps correct components of the metabolic syndrome, such as abdominal obesity, glucose tolerance, and blood lipid levels, even with only modest changes in body weight. Exercise Prescription for Obesity

Exercise programs must consider that people who a rc obese need to increase energy expenditure but may be unfit. Exercise should be low impact to prevent injury to load-bearing joints. Good examples include walking, low-impact or watcrbased aerobic activities, swimming, cycling, and Circuit-type resistance exercise using weight and stationary exercise machines. Each session should be long enough to require the exerciser to expend at least 300 kcal (1,260 kJ) of ene rgy, equivalent to at least 30 min of exercise per session. Since energy expenditure inc~aSl;'S with increasing duration of exercise, longer sessions approaching 60 min are recommended provided lhey do not lead to overuse injury or excessive fatigue . Exercise should be performed at least four times per week a nd daily if possible. Since people who are obese may be unfit to start with, low-intensity -programs that progress gradua.llv are recommended. "Exercise intensities in the ra~ge of 40% to 60% VO;nax or heart rate reserve, equivalent to opproximately 50"/" to 65",{, of agepredicted maximum heart rate (see chapter 11), are appropria te at the onset of an exercise program.

Hypenension In most d eveloped countries, hypertension or high blood pressure a ffects about 20% of adults; up to 60% of older adults may exhibit high blood pressure. The World Health Organization (vVHO) defines hypertension as a chronically elevated blood pressure reading above 140/90 nunHg. The fi rst number is systolic blood pressure and represents the pressure in Lhe arteries during the heart's contraction; the second number is diastolic blood pressure and represents the pressure in the arteries du ring relaxation of the heart betwe€n contractions. Hypertension is associated w ith increased ri'lk for stroke, heart disease, renal (kidney) failme, ether of neurons thrau gh synapses--

Cerebellum

the specialized junctions between nerve cdlsprovides the essential found ation for the human nervous system and allows the nervous system to provide effective two-way communication between its sensory receptors and its motor units.

Structure and Function 01 Neurons Spinal cord

\

Descending elfarent (motor)

pathways

Figure 14.1 The central neNOUS system consisting of the brain and the spina l cord with its ascending sensory and descending motor pathways

The neuron or nerve cell is the basic component of the neuromuscular system and is cs&entiaJ for receiving and sending met;sages (or information) throughout the entire s},stem. Vvbileneurons var}' substantially in both size and shape, depending on their specific functiun and location within the nervous system, most havl' a similar structure-a cell body to which are cOimected a singl~ axon and (typically) many dendrites (figure 14.2). Thc celt body, containing the nucleus, regulates tilt:' homeostasis of the neuron. The dendrites, collectively formed into il dendritic tree, connect with and receive infonnation from other neu rons and, in some cases, sensory receptors. The axon is responsible for sending information 8way from the neuron to other neurons. Collatcr81 branches off the axon permit communication of the nerve impulses from any particular neuron to more than one target neuron. Any single neuron can influence the activiLy of up to 1,()(X) other neurons and is itself

204

The Biophysical Foundations of Human Movement

Figure 14.2

A typical neuron or nerve cell.

influenced by the excitatory and inhibitory impacts of some 1))00 to 10,000 other neurons. There are several types of neurons; the structu re of each is dictated by its function (figure 14.3). The sensory or afferent neurons are relatively linear in s hucture with a single axon cormecting the sensory receptor ends to the cell body. The structure of the motor or efferent neurons varies accord ing to their location. TIle alpha motor neurons of the spinal cord possess many dendritic branches and a relatively long axon, also heavily branched to innervate multiple (100-1 5,000) skeletal muscle fibres. The gamma motor neurons, as we will see in the next section, innervate contractile (intrafusa l) fibres located within the muscle receptors. Consequently gamma motor neurons, which constitute about 40% of the total motor neurons within the spinal

cord, have smaller, considerably less branched, axons than do alpha motor neurons. The pyramida I cells, located within the motor cortex of the brain, are so named because of their shape, derived from their branching tree of dendrites that all fu nnel d own to a single slender axon. Pyramidal cells send motor commands over the long distances from the brain to the spinal cord and may have axons up to 1 m (3.3 ft) in length . The Purkinje cells within the cerebellum also have a single thin axon to which information is sent from an incredibly rich, systematically organized set of dendrites that provide these neurons with a characteristic treelike appearance. lnterneuron s have a variely of s hapes but typically have multiple dendri tes and branching axons that permit the cOlU1ection of multiple neurons with multiple other neurons. The structure of interneurons and thei r connections facilitates both the convergence of multiple inpu t messages onto a single output cell or set of cells and the divergence of a single input message to several different motor neurons. Interneurons o ri gina te and terminate within either the brain or spinal cord .All neurons wi thin the central nervous system are surrounded by, and outnumbered by, other cells called glia or glial celJs, which provide, among other things, the metabolic and immunological s upport for the neurons. Neurons ca rry Inessages f.rom theirdelldri tes to the terminal fibres of their axons through a series of electrical pulses, prod uced in the axon hillock (fi gure 14.2). The electrical pulse produced by the axon hillock is dependent on the spatia.! and temporal distribution of Ihe pulses impinging on the cell body from its dendritic tree. Signals arriving early and origina ting from dendrites close to the axon hillock carry more weight than signals from distant neurons arriving late. If the summed weight of the impulses reaching the neuron exceeds its threshold voltage, the axon hillock triggers a pulse (the cell "fires") and this pulse is propagated along the axon to its terminus. The rate at which it is transmitted varies, being greatest in axons of large diameter and in those that are insulated by the fatty substance, myelin. Each neuron is therefore more than simply a conductor of electrical signals, constantly undertaking complex proCl..ossing . on the input signals it receives from other sources.

SlrUClure and Funclion of Synapses The passage of information fTOrn one neuron to another occurs via the synapses (which are in many

Basic Concepts of Motor Control : Neurophysiological Perspectives

205

Figure 14.3 Functional types of neurons: (a) sensory neuron; (b) alpha motor neuron; (c) gamma motor neuron; (d) pyramidal cell neuron: (e) Purkinje ceil neu'on: and (f) interneuron.

ways the equivalent w ith in the nervous system to the joints in the m usculoskeletal system). The term synapse, coined by Sir Charles Shcrrington, originates from <J Greek word meaning " union." At the synnpsc, the axon of one neuron comes in dose p roximity to, but not direct physicnl contoct w ith, the receptor surfaces of one or more other (postsynaptic) nerve cells (figure 14.4) The electrical activity in the presynaptic neuron is t ransmitted across the gap (the synaptic cleft or junction) to the postsynaptic neuron either via the direct spread or electrical current or, more frequently, by the action of a chemical mediator,

ca lled a neurotransmitter. In the case of chemical transmiss ion, the nerve impu lse in the axon of the p resynaptic neuron trigge rs the release of a neurotransmitter from tin y stora ge sacs (vesicles) w ithin the presynaptic membrane into the synaptic d eft. Specialized receptors on the membrane of the p ostsynaptic neuron detect the presence of the neurotransmitter, triggering either a heightened excitatory or inhibitory response in the postsynaptic neuron, dependent on the nature of the specific neurotransmitter. The transmission of information from one neuron to another the refore typically requires lhe Lransduclion of an electrical signal

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The Biophysical Foundations of Human Movement

Figure 14.4 The microscopic struct ure of 0 typicol synapse.

to a chemical one (at the presynaptic neuron), the diffusion of the chcmicil l trilnsmitter i1cross the syoilptic d eft, and then the transduction of the chemical signal back to an electrical one (i1t the postsynaptic neuron). There are several different neu rotransmitters; acetylcholine (ACh) (an excitatory neurotransmitter) is the best known. The synaptic COI"Ulection can be either excitatory or inhibitory and Utis provides the fOWldation for more complex flUlctional connections within the nervous system such as reciprocaJ inhibition, wltich, as we w ill see in a subsequent section, form the comerstolle of many reflex actions.

Sensory Receptor Systems for Movement The main sensory information to guide the selection and control of movement comes from vision and proprioception. Visual information is derived from the light-sensitive sensory receptors loca ted within the retina of the eye. Pro prioception (from the Latin proprius, meaning "own") i s information abou t the movement and orientation of the body and body parts within space and is provided via kinaesthetic receptors located in the m uscles, tend ons, joints, and skin and vestibular receptors for balance located in the inner ear (kinesthesis is derived fTOm two Greek words meaning "to move" and "sensation").

Although Ule sensory receptors for the many facets of vision and proprioception, as well as the receptors for other senses such as hearing, taste, and smell, varydramaticaUy in their specific structme, all sensory receptors share the coounon function of transducing physical energy from either beyond the body (such as light or sound waves) or within the body (such as muscle tension) into coded nerve impulses. These nerve impulses can then be transmitted from one part of the body to another via the nervous system or in tegrated from one sensory system to another. In this regard the sensory receptors are very much like transducers in electronics, converting the information they receive into electrical pulses that can be transmitted along the body's ma ny neural pathways. Humans, like all other animals, are sensitive to only a limited range of the physical signals within the environment. We know, for example, that ultraviolet and infrared wavelengths of light are present in our su rround ing environment, but we do not perceive theilesign als w ithout the assistance of mecha nical devices because they fall beyond our usual range of visual sensitivity.

The Visual System rich visua l perception of the surroun ding environment is achieved through the unique anatom y of the eye and a very complex set of neural processes (figure 14.3). Light reaching the retina (the light-sensitive area at the back of the eyeball) passes t hrough a number of layers of cells to reach the photoreceptors. The photoreceptor cells (the rods and cones) contain chemicals that are sensitive to light, and they send off nerve impu lses through their axons to other cells in the retina. This pattern of nerve impulses is specific to the pa ttern of light falling on the photoreceptors. The rods are most sensitive to light and do not respond to colour, and are therefore the primary receptors for night vision. The cones, in contra!>t, require high levels of illumination to fWlction but enable us to have colour vision. The density of both types of pholoreceptors is higher aroWlcl the fovea, giving this area (corresponding to some 2° of the centre of our visual field) the highest le\'el of sensitivity (acuity). Nerve impulses arising from the photoreceptors are passed through a number of other layers of intemeurons before being sent to the brain via the optic nerve. The arrangement of nerves in the horizontal, bipolar, a macrine, and ganglion cell layers of the retina allows for early processing of

OUf

Basic Concepts of Motor Control: Neurophysiological Perspectives

207

Figure 14.5 Horizontal section of the eyeball (shown on left) with t he layered microstructure of the retina (shown on right)

the visual signaL especially in tenns of averaging signals over a range of photoreceptors and enhancing contrast between adjacent areas of the visual field . Visual signals from the retina are carriL>d via the op tic nerve along two major pathw ays, d istinct in structure and function. Some 70% of the COlUlections from the optic nerve go first to an area of the midbrain (ca ll ed the la tera l gen iculate nucleus) and from the re t o the v isual cortex, which is located toward the back of the cerebrum . This pathway, contributing to focal vision, is spec ialized for recognizing objects, distinguishing detaiL and assisting in the direct visual control of fine, precise movements (such as those involved in threading a needle). Most of the remaining nerve fibres from the optic nerve terminate in another section of the midbrain called the superior collicuH. This pathway, contributing to ambient vision, receives information from the wh olo: of the retina, including the peripheral retina, and is concerned with the location of moving objects within the whole visual field . This pathway is

especially im p licated in p rovid ing information about our position in space and our rate of movement through the environment. Damage to the focal vision pathway results in an inability to identify objects but not the ability to locate them. The converse is true of damage to the ambient vision pathway.

The Kinaesthetic System [n addition to information prov ided th rough vision, infonnation about the "sense of movement"

is also derived from spt'cialized receptors located w ith in the muscles, tendons, joints, and skin .

Muscle Receptors The principal source of so:nsory information from skeletal m us cle is provided by the muscle spindle. The muscle spindle is rnuque as a receptor in that it also contains m uscle fibres il nd hen ce also has movement capabilities. Muscle spindles are loca ted within aU s keletal mu scles, although they are particularly abu ndant in small m uscle:s (such as those in the hands) used to control fi ne voluntary

208

The Biophysical Foundations of Human Movement

movements, Muscle spindles provide the centra l nervous system w ith information about the absolute amount of stretch plus the rate of change of stretch in a particular muscle. This, as we will see in s ubsequent sections, is invalU50years) people as they performed a reaction time tosk. Consistent witt"! the EEG findings

of the Tuebingen grcup, the U.S. team discovered greater activation of a number of motor areas of the brain (c ontralatera l sensorimotor cortex, premotor and supplementary motor areas.. and Ipsilateral cerebellum) in the older partiCipants. Further, some a reas of the brain that were not activated for the younger participants in completing this task (ipsilateral sensorimotor cortex, basal ganglia, and contralateral cerebellum) were activated by the d der p articipants. Collectively, these findings suggest that even for the performance of simple movement tasks, additional a reas of the brain of older p:::lrticipants are recruited . Thts is possibly an adaptiVe mechanism to compensate for some of the inevitable neural function loss that occurs with aging. SOUecES • Mattay, V.S., F, Fera, A. Tessitore, A.R. Hariri, S. Dos. J.H. Callic ott. and D.R. Weinberger. 2002. Neurophysiolog ical correlates of age-related c hanges in human motor function . Ne urology 58,630-35 • Soiler, A., J . Dic hgans, and C . Gerloff. 2COJ. The influence of normal aging on the cortical proceSSing of a simple motor task. Neurology 55: 979-985.

Motor Control Changes Across the Ufe Spon

A prog ressive loss of nerve cells occurs throughou t life (at a rate of som e 10,000 per day) such that by age 65 to 70, some 20% of the total neurons present a t birth are lost. Glial cells, on the other hand, increase with age. The net effect is a d ecrease in brain weigh t in people who are elderly. Brain activity during the performance of simple movements (as mea silled using techniques such as electroencephalography an d functional magnetic resonance imaging) appea rs to be funda mentally different (or older peop le (see " Brain Mechanisms for Movement in Older People," p. 252). There is also a genera l s lowing of sensory and motor function with aging, resulting from a reduced impulse (signal) strength relative to background neural activity (noise).

Changes in the Sensory Receptors and Sensory Systems Maturation of Ule key sensory systems for movement (Le., the visual, kinaesthetic, and vestibular systems) occurs at different rates, each limiting the time at which mature movement control can first occur. The changes in movement "hardware" affect both information-processing capabili ty and observable motor performance. The Visual System

The eye, like the brain, undergoes most of its growth prior to birth, even though the size of the eye a t birth is only about half its final size at maturity. The retina is fairly well developed at birth; the myelination of the op tic nerve has commenced at this time and is complete some one to four months after birth. The neural pathways to the visual cortex are fWlctional at birth. Visu al acuity (sharpness of vision) is poor at b irth, and a t th e first month of life the acuity of the human infant for stationary objects is about 5% that in matu.re adults. (The infant of one month sees the same degree of visual detail from a d istance of 6 m [6.6yd ] that an adult would see from approximately 250 m [273 ydJ!) Acuity improves rapidly during the first yenrs of life; adult levels are attained by about age 10 for the viewing of stationary objects and age 12 for the viewing of moving objects (figure 16.9). This improved acuity results, to some degree, from an improved capability to adjust the shape of the eye's lens (calJed accommodation) but primarily from improved neuronal differentia tion in the retina and cOlm ections in the cortex. The visual system matures at a rate sufficient to provide the ~isual information needed to guide movement at each stage of development.

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With aging come a num ber of declines in dsual fWlction tha i can, in turn, adversely affect movement capability. By about age 40 there are clinically signilicant losses in the ability to accommodate to near objects. Material that can be read at 10 cm (4 in.) a t age 20 must be progreSSively moved away to distances of 18 cm (7 in.) a t 40, 50 cm (20 in.) at 50, and 100 em (39 in.) a t 70 years of age to retain sharp focus. In addition to acuity losses with aging., there is reduced sensitivity to glare, declining sensitivity to contrast, narrowing of the visual field size, and increased visual difficulty at low light levels. All of these changes make it increasingly difficult for people to perform movement tasks (such as dr iving or catching) that require precise visual informa tion and accurate judgment of the location and speed of moving objects The Kinaesthetic and Vesflbular System

Because of their central rotc in man y reflex systems essential for the survival of the newbom, the kinaesthetic receptors develop early and are functional essentially from birth. Cutaneous receptors in the mouth a re functional from as early as 7 to 8 weeks, and those in the hand from 12 to 13 weeks after conct!ption. Muscle spindles are evident in muscles of the upper arm from 12 to 13 weeks postconception, although the main deveJopment of the m uscle spindJes, as well as the Golgi tendon organs, joint receptors, and cutaneous receptors,

254

The Biophysical FoundaNons of Human Movement

occurs during the period of four to six mon ths after conception (three to five months before birth). The vestibular apparatus is completely formed two to tluee months after conception and may function reflexively from this point omvard. The kinaesthetic and vestibular systems are thus prepared early in life to support infant activity before the visual system matures. Although relatively little is known about the effect of aging on the kinaesthetic <md vestibular receptors, there are clear func tiona l losses in elderly persons with respect to balance, as detected tluough the vestibular system, and sensitivity to touch, vibration, temperature, and pain as detected thro ugh the cutaneous receptors. We know that some 40% of the vestibular receptors and nerve cells are lost by age 70. We also know that lhe number of Meissner's corpuscles in the skin (figure 14.8) decreases with age and that those remaining undergo changes in size and shape. In addition to a loss of receptors is a decrease of as much as 30% in the number of sensory neurons innerva ting the peripheral receptors (a condition known as peripheral neu ropathy).

Changes In the ElleClars (Muscles) Some aspects of the growth and aging of m uscle wl:!rl:! cunsidered in chapters 4 and 12. The number of muscle fibres increases prenataJly and also for a short period postnatally, approximalely doubling betw'een the last trimester of geslation and fo ur months after birth. Most fib res do not differentiate l.Ul.til the foetus is around seven months old Siow-hvitch (or type 1) fi bres begin to appear five to seven months after conception, and these fi bres constitute about 40% of all muscle fib res present at birth. Fast-twitch (or type IT) fi bres make up abou t 45/., of all fibres present at tenn. Slow- and fast-tv..'itch muscle fibres continue to increase in the first year of life, and the releltive distribution of fibre type appeelrs to reach steady stelte by the age of 3 years. As children mature, their muscle fibres become wider (figu re 16.10) and longer, and this comes about through an increase in both the number and length of the contractile units (the myofibrils) within each muscle fibre. Adult-sized muscle fibres are attained in adolescence. The reductions of muscle size that occur in the elderly appear to be a consequence of red uced effectiveness of the nerves that activate the muscles and the selective loss of fast-twitch muscle fibres and motor lmits

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p rimarily. Growing evidence indicates significant changes in the control properties of the motor units wi th aging. For in.s tance, discharge rates of motor uni ts become more variable in older people, and this contributes to a reduced capability to perform steady, submaximal muscle contractions.

Mofor Control Changes Across the Ufe Span

Changes In Reflex Systems Receptors and effectors, as we SJW in chilptcr 14, arc the key elements of reflex systems. The development, modification, and frequently the extinction of reOexes havea complexity greater than that of the maturational changes of the sensory and effector componen ts of the various reflex systems. Reflexes present at birth ur soon thereafter can be broadly distinguished wi th respect to whether their principal function is to assist survival of the newborn orto lay the foundations for thede\'elopment of voluntary movement control.

255

birth or soon thereafter. These reflexes dis.'ppear after some four to five months and before voluntary walking or swimming is attempted (figure 16.2). While the exact role of the postural and locomotor reflexes in the development of future volunlary movement control is not entirely d ea r, it appears that these refl exes collectively playa role in preparing the nervous system and its pathways for the emergence of the voluntary fundamental motor skills discussed earlier in this chapter. Localized reflexes such as the stretch reflex described in chapter 14 persist throughout the life span and appear to alter relatively little in old age.

Primitive Reflexes

The human newborn is extremely vulnerable because of its limited mobility and capacity for voluntary movement. Consequently, in the early s tages of life, infants must depend heavily on adult caretakers and some reflexes for survival and protection. Those reflexes present at birth that function predominantly for protection and survival are referred to collectively as primitive reflexes. Examples are the sucking reflex, which enables the newborn to instinctively gain n utrition from the mother 's breast; the searching or rooting reflex, which helps the newborn locate the nipple; a nd the Moro reflex, which as..ive the impression of having "all the lime in the world ." One important means by which expert performers overcome lime constraint::> is to anticipate likely events from any advance information availa ble to them. Experts are better able than lesserskilled players to predict actions in advance on the basis of infonnation available from the actions and postures of their opponents. For example, expert badminton players are able to make more accurate predictions of where an opponent's stroke will land fro m information available before the opponent

266

The Biophysical Foundations of Human Movement

Figure 17.5 A typical st ructured pattern recogn ition situation from the spomt of fie ld hockey. Photo copyright by the National Sports Informati on Centre. Australian Sports Commission. Reprinted with permission.

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strikes the shutllecock lhan can novices (ngu re 17.7). Experts are also able to pick up information from cues in addition to those used by novices. In the bad minton exampl e, experts are able to pick 1.1 p advance informiltion from the motion of the racket and the arm holding it whereas novices Ciln only use the racket as a cue (figure 17.8). Expert-novice differences in the ability to anticipate OCCUI even when the hvo skill groups may be looking at the same features of their opponent. This demonstrates that the limiting factor in the perceptual performance of untrained individuals is not the ability to pick up the necesEX"1ry sensory information but rather the ability to interpret, understand, and use it to guide decisiun making and movement execution.

Decision Making A capability to make decisions both quickly and acc urately wuuld be clearly beneficial for the

Motor Conlrol Adoptalions to Training

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S not only are more consistent on a trial-to-trial basis for expert performers but also show a clearer, more distinct pattt'm of force pulses. Expert performers make greater use of Ule external forces (SUdl as gravity and reactional forces) available within movements and restrict the injection of muscular force generated by the body to only those points in the movement w here it is needed and can act most effectively. The time course of power generated in movements consequently varies significantly between experts and novices. Novices tend to supply muscular force more frequently throughout a movement, often either inefficiently in opposition to external body forces or as an unnecessary supplement to external forces. It is therefore not surprising to observe that neuromuscular recruitment patterns (as revealed from electromyography) become more discrete with practice and that there is a general reduction in recruitment as muscular contraction extraneous to the movement of interest is diminated (figure 17.10).

Implications for Training The value of knowledge about expert-novice differences in information-processing capability is thaI it provides a guide to where energy and atten-

269

tion should be directed in practice and training. Training based on improving the information-processing factors known to be related to the expert's advan tage on a task would appear to be more sensible than training focu sing on factors providing little or no discrimination between experts and novices. This logic is unfortunately not always fully appreciated or considered in the design of p ractice and recommendations for practice. For example, the generalized visual and kinaesthetic training p rograms that become popular from time to time, and that are d esigned to improve motor skills through improving the general sensitivity of the sensory systems for movement, are most unlikely to be beneficial forskillieaming. The reason is that they do not, under most circumstances, train any of the limiting infoonation-processing factors for skill perfonnance. The available evidence on motor expertise also clearly suggl-"Sts that the training of perceptual and decision-making skills is, in many caSI-'S, just as important as the training of movement execution skills, if nol more so-yet this is also not frequently reflected in current training practices. Given thai good practice is clearly fWldamental to motor skill learning, the next section focuses on some of the major factors known to affect the learning of motor skiUs.

Figure 17.10 ComparIson of the electromyographic activity of selected muscles during the execu110n of on overarm hitting action by skilled and unskilled performers. Note that the bursts of muscle activity for the skilled performers are more discrete than those for the unskilled. Reprin ted from Sakurai and Ohtsuki 2CXX)'

270

The Biophysical Foundations of Human Movement

Factors Affecting the Learni ng of Motor Skills An age-old adage about skill leaming is tha t "practice makes perfect. " Studies by experts from a range of motor domains demonstrate the extraordinary amuuntsof deliberate, eifortfu l practice (typically >10,000 h) undertaken in the acquisition of expertise and show tha t the sheer volume of practice is one of the key d iscriminators between experts and lesser-skilled performers (figure 17.11). Altho ugh it is certa in ly tru e that extensive a mounts of practice are nffessa ry for high levels of skill to be developed, practice is a necessary b ut not J. sufficient condition for learning. The " practice makes perfect" ad age. therefore, while essentially true, needs to be qualified in a number of ways.

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ImperfeClabllity 01 Skills One importan t quali fication to the " p ractice makes perfect" adage involves the recognition that a lthough motor skills improve with pmctice, there is no reason to suggest tha t they ever become perffft~in other words, that the re is no room for fu rther improvement through learning. In even extremely simple tasks, like hand rolling cigars in a factory, improvements in performance are still apparent after as many as 100 million trials of practice (figure 17.12)! In more complex motor skills, which involve many more components that can be potentially improved w ith practice, improvements are likely to extend over an even greater timescale. lllere is no evidence thal skill iea rrung ever ceases, provided tha i prac tice is ongoing. The levelingoul of performance observed after a number of years of pr,)ctice or performance in variOllS motor skills is more likely attributable to either psychological or physiological factors or to measurement d ifficulties. In light of this, a more appropriate adage may be "practice ma kes better."

Necessity 01 Feedback lor Learning

Provincial

Although practice is necessary for learning, practice alone does not guarantee leanUng. In particular, learners must be abJe to regularly delive feedback about their p erformance in orde r for practice to be effective in improving learning. If learners a re not able to gain information about the success or lack of success of each attempt they make at a

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Figure 17.12 Time token to hand roll a cigar as a function of amount of practice for factory workers.

Reprinted from Helsen. Starkes, and Hodges 1998.

Reprinted from Crossman 1959.

Motor Control Adaptations to Training

271

Not all feedback is, of course, equally effective.

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new task, learning will b~ impaired and indeed may not occu r at all. In the study shown infigure 17.13, participants had to leanl to perform an accu rat.e arm-positioning movement on [he basis of kinaesthetic in formation alone. The p articipants were divided into fo ur groups w h o differed in the number of S11Ccess!v€' trials on which they were given feedback information (Group 0 had no fpPdback; Group 2 had feedback for the first 2 trials only; Group 6 for the first 6 trials; ,:md Group 19 for the first 19 ITiills). Performance was directly related to the amount of feedback given: Grou'p 19 performed the bcd perfonners wi th both the capaci ty to perform multiple tasks concurrently and improvements in efficiency that help delay Ule onsctof fatigue. C1earexpert-novicedifferences are evident in all three aspects of central information processing (perceiving, deciding, and acting), indicating that the nervous system responds to training through functional adaptations in much the same marmer as do other key systems for human movement. AJthough the precise neural mechanisms for learning are not yet well understood, it is apparent that the nervous system possesses considerable plasticity. Short-term changes in SYilllptiC efficiency and long-term changes in synaptic connectivity ilppear to be fundamenta l neural foundations for learning. Under the appropriate set of practice

conditions, and in the presence of suitable feedback, continuous refinement and improvement of all motor skills seem possible (even simple tasks performed by acknowledged experts). The challenge for researchers and practitioners alike is to wlderstand more fully the optimal practicecondilions for the continuous learning of differenl types of motor skills

Further Reading and References Abernethy, B., J. Wan", beneficial to encourage individuals to try a variety of activities to get a taste uf what mi ght be involved in regular exercise. People in thE' preparati on stage have taken some steps toward engaging in regular exercise. They may have contacted a physician, joined a gym, or bought a new pair of ru nning shoes or

297

the latest exercise gadget advertised on television. They are probably exercising irregularly but have p lan s of exercising three or more times per week beginn ing sometime in the next month. In this stage, people can benefit from information about goa l setting, su gges tions fo r safe ana enjoyablr.> activ ities, and the recognition of obs tacles to regular exercise. Those in the action stage have modified their behaviour and have begun to exercise regularly, but ha ve been doing so for less than six months. At this stage information about teclmiques for staying motivated , overcoming obstacles, and enhancing confidence can be useful. The maintena nce stage is dchievpd when there is little r isk of returning to sedentary behaviours, usually aftr.>r a period of six month.." of regular exercise. Effor ts, however, still ner.>d to bl:! made to avoid relapse-returning to an earlier stage of change. To p revent relapse, people can focus on refining specific types of exercise behaviour, injury avoid a nce, rewarding themselves for the attainment of goa l ~, and methods of reducing boredom . V\'hen interventions a re matched to the relevant stages of change, intervention p rograms have a much higher chance of success. This means thOlt "success" in some programs may b e movin g people from the p recontcmp lation stage to the contt'mplation stage. Research has demonstrated the effectiveness of basing exercise intervl:!ntions on the transtheorctical model with adolescents, college shldents, sedentary employ('('s, and adults 65 years and older.

Exercise Addiction Some people have no trouble adhering to exercise (achieving the maintenagues recognized that the sa me amount of work couJd be done on the sled if participants accelerated the sled to a predetermined height or stopped the sled after it had been released from that heighL The only difference was that the muscles had to perform concentrically in one situation and eccentrically in the other. Knowledge of the anatomy of muscle, tendon, and neural pathways allowed these researchers to tht'orize on how these structures would influence the stretch-shortening cycle. If they had not known about muscle sp indles, Golgi tendon organs, and the role of the la afferent neurons, the)' would not have been able to recognize that the increased work output following muscle stretch couJd be due to increased poten tiation of the muscles through a reflex arc. Thus to obtain a complete picture of the ways in which humans move, it is important to be

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The BiOphysical Foundations of Human Movement

eclectic and to investiga te problems from multiple perspectives using u vfin of Physical Education 10(3): 39-42. Figure 16.10 Reprinted, by permission, from R.J. M alina a nd C. Bouchard, 1991, Growth, maturation, and phy~ical activity (Champaign, IL: Human Kinetics),124. Figure 16.11 Reprinted, by p ermission, from P.J. Bairstow and].l. 1.aszlo, 1981, " Kinaesthetic sensitivity to passive movements and its relationship to molar d evelopment and motor control,"

Developmental Medicin e and Child Neurology 23: 606-61 6.

Fig ures 16.1, 16.2, and 16.3 Rep rinted, by permission, from J. Keogh and D. Sugden, 1985, Muvement skill developmen t (New Yurk: Macmillan), 32, 38,46.

Figure 16.12 Reprinted, by p ermission, from J. H odgkins, 1962, "Influence of age on the speed of reaction and m ovemen t in females," Journ al of Gerontology 17: 385.

Figure 16.4 Da ta from L. Von Wendt, H. Makinen, and P. Rantakallio, 1984, "Psychomotor development in the firs t year and mental retardation: A prospective study," Journal of Mental Deficiency R.esearch 21:\: 219-225.

Figure 16.13 Adapted, by permission, from J. Keogh and D.R. Sugden, 1985, Movement skill develapment (New York Macmillan), 337.

Figures 16.5 and 16.7 Reprinted, by permission, from D.L CaBahue, 1989, Understanding motor development: Infants, children, adolescents, 2nd ed. (New Canaan, CT: Benchmark Press), 239, 257.

Figure 16.14 Reprinted, by permission, from J.L Fozard, :\1. Vercryssen, S.L. Reynolds, EA. Hancock,and R.E.Quilte r, 1994, "Age differcnccs and changes in reaction time: The Baltimore Longitudinal Study of Ag.i.ng," Journal ofGeroHtology49(4): 179-189.

346

Credits

Figure 17.-1 Reprinted, by[Wrmission, from Neil R. Carlson, 1994, IJhysiolo~j of behavior, 5th ed. (Needh..1.m Heights, MA: Allyn and Bacon), 436.

Egure 17.11 Reprinted, by permission, from W.E Helscl1, J.L. Starkcs, and N.r. Hodges, 1998, "Team sports and the theory of deliberate practice," Jour/Jal of Sport and Exercise Psychology 20: 21.

Figure 17.2 Rep rinted, by permission, from A. Shumway-Cook and M.H. Woollacott, 2oo1,Mo!or control: Theory and practical applications, 2nd ed. (Baltimore: Lippincott WHliams & Wilkins), 92.

Figure 17.12 Reprinted, by p ermiss ion, fro m E.R.F.W. Cms~mfln, 1959, "A theory of the acquisition of speed skill," Ergonomics 2: 157.

Figure 17.3 Reprinted, by pennission, frum D.N. Spinelli, !-I.E. Jensen, and G.V. Di Prisco, 1980, "Early experience effect on dendritic in normally reared kittens,"

Figure 17.13 Reprinted, by permission, from E.A. Bilodeau, l.M .llilodcau, and D.A. Schumsky, 1959, "Some effects of introducing and withdrawing knowledge of results early and late in practice,"

ogy 62: I -Ii.

Jmnnal oj Experimental Psychology 58: 143.

Figure 17.5 Photo copyright by the National Sports Information Centre, Australian Sports Commission. Reprinted with permission.

Figure 17.14 Reprinted, by permission, from J.B. Shea and R.L. Morgan,1979, "Contextual interference effects on the acquisition, retentio n, and transfer of a motor ~kill," Journal oj Expaimental

Figure 17.6 Reprinted, by pennission, from J.L. Starkes, 1987, "Skill in field hockey: The n ature of the cognitivc advantage," Journal ojSport Psycho/o:?!} 9: 152. Figures 17.7 and 17.8 Reprinted, by permission, from B. Abemethy and LJ.G. RusscU, 1987, "Expertnovice differences in an applied selective attention task," Juurnal uj Sport Psychology 9(4): 331, 338.

Psychology: Hurmm l£tlrnillg and Memory 5: 183. Figure 20,] Data frum E. MCAuley, B. Blissmer, D.X. Marquez, G.J. Jerome, AF. Kramer, and l Katula, 2000, "Social relotions, physica l activity, nnd w ell-bcingin older adults," Prf"Ul:nt1Vi? Medicine 11: h08-617.

ligure 17.9 Data from table 1 in lV. Crosby and S.R. Parkinson, 1979, "A dual task inves tigation of pilots' skill level," Ergonomics 22: 1301 -1:11.1.

fig ure 22.1 Ada pted, by permission, fro m A.M. Williams andA. Franks, 1998, "Talent identification in soccer," Spurts Exercise (llld Illju ry 4(4): 159-165.

Figure 17.10 Reprinted, by permission, from S. Sakurai and T. Ohtsuki, 2000, "Muscle activity and accuracy of performance of the smash stroke in badminton with reference to skill and practice," Juumal of Sports Sciences 18: 910.

Figure 22.2 Reprinted, by pcrmi8sion, from M.A. Molciro and F.y. Cid. 2001. " Effects ofbiofecdback training on voluntary hear t rate control during dynamic exercise," Applied Psychuphysiology u;,t/ Biofeedback 26(4): 279-292.

Note: The italicized f and t following page numbers refer Lo figures and tables, res pectively. A acceleration 68-6~ angula r 70 centripetal 71 definition of 68 tangentia l 71 accomm odatio n 2S1 acetylcholine (Ach) 206 achievement goal orienta tion 282,283 actin filamen ts 27 action 227,231-232 adap tations to training 267-268 changes across life span 257-258 limitations to 232 underlying processes 231-232 action stage 297 activa tion heat 90 Acth'€ Australia 326-327 adaptations to tmining of bone 34-56 of ca rdiorespirato ry system 147-150 in children 168-1 70 definition of 1 information-p rocessing capabilities 264-269 injm)' and Ill-IE; of joints 56-58 metabolic 143-147 mutor skills 260-261 of m uscle-tendon units 58-59 of muscular system ]50-153 ncuromw:;cul;r 107-1J1, 261-264 of personality 309-311 to strength and sprint training 145-146 addiction 297-298 adenosine triphosphate (Arr) 123-127 adolesce nce, peer relationships in 302-303 uprobic energy system . Sec oxidative system aerobic exercise capacity. Sf'C endu ra";ce exercise caprvai training 156 aerobic po wer 129 afferent neurons 203 in reflexes 214