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Winter EM, Fowler NExercise defined andquantified according to the SystemeInternational d'Unites. J Sports Sci 27(5): 447-460

ARTICLE in JOURNAL OF SPORTS SCIENCES APRIL 2009

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Sheffield Hallam University

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Manchester Metropolitan University

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Available from: Neil Fowler

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Exercise defined and quantified according to the Systeme International

dUnites

EDWARD M. WINTER1 & NEIL FOWLER2

1Centre for Sport and Exercise Science, Sheffield Hallam University, Sheffield and

2Department of Exercise and Sport Science,

Manchester Metropolitan University, Alsager, UK

(Accepted 1 December 2008)

AbstractSport and exercise scientists have a common focus: the scientific study of factors that influence our ability to perform exerciseor physical activity. As a result, this ability is assessed and hence quantified. Accordingly, definitions of exercise and related

terms and nomenclature that describe the performance of exercise must adhere to principles of science and satisfy theSysteme International dUnites (SI) that was adopted universally in 1960. Frequently, these requirements are not met. Theaims of this review are twofold: (1) to identify instances of non-compliance and (2) propose universal definitions of exerciseand related terms and nomenclature that do conform to the SI and apply to exercise and physical activity that encompasseselite-standard competitive sport, activities of daily living, and clinical applications in rehabilitation and public health. Adefinition of exercise is offered: a potential disruption to homeostasis by muscle activity that is either exclusively, or incombination, concentric, eccentric or isometric.

Keywords: Exercise, definition, quantification, SI

1. Introduction

Irrespective of discipline interests, or indeedwhether or not interests are health- or sport-

related, those who teach, research or provide

consultancy in the sport and exercise sciences have

a common focus: the scientific study of factors that

influence the ability to perform exercise. This, in

turn, leads to two related questions: what is

exercise and how can the performance of exercise

be quantified?

Answers to these questions should apply to all

circumstances and adhere to the Systeme Interna-

tional dUnites (SI). The SI was adopted in 1960 as

resolution 12 of the 11th General Conference on

Weights and Measures hosted by the BureauInternational des Poids et Mesures (http://www.

bipm.org). Hand (2004) provided a detailed history

of various systems of measurement that have been

used during the last eight millennia, but especially

conflicts in the eighteenth and nineteenth centuries

that arose from the use of measures that were based

on disparate metric and imperial units. The intro-

duction of the SI marked the establishment of

internationally agreed quantities, units, and symbols

to be used in all measurement. Scientists are duty-

bound to abide by this system.In spite of notable attempts to identify and prevent

further misuse of terms (Caspersen, Powell, &

Christenson, 1985; Faulkner, 2003; Knuttgen,

1978; Rogers & Cavanagh, 1984, 2008) and clear

guidance on appropriate use (Royal Society, 1975),

terms and nomenclature that are commonly advo-

cated or used to describe exercise and related

performance are either used inappropriately or are

simply incorrect because they fail to follow rules and

principles. Support for this observation is exempli-

fied in the November 2008 issue of Medicine and

Science in Sports and Exercise (MSSE), which con-

tained 17 articles that had been through the processof peer review. Sixteen of the articles were specific to

descriptions of exercise or accompanying perfor-

mance, 12 of which had irregularities in terms,

nomenclature or units. Correct terms were used

inappropriately and other terms were simply wrong.

These irregularities are exemplified by the use of

units of mass to indicate weight and use of the term

work when mechanical work done by exercisers

Correspondence: E. M. Winter, Centre for Sport and Exercise Science, Sheffield Hallam University, Collegiate Crescent Campus, Sheffield S10 2BP, UK.

E-mail: [email protected]

Journal of Sports Sciences, March 2009; 27(5): 447460

ISSN 0264-0414 print/ISSN 1466-447X online 2009 Taylor & Francis

DOI: 10.1080/02640410802658461

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was not assessed, as well as workload, which is

inapplicable.

This situation occurs in spite of Knuttgens (1978)

formative publication in MSSE that led to the

adoption by the journal of the American College of

Sports Medicines (ACSM) guidelines shortly after

and which are illustrated in Table I. These guidelines

were included in the journals Information forAuthors but are no longer provided either on the

MSSE website (2009) or in the January 2009 issue of

the journal (pages ivii). This could be one explana-

tion why 30 years after Knuttgens (1978) original

paper, scientific infelicities prevail. It should be

noted that instances are not restricted to MSSE;

regrettably, they can be found in most of the major

journals in sport and exercise. Of particular concern

is the way that journals that have science in their

title present transgressions and that these transgres-

sions have avoided detection in the process of peer

review.

Science (Chalmers, 1999; McNamee, 2005;

Thomas, Nelson, & Silverman, 2005) is characterized

by: research-question, hypothesis-driven, randomized-

controlled-trial-approaches to epistemology (knowl-

edge acquisition); and precision and accuracy in

measurement (Hand, 2004). Science also requires

adherence to the SI. The purpose of this review is to

propose improved definitions of exercise, related

terms, and nomenclature that are universal, consis-

tent with the SI, and hence scientific. In doing so, we

will present examples of ways in which principles of

science are contravened. Importantly, we will also

provide solutions.

2. Exercise

It is often assumed that exercise involves only

movement represented by activities such as walking,

running, jumping, and swimming. Indeed, by theirimprecise titles, respected texts (Bartlett, 2007;

Winter, 2004) either wittingly or unwittingly pro-

mulgate this assumption. Exercise can also involve

movement assisted by machines or other devices

such as those found in cycling, wheelchair racing,

kayaking, rowing, skiing, and skating. During these

activities, energy is expended up to and beyond

120 kJ min71 (2 kW), equivalent to an oxygenuptake of 6 litres min71, compared with restingrates of approximately 5 kJ min71 (83 W), equiva-lent to an oxygen uptake of 0.25 litres min71. As aresult, a much-used definition of exercise is the one

proposed by Caspersen et al. (1985): planned,

structured and repetitive bodily movement

(p. 127).

However, there are activities that also require

substantial expenditures of energy but in which little

or no movement occurs. The Crucifix and other

examples of quasi-static balance and suspension in

gymnastics are illustrations. In competition, move-

ment is actually deprecated and marks are awarded

for stillness. In both codes of rugby, it is possible

for 16 or 12 players to exert maximum or

Table I. The American College of Sports Medicines guidelines for terms and nomenclature used in exercise.

Term Guideline for use

Exercise Any and all activity involving generation of force by the activated muscle(s) that results in disruption of a

homeostatic state. In dynamic exercise, the muscle may perform shortening (concentric) contractions or be

overcome by external resistance and perform lengthening (eccentric) contractions. When muscle force results in

no movement, the contraction should be termed isometric.

Exercise intensity A specific level of maintenance of muscular activity that can be quantified in terms of power (energy expenditure or

work performed per unit of time), isometric force sustained, or velocity of progression.

Endurance The time limit of a persons ability to maintain either a specific isometric force or a specific power level involving

combinations of concentric or eccentric muscular contractions.

Mass A quantity of matter of an object, a direct measure of the objects inertia (note: mass weight 7 acceleration due togravity; unit: gram or kilogram).

Weight The force with which a quantity of matter is attracted towards Earth by normal acceleration of gravity.

Energy The capability of producing force, performing work, or generating heat (unit: joule or kilojoule).

Force That which tends to change the state of rest or motion in matter (unit: newton).

Speed Total distance travelled per unit time (unit: metres per second).

Velocity Displacement per unit time. A vector quantity requiring that direction be stated or strongly implied (unit: metres per

second or kilometres per hour).

Work Force expressed through a distance but with no limitation on time (unit: joule or kilojoule). Quantities of energy and

heat expressed independently of time should also be presented in joules. The term work shouldnotbe

employed synonymously with muscular exercise.

Power The rate of performing work; the derivative of work with respect to time; the product of force and velocity (unit:

watt). Other related processes such as energy release and heat transfer should, when expressed per unit of time, be

quantified and presented in watts.

Torque Effectiveness of a force to produce axial rotation (unit: newton metre).

Source: McArdle et al. (2000, p. 646).

448 E. M. Winter & N. Fowler

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near-maximum effort such as in a scrum, yet no

movement occurs. The same can be seen in tug-of-

war. Other tasks such as rifle and pistol shooting and

archery also illustrate activities in which lack of

movement is a principal aim of the participants.

In endurance sports such as sailing and surfboard-

ing, there are extended periods of isometric or near-

isometric muscle activity in large muscle groups(Spurway, 2008). In ice-sports such as the skeleton

and other forms of bob sleigh, isometric muscle

activity that creates and maintains streamlined

postures of the body is decisive. Even in dynamic

exercise such as running and swimming, isometric

muscle activity in fixator and stabilizing muscles

contributes to performance (Rasch & Burke, 1967).

Clearly, exercise does not always require or involve

movement, so if a definition is to be universal, it

must acknowledge that movement is not necessarily

an outcome.

At this juncture, it is worth highlighting that the

term physical activity is frequently used as a proxy

for exercise that includes activities of daily living that

arise from occupational tasks and recreative pursuits.

For many, competitive sport is not a principal focus,

gym-based exercise can be intimidating, and exercise

is perceived to be hard work, vigorous, and possibly

unpleasant (Biddle & Mutrie, 2008). As a result, the

term physical activity has been adopted. This is

user-friendly and together with active living and

active lifestyle (Killoran, Cavill, & Walker, 1994;

Quinney, Gauvin, & Wall, 1994) has entered the

lexicon of sport and exercise science and indeed

general vocabulary. This has occurred in an effort toproduce more acceptable and cost-effective messages

(Sevick et al., 2000). It is worth considering the

background.

Caspersen et al. (1985) defined physical activity in

terms of the following three elements:

1. Movement of the body produced by skeletal

muscles.

2. Resulting energy expenditure that varies from

low to high.

3. A positive correlation with physical fitness.

As far as health outcomes are concerned, theintensity, frequency, and duration of exercise has to

be such that metabolic energy expenditure is usually

well above that experienced at rest (Bouchard &

Shephard, 1994). Consequently, homeostasis (i.e.

stability of physiological processes) is disrupted and

adaptations can occur at cellular, organ, systemic,

and whole-body levels of organization. Often, ex-

ercise refers to structured leisure-time physical

activity such as participation in jogging, swimming,

keep-fit activities, and recreational sports (Biddle

& Mutrie, 2008) rather than other unstructured

activities of daily living such as stair climbing and

walking during occupational and leisure-related

tasks.

A key problem with elements outlined by

Caspersen et al. (1985) is that they ignore types of

activity frequently performed by specific and nota-

ble groups such as the elderly or infirm, who are

among those for whom the term physical activityis intended. For example, consider seated exercise

that involves single or repetitive raising and low-

ering of the arms either symmetrically or asymme-

trically. With the palms of the hands supine,

participants flex their elbows and raise their fore-

arms until the tips of the fingers touch the clavicles.

They then lower the arms until the elbow is at right

angles, hold that position, and repeat the pattern

several times. There are clearly three distinct

phases: first, concentric activity as the forearms

are raised, eccentric activity as they are lowered,

and isometric activity as they are held. In spite of

the widespread use of this type of arm exercise, the

last of the phases is excluded from Caspersen and

colleagues (1985) criteria because movement does

not occur. Moreover, recent work has demonstrated

that standing and hence the recruitment of large

muscle groups in the trunk and legs can make a

contribution to health-related benefits (Hamilton,

Hamilton, & Zderic, 2007).

Caspersen et al. (1985) attempted to distinguish

between physical activity and exercise by considering

possible sub-components of activity. They defined

exercise as:

1. Body movement produced by skeletal muscles.

2. Resulting energy expenditure varying from low

to high.

3. Very positively correlated with physical fitness.

4. Planned, structured, and repetitive bodily

movement.

5. The objective is to maintain or improve

physical fitness.

Clearly, according to these suggestions, there is little

if any practical difference between exercise and

physical activity, and Caspersen and colleagues

(1985) exclusion of isometric activity in both is amajor problem. Moreover, according to Bouchard

and Shephard (1994), exercise also has as objectives

the enhancement of health or improvement in

performance. So, too, does physical activity. Finally,

strictly speaking, none of the above elements

provides a precise definition either of exercise or

physical activity. Indeed, the distinction between the

two terms is dependent on an interpretation of the

motivation or intent of the participant; this could

give rise to one persons exercise being another

persons physical activity. While the vocabulary is of

Exercise defined and quantified according to SI units 449

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great interest to the behavioural scientist, it does not

help in the evaluation of the activity itself.

For the sake of consistency, this review will use the

term exercise but the two terms are interchangeable.

Accordingly, and importantly, the use of exercise

or physical activity depends simply on the

circumstances and context.

What is clear is that exercise involves the use ofmuscle, although it must be acknowledged that there

are three main types of this tissue: striated, smooth,

and cardiac. Typing, playing electronic games, and

using a television remote control involve muscle

activity but these activities cannot be considered to

be exercise in the context of marked disruptions to

metabolism and hence homeostasis characteristic

of the recruitment of major muscle groups. With

such recruitment, exercise is likely to lead to

increases both in breathing rate and heart rate.

However, it must also be acknowledged that gentle

perturbations to smaller or local muscle groups can

occur continuously to maintain or extend functional

capability.

Before defining exercise, it is important to

consider the precise function of muscle because this,

too, is frequently misunderstood. The fascinating

historical background to studies of the anatomy and

physiology of muscle (Needham, 1971) tends to be

overlooked and this is a major source of current

misunderstandings.

3. The function of muscle

There are biblical references to muscle as the flesh oframs and there is a remarkable history of attempts to

explain how muscle functions (Needham, 1971).

These attempts date back at least to Hipprocates

(460377 B.C.) and probably even before that; it was

only in Hippocratic times that written records

emerged. Etymology of the term muscle reveals

a quaintness. The word derives from the Latin

musculus, a diminutive mouse, because of the way

in which active muscle resembles mice running

under the skin.

For the sake of simplicity, we will move forward

some 500 years and begin detailed consideration

with Galen (c. A.D. 129216). Galen was a physicianfrom Pergamum, now Bergama in Turkey. He was

influential in the thinking of medicine for some one

and a half millennia. Like Hippocrates, Galen had

interests in sport and exercise and, among his other

positions, he was appointed chief physician to the

gladiator school in Pergamum by Roman Emperor

Marcus Aurelius (A.D. 121180). Galen was guided

by the humoral school of thought that considered

human function and behaviour to be attributable to

four humors: blood, phlegm, yellow bile, and black

bile. These gave the characteristic moods of

sanguine, phlegmatic, choleric, and melancholic

respectively (Porter, 1999).

Notably, both Hippocrates and Galen were for-

mally appointed by the state as physicians to

contribute to the welfare of athletes and gladiators

respectively in similarly state-sponsored centres.

These centres had professional trainers and were

intended to improve performance, thus the spectaclefor spectators. Current sport and exercise science

and medicine are simply reinterpretations of what

has been established for some two millenia

(McArdle, Katch, & Katch, 2007; Winter, 2008).

Regarding muscle in particular, Galen proposed

mechanisms in attempts to explain how force was

exerted. In one of these, he claimed that when

muscle became active, it was infused with spiritus

animalis vital spirit and expanded. This is

seemingly consistent with the increase in girth that

tends to accompany muscle activity. The pervasive-

ness of this function-by-expansion theory was such

that it endured into the seventeenth century before it

came under closer scrutiny in what has been called

the first recorded experiment in neurophysiology.

This was performed by Swammerdam in 1663,

although the outcome was not published for another

60 years or so (Needham, 1971), and is illustrated in

Figure 1.

Swammerdam took an isolated muscle and sus-

pended it in a glass tube that was sealed at the

bottom and drawn out to a capillary at the top. This

capillary was sealed by a water droplet labelled e.

Upon stimulation by a wire c, the muscle

twitched. If Galens function-by-expansion theorywas correct, the water droplet should have risen. In

fact it remained stationary. At a stroke, Galens

postulate was overturned.

However, what has seemingly been overlooked is

the clear demonstration that when stimulated,

muscle does not reduce in volume either (i.e. it does

not contract), yet this is precisely the term that still

describes the active response of muscle. According to

Hierons and Mayer (1964), Goddard performed

in vivoexperiments on humans that were recorded in

the Register of the Royal Society in 1669. These

experiments suggested that there was a small

reduction in volume of active muscle. This reductionis consistent with the volume of blood that is forced

out by the high intra-muscular pressure that col-

lapses the associated vasculature. Nevertheless, the

fact remains: when stimulated, muscle does not

contract. What tends to happen is that muscle

shortens that is, the outcome is concentric activity.

However, this is not always the outcome.

When muscle exerts force, it does not necessarily

shorten. This response might be deliberate in that an

object is simply supported or imposed because

moving the object is beyond the force-generating


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capability of recruited muscle(s). This is termed

isometric activity. Also, muscle often increases in

length when it is exerting force. This occurs, for

instance, when an object is lowered; the muscle

activity is termed eccentric. Notably, Schneider

and Karpovitch (1948) expressed the term as

excentric. For reasons that are unclear,

eccentric is the form that is now commonly used.

Nevertheless, these forms of muscle activity give

rise to a useful definition:

The function of muscle is to exert force and it does so by

attempting to shorten.

This definition acknowledges that the attempt is not

necessarily successful because the outcome can be

either in isolation or combination, concentric,

isometric or eccentric activity. Importantly, the

definition applies irrespective both of the type of

muscle involved and whether the context is exercise-

or physical-activity-related.

It has to be acknowledged that the term activity

does not have universal approval (Faulkner, 2003)

because the active could refer, for instance, to the

innervation of muscle or release of calcium ions from

the sarcoplasmic reticulum, events that precede the

attempt to shorten. Furthermore, Atha (1981)

suggested that there were 64 combinations of

isometric, concentric, and eccentric activity depend-

ing on the task performed and hence the order and

speed in which these types of action occur. Never-

theless, the absence of the term contraction is thekey advantage of this definition.

Having clarified the function of muscle, we are

now in a better position to evaluate proposed

definitions of exercise. In 1978, Knuttgen recognized

that movement was not necessarily a characteristic of

exercise, and in a series of articles Winter (1990,

1991a, 1991b) supported Knuttgen (1978) and

proposed terms and nomenclature to describe

exercise and related performance. The definition of

exercise suggested by the ACSM illustrated in

Table I is laudable but while this definition is an

improvement over the one proposed by Caspersen

et al. (1985), it lacks simplicity and uses the term

contraction, the shortcomings of which were

highlighted above and by Rogers and Cavanagh

(1984, 2008).

Striated, smooth, and cardiac muscle are all

fundamentally involved in exercise, so we propose

the following definition of exercise that simplifies and

improves the precision of the one suggested earlier by

ACSM presented in Table I:

A potential disruption to homeostasis by muscle activity

that is either exclusively or in combination, concentric,

isometric or eccentric.

This definition acknowledges that perturbation to

metabolism is likely and movement is not necessarily

an outcome. Importantly, the requirement for uni-

versality is satisfied the definition can be applied to

all situations.

During exercise, metabolic demand is increased

and this increase will become useful when shortly we

consider how best to quantify either the intensity at

which exercise is performed or the amount of

exercise that is accomplished. Assessments of these

can be based logically on some marker or proxy

marker of metabolic demand.

4. Quantifying the ability to perform exercise

The next challenge is how to measure the ability to

perform exercise, as often attempts to meet this

challenge simply do not align with the SI and hence

science. Approaches to quantifying exercise can take

either a cause-or-effect focus (i.e. they can quantify

minimum requirements to complete a task) or one of

effects of the task on a participant. The former

commonly consider mechanics of a task, whereas in

Figure 1. Swammerdams experiment (1663). Reproduced with

the kind permission of Cambridge University Press.


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the latter physiological responses of the participant

provide the interest. We will begin by considering the

basic although often abused mechanical terms

force, work, power, and energy, and

highlight examples of their correct and incorrect

usage. Consideration will then be given to velocity,

impulse, efficiency, and economy and instances of

their appropriate and inappropriate use will behighlighted. Finally, solutions will be proposed that

present terms that are fully consistent with the

principles of science. These solutions are remarkably

simple.

4.1. Force

We have seen that the principal function of muscle is

to exert force. In 1687, Isaac Newton published his

three-volume Philosophi Naturalis Principia

Mathematica (Mathematical Principles of Natural

Philosophy) on classical mechanics. It is often

referred to simply as Principia or Principia Mathema-

tica and contains his proposed three laws of motion.

The concept of force comes from the first of these

laws. The expression of this law in its original Latin

text together with the translation (Cajori, 1960) is:

Lex I:Corpus omne perseverare in statu suo quiescendi

vel movendi uniformiter in directum, nisi quatenus a

viribus impressis cogitur statum illum mutare. Every

body perseveres in its state of being at rest or of

moving uniformly straight forward, except insofar

as it is compelled to change its state by force

impressed.

The SI unit of force is the newton (N). For linear

motion, if force is applied to a stationary or moving

object it tends to accelerate that object. The

reluctance of the object to accelerate is attributable

to its mass. The SI unit for mass is the kilogram (kg).

From Newtons first law, the mass of an object

represents its inertia that is, the bodys reluctance

to change its state of motion. Because of the effect of

gravity, this mass exerts a force and this force is the

weight of the object. Weight and mass are still

sometimes confused, especially in the contexts of

body weight and cycle ergometry. Body weightshould be reported in newtons, body mass in

kilograms.

In many instances, we are interested not only in

the linear effect of the forces that are acting but also

on the angular effects they produce. The moment of

a force (i.e. its torque) is the product of the force and

its perpendicular distance from the axis about which

either the body rotates or attempts to rotate. The

reluctance of a body to change its state of angular

motion is its moment of inertia and this property is

related both to the bodys mass and the distribution

of this mass about the rotational axis. This is

important in the context of exercise because the

action of muscle is commonly experienced as a

moment about the related joint and not as a linear

force.

A principal interest of physiologists is mechanisms

that explain muscles ability to exert force and, in

particular, those that explain or accompany fatigue inwhich muscles ability to exert force is reduced.

4.2. Work

Mechanical work done is a concept from classical

mechanics outlined in NewtonsPrincipiaand occurs

when:

A force moves its point of application such that

some resolved part of the displacement lies along

the line of action of that force.

Displacement is a vector quantity in that it has

both magnitude and direction, whereas distance is

a scalar quantity that has magnitude but without

specification of direction. With that note of

caution, mechanical work done tends to be

considered as the distance through which the point

of application of a force moves. Consequently, it is

calculated as the product of the force and the

distance over which that force is applied. The SI

unit of distance is the metre (m) so, as indicated in

Table I, mechanical work done is represented as

N m and the SI unit is the joule (J). One joule of

mechanical work is done when a force of 1 Nmoves through a distance of 1 m.

Consider, then, isometric muscle activity. A great

force could be exerted by the biceps to oppose the

weight of an object with the hand. Since the hand will

not move if the weight is beyond the capability of the

muscle to act through its associated lever system or if

the degree of muscle activation is moderated to

produce a moment equal in magnitude but opposite

in direction to that produced by the weight,

mechanical work done is zero.

During consideration of mechanical work done,

boundaries of the related energetic or thermody-

namic system have to be specified. The work donerepresents the change in net energy of the entire

system. For example, during a movement the system

might be defined as the entire body or, alternatively,

as only a single limb or segment. Conventionally, for

exercise a distinction is made between the concepts

ofinternaland external work (Winter, 2004).

Internal work is the mechanical work done to

change the mechanical energy of different parts of

the system (e.g. to move one or more limbs), with no

change in the energy of the total system. External

work is that done which does bring about a change in


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the total energy of the system. However, this does not

capture all of the mechanical work that might be

done. For instance, during the initial stages of

activation and force development, parallel and series

elastic components in muscle stretch as the active

muscle shortens. When all of the elastic components

have been stretched to their limit, no further short-

ening of muscle occurs. Strictly speaking, the muscleis performing mechanical work while the elastic

components are stretched. This can be termed

internal work. Nevertheless, external mechanical

work done remains zero throughout. In activities

such as running and cycling, where movement does

occur, we should differentiate between the mechan-

ical work done to move the limbs and that required

to move the whole body or external object.

The assessment of internal mechanical work is far

from easy (Winter, 2004) because it requires assess-

ment of the distance through which the associated

muscle(s) shorten and, similarly, the distance

through which series and parallel components

increase as well as the determination of associated

tissue-stiffness forces. We shall return to consider

internal and external mechanical work below in sub-

section 4.7 on efficiency.

As Table I illustrates, the terms work and

exercise cannot be used synonymously yet fre-

quently, and incorrectly, they are. Indeed, as

exemplified by isometric muscle activity, it is possible

to incur a metabolic demand and thus be considered

to be exercising but no internal or external mechan-

ical is done.

4.3. Power

Like work, power is a mechanical construct from

classical mechanics and is a term that is frequently

misapplied to sport and exercise. The history of its

use can be traced to James Watt (17361819), who

developed the atmospheric steam engine from the

original design of Thomas Newcomen (16631729).

Watt proposed a means to assess the effectiveness of

steam engines that were proliferating at the begin-

ning of the Industrial Revolution. These engines

were replacing horses to drive industrial processes

and so keep pace with demand for outputs. He wasreportedly the first to use the term horsepower so

as to compare the capabilities of engines with their

equine equivalents. Power is:

The rate at which mechanical work is done.

The unit of power is the eponymous watt (W), i.e.

J s71. It is important to acknowledge that the originsof power are firmly rooted in steam engines and it

is still used to indicate the capability of two-stroke,

four-stroke or diesel internal-combustion engines

that are used in automobiles, locomotives, and ships.

One horsepower derived from Watts original work is

equivalent to 745.7 W, whereas the metric horse-

power is 735.5 W. The former tends to be used.

Even in the context of engines, power on its own

does not necessarily provide an adequate evaluation

of suitability. The torque an engine produces is an

important factor. This refers to the moment appliedto an engines crankshaft and manufacturers usually

attempt to achieve high torque throughout the range

of revolutions per minute (rev min71). As a result,revolutions per minute can be kept comparatively

low at, for example, 30004,000 rev min71,whereas Formula 1 engines rev to a regulation-

restricted maximum of 19,000 rev min71.From its origins in classical mechanics, the

construct of power is now used to assess rates at

which one form of energy is converted to another

(Royal Society, 1975). This includes rates of energy

expenditure during exercise that are determined

from analyses of expired air. The term work rate

is used frequently to describe performance during

exercise but this is colloquial and should be avoided

(Rogers & Cavanagh, 1984, 2008). Besides, as the

definition clearly indicates, work rate must be power

output. The term work rate is not recognized by

the SI.

Athletes in explosive events such as horizontal and

vertical jumping, sprinting, throwing, and bobsleigh

are often said to be powerful. Indeed, commenta-

tors and authors frequently use this emotive term,

but we will see in sub-section 4.6 on impulse that in

most cases they are incorrect (Adamson & Whitney,1971; Smith, 1972).

4.4. Energy

Ultimately, we are heliodependent; our energy

derives from the sun. However, energy is expressed

in various forms, including heat, light, electricity,

chemical reactions, sound, and movement. The last

of these is also termed kinetic energy. Forms of

energy are converted from one to another. For

instance, the food that we eat is digested and, in so

doing, complex insoluble material is converted into

simple soluble substances. These substances canthen be transported around the body and taken up by

cells. When enzymes act on these substances, the

substances are termed substrates and the interaction

of substrates and enzymes releases energy. The

currency of energy in our cells is adenosine tripho-

sphate (ATP).

During exercise, ATP provides the chemical

energy for muscle to exert force or, if movement

occurs, to convert chemical energy to kinetic energy.

This change in energy indicates that work must have

been performed to effect changes in the energy either


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within or between bodies. In conversions of chemical

to mechanical energy, heat is also released and heat

production thermogenesis is an accompaniment

to exercise.

When energy release is aerobic, substrate reacts

with oxygen. In anaerobic reactions, the energy is

released without the use of oxygen even though there

might be abundant supplies of oxygen available(Connett, Gayeski, & Honig, 1986). It is likely that

the fundamental challenge in exercise is to ensure

that energy requirements are met by energy avail-

ability. That is not, of course, to deny the importance

of other important factors such as mental skills and

technique.

Conventionally, energy is said to provide the

capacity to do mechanical work but, in the context

of exercise, this definition can create confusion

because as we identified earlier, the conversion of

metabolic energy need not necessarily lead to the

expression of external work but might result instead

to some other energy exchange such as heat.

Mechanical work and hence movement is not always

an outcome. In the context of exercise, a useful

definition of energy is:

That which must be expended to perform exercise.

4.5. Velocity (v)

Quantities such as time (s) and distance (m) are

commonly used to measure exercise performance.

Similarly, mean speed (distance/time, m s71

) can beused to assess both the capability to perform exercise

and the intensity of exercise. These are scalar

quantities in that they indicate magnitude only.

When quantities indicate magnitude and direction,

they are said to be vectorquantities.

Authors sometimes claim that mean running

velocity or swimming velocity was some measure of

metres per second. This is often neither scientific nor

relevant. Consider, for instance, a 400-m runner in

the inside lane of a 400-m track. The runner finishes

at the same point as that from which he or she

started, so their mean velocity is 0 m s71. So, too, is

the mean velocity for a 10,000-m runner on the sametrack. The same applies to the 100 m, 200 m, and

other distances that are even-multiples of 50 m for

swimmers who compete in 50-m pools.

In all of these cases, it is not mean velocity that is

meaningful; speed is actually the informative and

correct term. At any moment in time, instantaneous

velocity defines the rate and direction of movement,

but to quantify an overall effect, a composite measure

(i.e. speed) is required. It might be that velocity

sounds much grander but the grandness leads the

unwary to grandiloquence.

4.6. Impulse

Impulse is another mechanical term from classical

mechanics and emanates from Newtons second law.

In Newtons Principia, this law is stated as:

Lex II: Mutationem motus proportionalem esse vi

motrici impressae, et fieri secundum lineam rectam quavis illa imprimitur. The change of momentum of a

body is proportional to the impulse impressed on

the body, and happens along the straight line on

which that impulse is impressed.

Impulse is fundamental to exercise, especially when

projectiles are involved. These projectiles could be

implements such as shot, javelin, and discus or the

body in horizontal and vertical jumping. In spite of

its fundamental nature, impulse is frequently either

completely overlooked or eclipsed by power

(Adamson & Whitney, 1971; Smith, 1972). For

linear motion although the principle applies to

angular motion as well Newtons second law

states that the change in momentum of a body

either as an increase or decrease depends on the

size and direction of the force applied and the

duration for which the force acts. This can be

expressed as:

Fa a

where F is the applied force and a is the resulting

acceleration. This proportion expression can be

changed into an equation by introducing aconstant,m:

Fm a

where m is the mass of an object.

Acceleration is the rate of change of velocity, so the

equation can be expressed as:

F m vu=t

where v is final velocity, u is initial velocity, and t is

the time over which the change in velocity occurs.

Consider an activity such as vertical jumping inwhich initial velocity is 0. The expression now

becomes:

Fm v=t

This can be rearranged to:

F t m v

whereF tis the impulse of the force and m v is theresulting momentum of the body. This is why the


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expression is often referred to as the impulse

momentum relationship. In most exercise settings,

it is reasonable to assume that there will be no

meaningful change in mass, so as impulse increases it

is velocity that changes. Not only is this relationship

fundamental in activities such as jumping (both

horizontally and vertically), throwing, and sprinting,

it is also important in multiple-sprint-type sportswhere cutting and similar changes in direction are

important.

In jumping and throwing in particular, the

principal factor that determines performance is

velocity at take-off and release respectively. Here

velocity is appropriate because it implies direction

and hence angle at release or take-off and height. At

this point, velocity is determined as:

F t=m v

Accordingly, it is the preceding impulse the force

time integral that determines performance. The

performer has to maximize impulse through appro-

priate technique to manipulate the force he or she

applies and the duration for which the application

occurs. In events such as the shot putt, javelin, and

discus, techniques are designed to allow the athlete

to apply force for as long as possible. Precisely the

same applies to jumping and the push-phase in

bobsleigh. In many circumstances, it is optimization

of the product of force and time that is critical for

success and which determines the limits of

performance.

Moreover, this force that is, of course, a meanvalue throughout the activity should also be as high

as possible so that the resulting product of force and

duration is maximized. A key and largely unan-

swered question is, which is the more important,

magnitude of forces or duration of application? Short

durations could give insufficient time for muscles to

become fully activated and so lead to injury, whereas

long durations could be disadvantageous because

stretchshortening cycles might not be fully

harnessed.

In multiple-sprint activities such as rugby, hockey,

association football, tennis, squash, badminton, net-

ball, and basketball, sidestepping and cutting during which accelerations and decelerations occur

are important. These accelerations and decelera-

tions require impulse.

Forcetime profiles, or force histories as they are

sometimes called, can be secured from force

platforms, accelerometers or kinematic analyses.

The first two of these tend to be the preferred

approaches because of errors inherent in the

double-differentiation of displacement-time data

that is required to derive acceleration using

kinematics.

We can return now to sub-section 4.3 on power.

The Sargent or similar type of vertical jump is

often reported as a measure of lower body

power. In impulsive activities such as jumping,

power is at best a distraction and at worst

irrelevant; its use in this context is simply

incorrect. The unit of performance is metres and

not the required watts and it is the impulse-generating capability of muscle that is the key

determining factor, not its power-generating cap-

ability. Maybe to be impulsive one needs to be

powerful, but it is impulse that is decisive. It is

probable that the physiological mechanism for

which the term power is often incorrectly used as

a proxy is the rate of force development. The more

rapidly a muscle can reach the desired force, the

greater will be the impulse for the same given

activation time.

The psychologist is probably interested in how

mental skills can harness impulse-generating me-

chanisms, the physiologist probably wants to identify

these mechanisms, and the biomechanist probably

wants to identify the technique that is most effective.

Clearly, all are interested but their interests are from

different perspectives.

It is possible from forcetime profiles mathemati-

cally to integrate the curve and so develop a velocity

time curve. This can be superimposed on the force

history and when force and velocity are multiplied,

the units of power (W) emerge. While at first sight

this appears plausible and is dimensionally correct,

the approach is misapplied because of the context.

No account is taken of values that preceded orfollowed the calculated maximum.

It should also be noted that power, as derived

from forcetime data or other similar methods,

represents the rate at which external mechanical

work of the whole body is performed. Since in

virtually all instances movement is a consequence

of the action of many co-active and co-ordinated

muscular actions, the calculation of a single net

figure to represent this has little if any relevance to

the rate at which metabolic or mechanical work is

done by an individual part of the system, or to the

sum of the parts to indicate whole-body work. For

example, consider someone who stands and mi-mics the arm actions of sprinting. Symmetrical

movement of these limbs means that the position

of the bodys centre of mass does not change

(Winter, 1979). Similarly, while this centre of mass

does move during stair climbing, for example, its

path does not provide the information required to

calculate both internal and external mechanical

work done. These examples illustrate why it can be

misleading to assess work done and hence power

output based solely on the position and path of the

bodys centre of mass.


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4.7. Efficiency

According to Winter (2004, p. 122),

The term efficiency is probably the most abused and

misunderstood term in human movement energetics.

What is the justification for this condemnatorystatement? As before with power, the origin of the

term stems from Watts work on engines and

classical mechanics. It is assessed as:

Some measure of mechanical work done

Some measure of energy input

Then, as now, engineers were concerned with how

well engines were using fuel to assess the cost-

effectiveness of outputs. According to the SI, both

the numerator and denominator in this expression

are measured in joules. Normally, efficiency is

expressed as a percentage, so the outcome of the

product above is multiplied by 100.

As Winter (2004) examined at length, identifica-

tion both of the numerator and denominator is

especially challenging because each contains several

considerations that are frequently overlooked. What

is the problem?

First, the denominator. When exercise is sup-

ported wholly by aerobic metabolism (i.e. it is

performed sub-maximally at steady state), this can

probably be estimated, but estimation is not

straightforward. Oxygen uptake can be determinedand then multiplied by an energy-equivalent that is

based on the respiratory exchange ratio (McArdle

et al., 2007). This equivalent varies and usually

assumes no contribution from protein, but if precise

values were required, the contribution from protein

would have to be identified. If non-protein equiva-

lents will suffice, substrates are, in the extreme,

exclusively fat or carbohydrate but, more likely, fuel

for exercise is a mixture of the two.

Non-steady state exercise (i.e. for which there are

contributions from anaerobic metabolism) markedly

complicates matters. Such contributions could be

either self-evident during what is clearly non-steady-state exercise or subtle as in exercise that results in

the slow-component of oxygen uptake that is super-

imposed on the anticipated steady-state profile

(Jones & Poole, 2005). Attempts to measure oxygen

uptake after exercise until baseline resting values

return and then use this supposedly to estimate the

contribution from anaerobic metabolism are fraught

with difficulties (Bangsbo, 1996; Medb, 1996).

Consideration has to be given to energy expended at

rest and to move the limbs such as the legs in

unloaded cycle ergometry. One or the other, both or

neither of these could be subtracted from the gross

energy expenditure to obtain a net value or, for

that matter, a work or delta (change) response

as the intensity of exercise changes (Cavanagh &

Kram, 1985a, 1985b).

As if identification of the denominator wasnt

challenging enough, identification of the numerator

is even more difficult. In friction-braked or electri-cally braked cycle ergometry, providing pedalling

rate is held constant, a calculation of external

mechanical work done is possible. For friction-

braked ergometers, if pedal rate and gearing from

the pedal-crank sprocket to the flywheel are

known, the distance travelled by an imaginary point

on the flywheel for one pedal revolution can be

determined.

Multiplication of this distance by the applied force

on the ergometer and revolutions pedalled in the

time that corresponded to the collection of expired

air for the determination of oxygen uptake, produces

mechanical work done during the period of interest.

In cardiac and pulmonary physiology, this is typically

one minute. Care has to be taken to recognize that

this time interval does not adhere to the SI unit of

time (i.e. seconds), although common usage makes

the minute allowable.

However, this gives only the external mechanical

work done and does not account either for frictional

and other losses in the system or variations in the

applied force that arise from oscillations of the

resistance. According to the manufacturers, Monark

ergometers have a loss of approximately 9% from

input at the pedals to output at the flywheel. Thisloss is attributable largely to friction in bearings and

between the chain and sprockets. Devices are

available to calibrate friction-braked ergometers by

comparing input at the pedal crank with indicated

output at the flywheel, and force transducers can be

mounted in series with the load and friction belts to

record resistive forces. As with all instruments, this

type of calibration ensures both precision and

accuracy of recordings.

Another amount that tends to be overlooked is

internal mechanical-work-done to move the legs and

arms. This concept was introduced in sub-section

4.2 and becomes especially important if externalpower output is held constant by reciprocal variation

of pedalling rate and resistive load; this is usually the

case in attempts supposedly to investigate the effects

of different cycle rates on efficiency, such as during

wheelchair propulsion. Internal mechanical work

done can be identified through kinematic analysis

of the limbs. In essence, this involves summation for

limbs of changes in potential and kinetic translational

and rotational energies for each of the limb segments

(Winter, 2004). It is an involved procedure but any

attempt to assess the effects on efficiency of changes


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in cycling rate will be fundamentally flawed if

internal-mechanical-work-done is not identified.

A seemingly simple task such as cycling presents

several challenges. The assessment of other locomo-

tor tasks such as running presents even more,

although the principles are exactly the same (Winter,

2004). For constant-speed locomotion on a level

surface or on a treadmill, there is a strong case to bemade for the external work to be zero and thus the

efficiency is incalculable. Only air resistance and

internal friction such as in skeletal joints provide

opposing horizontal forces.

It is perhaps now clearer why the term efficiency

is misused and the implications of its use are

underestimated.

4.8. Economy

Unlike efficiency, economy is a flexible term because

its use is not as demanding. It tends to be used to

describe oxygen uptake or heart rate responses to

exercise; these are proxies for energy expenditure. As

a result, the evaluation of training, for instance, could

include assessments of economy. For example,

in endurance-running events, training-induced

reductions in oxygen uptake or heart rate at set

speeds suggest that economy has improved. If

maximal physiological responses had increased, the

relative improvement in economy might have been

even better. The most common expression of

economy in running is oxygen uptake reported as

ml kg71 km71 (Jones, 2008). This standardizes the

expression of energy cost per unit distance and thusallows different speeds or modes of locomotion to be

compared.

4.9. Effectiveness

Effectiveness is a term that can be independent both

of efficiency and economy. For a 100-m sprinter, for

instance, neither efficiency nor economy is a

principal concern; the athlete simply wants to run

down the track as quickly as possible. After all, the

event lasts only 10 s or so. Where economy of effort

does become important, perhaps even efficiency if

correctly assessed, is in endurance events. Here,energy cannot be wasted because exercise might have

to be sustained for hours. As a result, athletes have at

least to be economical to be effective. Care has to be

taken to ensure appropriate use of the terms

effectiveness, efficiency, and economy.

In cycling and wheelchair propulsion, the term

effectiveness has been used to describe the propor-

tion of total applied force that acts to create the

torque that drives the pedal or wheel. The ratio of the

so-called effective force to total force has been

cited as a proxy value for efficiency (Dallmeijer, van

der Woude, Veeger, & Hollander, 1998). However,

care is needed to determine components of the force

that are actually useful. For instance, in the case of

wheelchair propulsion, it is reasonable to assume that

some mediolaterally oriented forces are necessary to

develop the necessary contact friction to allow the

transfer of the propulsive tangential force to the

wheel. However, these forces are not conventionallyincluded in the calculation of effectiveness.

4.10. Cycle ergometry

Cycle ergometry can be a source of several pitfalls, so

warrants particular attention. Some of these pitfalls

were identified in sub-sections 4.1 (force) and 4.3

(power). Power can be a useful measure of exercise

capability and the internal-combustion or other type-

of-engine analogue is useful and appropriate. For

instance, a prerequisite for successful performance in

track and road cycling is the ability to sustain high

power output. In sprint finishes, even higher power

outputs are decisive.

Cycle ergometry can also provide a useful indica-

tion of muscle function even though cycling might

not be the athletes principal mode of exercise. This

introduces another consideration: forcevelocity

relationships outlined by Hill (1938) that are as

relevant today as they were then. Basically, there is

an optimum speed of shortening of muscle that

maximizes power output. In turn, speed of short-

ening depends on the force being generated and

hence the load that is moved. However, power as a

measure of performance ceases to have any physio-logical meaning when the model is extended beyond

the most simple single-muscle model.

In Wingate-type tests, for instance, the commonly

applied force of 7.5% of body weight might not be

great enough to achieve optimized peak power output

for all of the muscles involved in the activity. This is

not necessarily a problem because there are several

alternative techniques that do optimize values

(Winter & MacLaren, 2009). However, the ways in

which applied forces are expressed are often incor-

rect. For example, 7.5 g kg71 body mass is wrong.Grams and kilograms are units of mass not force and

hence applied force. The expression 0.075 N kg71

body mass is still wrong because it confuses the unit

for force with the unit for mass and besides, it doesnt

strictly represent 7.5% of body weight. The expres-

sion could be 0.075 N N71 body weight but becausethe unit is common to both, even that can be

improved to: in ratio 0.075:1 with body weight. The

simplest of all is where we started: 7.5% of body

weight.

It is also common to see an expression something

like, to overcome the inertia of the system,

participants were given a rolling start. This means


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that participants could then achieve the impossible

and accelerate the system without the application of

any more force. While it is indeed difficult to set the

system in motion, this is attributable to the work that

has to be done to overcome frictional and other

factors to increase the angular momentum of the

flywheel. The inertia of the system remains.

4.11. Workload

Finally, consideration is given to this term, a term

that blights sport and exercise science. It has been

the feature of an editorial in this Journal (Winter,

2006). In brief, it is nonsensical. It could mean the

load of work that was performed, in which case the

accompanying unit should be joules. It could be the

opposing force in the work, in which case the unit

would be newtons. It cannot be power output (W),

as it is frequently claimed, nor can it be running

speed (m s71) with which it is frequently althougherroneously associated. In short, as Winter (2006)

stated, the term should be banished from the lexicon

of exercise sciences.

5. Solutions

Having given due consideration to potential pitfalls,

how can the ability to perform exercise be described

and quantified in ways that conform to the SI and

adhere to science? The following will propose

methods to do so that are remarkably simple.

5.1. Measures

Let us now consider the expressions exercise

performance and exercise capacity.

5.1.1. Exercise performance. To assess performance,

scalar quantities such as time (s), distance (m), and

speed (m s71) might be all that is needed todescribe how well someone is exercising. In fact, all

one might need is a competition, for example in

running and swimming, to identify who finished first,

second, third, and so on and hence produce ordinal

data. Nevertheless, accompanying measures add a

precision that is probably more informative. Time tocomplete a set distance, the distance one can jump or

project an implement, or the mean speed during for

example running, cycling or swimming, are all

simple, suitable measures. Vector quantities such as

force and velocity might also suffice and, providing

the use and context are correct, so too might power,

but the instances where power is relevant are

probably few.

5.1.2.Exercise capacity.This is slightly more involved

yet, paradoxically, the outcome measure can be

remarkably simple: time. Usually, it is the duration

for which one can exercise to volitional exhaustion,

either at a set percentage of performance capability

or at a percentage of a physiological maximum.

Maximum oxygen uptake ( _VO2max) and maximum

heart rate tend to be the most common physiological

measures. Care is needed with _VO2max because of

the slow component (Jones & Poole, 2005) and thereis enthusiastic debate about whether it is possible to

have steady-state exercise at challenges that exceed

about 75% _VO2max.

5.1.3. Intensities and domains. A principal require-

ment is often the need to know how hard someone is

exercising and this is where the term intensity,

expressed in Table I, becomes useful. Its use stems

from its universality: it can be applied to all situations

and this includes those situations that span dis-

ciplines. All that changes is the unit used to quantify

intensity (Knuttgen, 1978). For instance, isometric

force could be expressed in newtons; running,

swimming or cycling speed could be expressed in

m s71; and power where appropriate could beexpressed in watts. Moreover, these units could be

expressed absolutely or as percentages of their

respective maxima. Similarly, intensity of exercise

could be expressed as equivalents to percentages of

physiological maxima. Intensities could be described

as low, moderate or high as appropriate. When

exercise is performed all-out, it is maximal. This

should prevent the nonsensical term supra-

maximal being applied; it is simply not possible to

exceed ones maximum. It should be acknowledgedthat maximum-intensity exercise can exceed the

intensity required to elicit _VO2max by a factor of

three or four (Williams, 1987).

The important point is that all are intensities of

exercise and can be described as such, only the

units differ. This means that the possibility of

besmirching science is at least reduced and perhaps

eliminated; expressions such as workload and work

rate are immediately and correctly abandoned

together with the confusion and transgressions that

they create.

According to physiological responses, intensities

can be categorized into domains such as moderate,heavy, very heavy, and severe (Whipp, 1996) and,

indeed, extreme (Jones & Poole, 2005). This provides

further elegance and simplicity. In addition to these

objective measures, account can be made of sub-

jective responses that is, perceptions of exertion

(Borg, 1998). These provide important assessments

of how participants feel as they perform exercise.

These assessments of feeling might or might not be

accompanied by other supposedly harder measures;

they could stand on their own, support or be

supported by others.


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6. Summary

It should be acknowledged that exercise and physical

activity do not always result in movement, yet energy

expenditure can be prodigious. A definition of

exercise should acknowledge this. The one proposed

here does just that. In the context of muscle function,

the term contraction should be used cautiously.Improved accuracy arises from attempts to short-

en, but in spite of clear historical precedents to the

contrary, contraction will probably continue to be

used. Terms such as workload and work rate should

be abandoned and the terms intensity of exercise and

domains of exercise should be adopted because of

their clarity and universal applicability. It should be

acknowledged that the use of terms should be

undertaken with care because descriptions of ex-

ercise can transgress principles of science.

The continued development of sport and exercise

science demands that practitioners and theoreticians

do not commit such transgressions. Accordingly, thefollowing definitions and terms are intended to

uphold principles of science and adhere to the SI:

. Function of muscle: the function of muscle is to

exert force and it does so by attempting to

shorten.

. Exercise: a potential disruption to homeostasis

by muscle activity that is either exclusively, or in

combination, concentric, eccentric or isometric.

. Intensity of exercise: this expression should be

used instead of workload or work rate to

indicate physiological, psychological or biome-

chanical demand on the participant by theperformance of exercise.

Acknowledgements

We are grateful for the advice given by Professors

S. J. H. Biddle and H. G. Knuttgen, D. Broom PhD,

F. B. C. Brookes and three anonymous referees.

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