Basic Exercise Physiology

Chapter 1
Basic Exercise Physiology
HOWA R D G . K NU T T G E N
Introduction
The performance of sport, as with all physical
exercise, is dependent upon the coordinated activation of the athlete’s skeletal muscles. The
muscles constitute the sources of the forces and
power required for skilled movement. Unfortunately, the description and quantification of exercise is frequently made awkward, if not difficult,
by a variety of terms, some of which are confusing or inaccurate. Through the years, terms have
been regularly misused and units of measurement inappropriately applied.
Exercise
The term exercise, itself, has been defined in different ways by different sources in the literature.
For the Encyclopaedia of Sports Medicine series of
publications, the definition has been accepted as
‘any and all activity involving generation of force
by activated skeletal muscle’ (Komi 1992). This
would include activities of daily living, activities
of labour, activities for physical conditioning and
physical recreation, as well as participation in
sport competition. In the Encyclopaedia of Sports
Medicine series, a sport will be considered as
any organized activity that involves exercise,
rules governing the event and the element of
competition.
To bring about movement of the body parts
and coordinate the skills of a sport, the central
nervous system activates the striated, voluntary
muscle cells which are the principal constituents
of the various structures called skeletal muscles.
The response of muscle cells to neural stimulation is to produce force.
In order to develop force, skeletal muscle cells
are activated by electrochemical impulses arriving via efferent neurones, the cell bodies of which
are located in the anterior horn of the gray matter
of the spinal cord. When the threshold of excitation of the muscle cells of a motor unit has been
attained, electrochemical events within each
muscle cell (fibre) result in the cylindrical fibre
generating force along its longitudinal axis in
order to draw the ends of the cylinder towards
its midsection. In this way, the activated fibres
develop force between the attachments of the
muscle in which they are contained. It has been
proposed that this process be referred to as a
muscle ‘action’ (Cavanagh 1988) rather than
‘contraction’ due to the fact that any activated
individual fibre and, indeed, an entire muscle
may: (i) shorten the distance along its longitudinal axis, (ii) be held at the same length by an
opposing force, or (iii) be forcibly stretched in
length by an opposing force. The term action has
the advantage of being independent of a change
in length or of direction. By definition, contraction
means shortening only.
The terminology employed to identify the
three actions thus deserves discussion and explanation. The interaction of muscle force development and the external forces will result in actions
that produce static exercise (no movement about
the related joints) or in dynamic exercise (resulting in a change in joint angles). Static exercise of
3
4
nutrition and exercise
Table 1.1 Classification of exercise and muscle action
types.
Exercise
Muscle action
Muscle length
Dynamic
Dynamic
Static
Concentric
Eccentric
Isometric
Decreases
Increases
No change
activated muscle is traditionally referred to as
isometric. Force is developed but, as there is no
movement, no work is performed. All other
muscle actions involve movement and are
termed dynamic. The term concentric is traditionally used to identify a shortening action and the
term eccentric is used to identify a lengthening
action, although the origin of these terms is
obscure (Table 1.1) (Knuttgen & Kraemer 1987).
The International System
Some years ago, the world of science adopted an
International System of Measurement (Bureau
International des Poids et Mésures 1977; Le
Système International, abbreviated as SI) to quantify all physical entities and processes. The unit
of force in the SI is the newton (N). One newton is
quantified as the force which imparts to a mass of
1 kilogram an acceleration of 1 metre per second
per second. To develop force, a muscle cell
requires energy, and the SI unit for energy is the
joule (J).
When force is expressed through a displacement (i.e. movement of body parts is occurring),
work is measured as force (N) multiplied by the
distance (m) of the displacement, and work can
be calculated as force ¥ distance: 1 N ¥ 1 m = 1 J.
During movement, the performance of work
involves conversion of one form of energy (J) to
another. The SI unit for energy is the same unit
used to quantify work. One joule is the energy of
1 newton acting through a distance of 1 metre.
Any energy used by the muscle for force development that does not result in work becomes
heat, the SI unit for heat also being the joule.
Obviously, direct relationships exist among
energy, work and heat and they are quantified
with the same unit, the joule. Throughout this
publication, the term energy will most often refer
to metabolic energy.
When time [SI unit, the second (s)] becomes a
factor in quantifying energy release, the performance of work or the generation of heat, then the
rate of energy release, work performance or heat
generation is presented as power, the SI unit for
which is the watt (W) (1 J ¥ 1 s–1 = 1 W). In exercise
in which 150 W of external power is produced at
a metabolic cost of 750 W, then the rate of heat
production is 600 W.
Attention should be called at this point to the
fact that, when describing exercise and sport,
physiologists and nutritionists can be interested
in the available energy content that can be
metabolized from the food ingested (J), the total
stored energy available for the muscle cells (J),
the total energy utilized during a conditioning
session or sports performance (J), or the rate at
which muscle cells are called upon to produce
power (W).
The joule and the calorie
As described above, the joule is the SI unit used
to quantify energy, work and heat. This provides
a simple and efficient basis for describing the
relationship among nutrition, exercise performance, body heat generation and heat dissipation, both in terms of total amounts (in joules) or
as power (in watts). Unfortunately, the calorie
and its multiple, the kilocalorie (kcal), have been
utilized for so long in nutritional circles that a
change to the description of the energy content of
foods in joules is being implemented very slowly.
Instead of utilizing the convenient relationships
among newtons, joules, seconds and watts,
conversion factors need to be employed. For
example, 1 cal = 4.186 J and 1 kcal = 4.186 kJ.
When a mechanically braked cycle ergometer
is used for an exercise bout, one method of
obtaining the desired power production would
be to have the subject cycle at a pace that would
produce a ‘velocity’ of the flywheel rim of 5 m · s–1
and provide an opposing force (sometimes
termed ‘resistance’) of 60 N. The simplest way of
basic exercise physiology
quantifying exercise is with SI units. The bout of
exercise can be described as follows:
Power developed on the ergometer: 300 W
Duration of exercise: 600 s (10 min)
Metabolic power (derived from oxygen uptake):
1500 W
Total metabolic energy utilized
= 1500 W ¥ 600 s = 900 000 J = 900 kJ
Mechanical efficiency
= 300 W/1500 W ¥ 100 = 20%
If work is calculated by using a ‘kilogram of
force’ (an improper unit of measurement!), a
kilogram-metre can be utilized as an unsanctioned unit to quantify work. Conversion factors
would be utilized to convert kilogram-metres
per unit of time into the correct unit for power,
the watt. If the calorie is used to quantify metabolic energy, conversion factors must be utilized
to obtain a measurement of metabolic power that
can be compared to the power transferred to the
cycle ergometer. It is far easier to utilize SI units
throughout all research activity and scientific
writing: the newton, the metre, the second, the
joule and the watt. (It is important to call attention to the fact that a kilogram-metre [kg-m] in
the SI is actually the correct unit of measurement
for torque.)
There are an infinite number of configurations
of force and velocity (determined by cadence on
the ergometer) that can produce the desired
external power produced and therefore metabolic power desired.
In this volume, the editorial decision was
made to acknowledge the continued and extensive use of the kilocalorie (kcal) in much of the
scientific literature for the quantification of the
energy content of foods and therefore to permit
the use of this unit of measurement in the various
chapters where considered expedient.
Energy for muscle activity
The mechanical and biochemical events associated with muscle cell force development are
5
described in detail in Chapter 2. However, it is
worth making the following general comments
and observations as related to nutrition for sport.
The immediate source of energy for muscle
force and power production is adenosine
triphosphate (ATP). ATP is the final biochemical
carrier of energy to the myofilaments for the generation of force. The breakdown of phosphocreatine (PCr) serves to reconstitute ATP when other
sources contribute little or no energy. Each
muscle cell then becomes dependent on fat (fatty
acids), carbohydrate (glucose and glycogen) and,
to a very limited extent, protein (amino acids) as
the sources of energy to resynthesize ATP and
PCr during exercise. All persons concerned with
the nutrition of the athlete must consider the
nutritional demands of the long-term conditioning programme, the preparation for competition
and the competitive event itself, when planning
individual meals as well as the weekly and
monthly dietary programmes.
It is generally accepted that the muscle cells
obtain all the energy needed for short-term sport
performance of a few seconds (as in the throwing
and jumping events of track and field, weightlifting and springboard and platform diving)
from ATP and PCr (Fig. 1.1). These compounds
are then resynthesized during recovery. When a
sport performance lasts approximately 10 s (e.g.
the 100-m run), other energy sources, including
especially anaerobic glycolysis (resulting in lactic
acid formation in the muscle), must also contribute to the resynthesis of ATP. The lower the
intensity and the longer the event, the better able
is aerobic glycolysis to contribute energy. It is
also assumed that, during events that are still
considered ‘sprints’ but that last longer than a
few seconds, aerobic metabolism begins to make
a contribution to ATP resynthesis.
As the duration of the exercise period
increases still further, the energy from the oxidation of a combination of fat and carbohydrate
becomes a significant source of energy. If exercise
lasts 15 min or longer, such intensities demand a
steady-state of aerobic metabolism (i.e. lower
than maximum aerobic metabolism) except for
any final effort that calls forth all the power the
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nutrition and exercise
Fig. 1.1 Olympic weightlifting is
an example of a sport in which the
competitive performance is so
short that all of the energy for the
lift is provided by the high-energy
phosphates, ATP and PCr. Photo
© Allsport.
athlete can generate. The final burst of power (or
‘kick’) results from a combination of high utilization of both anaerobic glycolysis and aerobic
power. In the range of events that last between
30 s and 12 min, a combination of anaerobic glycolysis and oxidative metabolism provides most
of the energy necessary to resynthesize ATP and
permit the athlete to continue. The lower the
demand for power, the better the oxidative
metabolism can provide the energy for ATP
resynthesis. Anaerobic glycolysis involves only
carbohydrate and, at these high intensities, even
aerobic metabolism draws upon carbohydrate in
preference to fat.
An athlete who performs to exhaustion
in approximately 3–12 min challenges the cardiorespiratory and metabolic mechanisms so
that aerobic metabolism eventually attains its
highest level. When this occurs, the oxygen
uptake is identified as either ‘maximum oxygen
.
uptake’ (Vo2max.) or ‘maximum aerobic power’.
It is not uncommon to read and hear the term
‘maximum exercise’ used to refer to intensities
that result in maximum oxygen uptake. The term
is completely misleading, given the fact that the
athlete can produce power anaerobically for
short periods of time that is four to five times as
great as that which can be developed utilizing
maximum aerobic power.
Fat is stored to a limited extent inside the
muscle cells but can be mobilized during exercise
from depots around the body for transport by the
circulatory system to active muscle cells. Carbohydrate is stored inside the muscle cells as glycogen but can also be mobilized as glucose from
glycogen stored in the liver.
Power, energy and endurance
The information presented in the three panels of
Fig. 1.2 provides vivid examples of the relationships among human metabolic power production, the sources of energy and the ability to
endure at specific exercise intensities. In panel A,
the relationship between endurance (or time to
exhaustion) is plotted vs. metabolic power. For
the sample athlete, power production of about
5000 W can be assumed to come solely from
energy stored in skeletal muscle ATP and PCr.
In the range of 2000–4000 W, anaerobic glycolysis assumes major responsibility for the provision of energy. This results in the production of
large amounts of lactic acid and lowered pH in
the sarcoplasm, which are believed to eventually
hinder force and power development by the
muscle fibres. Lactic acid values in the blood rise
commensurate with muscle concentrations.
For the athlete in the example, oxidative
metabolism begins to make a major contribution
of energy for ATP and PCr resynthesis once the
Endurance time to exhaustion (min)
basic exercise physiology
40
7
*
30
20
*
*
10
* *
0
0
*4000 *
2000
*6000
Metabolic power (W)
(a)
1.0
%
CHO
100
%
Fat
0
75
25
50
50
25
75
0
100
0.9
2
RQ
.
V O2 (mmol.s–1)
3
0.8
1
M
0.7
Rest
(b)
500
1000
1500
2000
6000
Metabolic power (W)
Rest
(c)
500
1000
1500
2000
6000
Metabolic power (W)
Fig. 1.2 The relationships of (a) endurance time, (b) oxygen uptake in steady state, and (c) respiratory quotient
(RQ) and percentage substrate utilization to human metabolic power production. Values presented for power are
representative for an 80-kg athlete.
power output falls to approximately 2000 W.
It is at power productions of 1500–1800 W that
.
the maximum oxygen uptake (Vo2max. of 2.7
mmol · s–1) is elicited for this athlete during the
final stage of an exercise bout. At 1500 W, the
athlete could sustain exercise for approximately
8 min but at 1800 W, for less than 5 min.
Below 1500 W (Fig. 1.2b), the athlete is able to
sustain exercise for extended periods with completely or nearly completely aerobic metabolism,
utilizing fat and carbohydrate to resynthesize
ATP and PCr. The letter ‘M’ is placed on the
abscissa to indicate the power production corre.
sponding to about 75–80% Vo2max. that the
athlete could sustain for a marathon (42.2 km). At
any higher level, the athlete would enlist anaerobic glycolysis, accumulate lactate and lower pH
values in the skeletal muscle cells, and be forced,
eventually, to reduce power or stop.
Note the relatively narrow range of power production that can be produced completely aerobically by comparing Fig. 1.2b with Fig. 1.2a.
Marathon pace in this example would constitute
approximately 24% of maximum power produc-
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nutrition and exercise
Fig. 1.3 Marathon pace for a
runner requires approximately
75–80% of maximal aerobic power
and approximately 24% of the
anaerobic power the same
muscles could produce for a
strength exercise. Photo ©
NOPP / Larry Bessel.
tion and the range for maximum aerobic power
would constitute approximately 30% of maximum power production (Fig. 1.3).
In Fig. 1.2c, the relationship of respiratory quotient (RQ) as determined from steady-state respiratory exchange ratio (RER) to metabolic power
is presented. RER, which compares oxygen
uptake to carbon dioxide removal in the lungs,
attains steady state at lower levels of power production (in this example, less than 1500 W). The
values for RQ vs. power production are modified
from Åstrand and Rodahl (1986). A range is presented to accommodate different values that
might be obtained during different days as a
result of variations in the athlete’s diet. Utilizing
both the left-side and the right-side ordinates, the
observed RQs indicate a high utilization of carbohydrate from approximately 75% of maximum
aerobic power and upwards. The higher
the intensity, the greater is the contribution of
carbohydrate.
An athlete maintaining a diet high in carbohydrate will maintain a higher RQ at all levels of
aerobic exercise, whereas the RQ of an athlete
with a low intake of carbohydrate will remain
remarkably lower. During long-lasting events
and training bouts, the RQ will become lower at
any chosen intensity the longer the exercise lasts,
as it is related to increasing free fatty acid avail-
ability and falling levels of glycogen in the active
muscles. RQ can also be affected by the ingestion
of a substance such as caffeine which results in an
enhanced utilization of fatty acids for the energy
demands of exercise.
Skeletal muscle
A skeletal muscle is made up predominantly of
extrafusal skeletal muscle fibres, long cylindrical
cells which run the length of the muscle, be it
short or long (e.g. 1–300 mm). Intrafusal fibres
are the small skeletal muscle cells found in the
muscle spindles which assist in controlling the
body’s coordinated movement. The muscle also
includes connective tissue which provides some
organization to the muscle’s internal structure
(white connective tissue) and elasticity (yellow
connective tissue). Arteries, veins and capillaries
made up of smooth muscle, connective tissue
and epithelial cells are found throughout each
muscle, serving as the combination delivery/
removal system. Afferent and efferent neurones
connect each muscle to the central nervous
system to provide the muscle with motor control
and send sensory information to the central
nervous system. Fat is found within and between
muscle cells in quantities that become reflected in
the person’s total body composition and percent-
basic exercise physiology
age body fat. Therefore, each muscle is made up
of cells representing the four basic tissue groups:
muscle, connective, nervous and epithelial.
Extrafusal fibres can be further divided among
groups based upon the interrelated twitch characteristics and metabolic capabilities. Fibres of
a particular motor unit (defined as a motor
neurone together with the extrafusal fibres it
innervates) that attain peak force development
relatively slowly are routinely termed ‘slow
twitch’ or type I fibres. Fibres that attain peak
force relatively more rapidly are termed ‘fast
twitch’ or type II fibres and further subdivided
into type IIa, type IIab and type IIb groups, as
based on myosin ATPase staining (Fig. 1.4).
Type I fibres are characterized by high mitochondrial density, high myoglobin content, high
aerobic metabolism and modest glycolytic capacity. Early anatomists described muscles with
what we now identify as high type I fibre population as ‘red muscle’ because of the darker colour
caused by the high myoglobin content. Type II
fibres have high glycolytic capability, low mitochondrial density and low capacity for aerobic
metabolism. Types IIa, IIab and IIb fibres are low
in myoglobin content, the reason for their being
identified many decades ago as white muscle
fibres.
The total number of muscle fibres in a particu-
Fig. 1.4 Cross-section of human
muscle showing the mosaic
pattern of fibres: darkest stain =
type I; lightest stain = type IIa;
medium light stain = type IIab;
medium dark stain = type IIb.
Photo courtesy of William J.
Kraemer.
9
lar muscle and the proportion identified as type I
and type II appear to be genetically dominated,
with small changes occurring through conditioning, injury, ageing, etc. It should also be mentioned that, while type II motor units are termed
fast twitch and type I motor units are termed
slow twitch, the comparison is on relative terms
and all extrafusal muscle fibres attain peak force
and shorten extremely fast. The differences
among them are great, however, and the shortening velocity is generally considered to be 4–10
times faster for the type II fibres than for type I
fibres.
The maximum force that can be developed by
an activated muscle is directly related to the
physiological cross-section of the muscle, a term
that describes the collective cross-sectional area
of the muscle cells, excluding the connective
tissue (including fat), nervous tissue and blood
vessels. The larger the physiological crosssection of muscle, the greater is the muscle’s
ability to generate peak force (strength). Considerable evidence exists to confirm the importance
of the type II fibre population of a muscle to its
ability to develop high force and power.
A high type I fibre population and the accompanying increased capillarization to supply
oxygen has been shown to be important for sustained, rhythmic exercise which depends on
10
nutrition and exercise
aerobic metabolism. For example, a marathon
runner can utilize over 12 000 repeated muscle
actions of each leg in completing the 42.2-km
course.
The characteristics and capabilities of the
muscle fibres can be substantially modified by
specific training programmes. Athletes engaging
in sports which involve wide ranges of power
and continuously varying amounts of aerobic
and anaerobic metabolism must utilize a programme of conditioning that raises both the
anaerobic and aerobic capabilities of the three
fibre types. Examples of such sports are soccer,
basketball and tennis.
Physiological support systems
While muscle cells may obtain energy for force
and power production from both anaerobic
sources (the breakdown of ATP and PCr; anaerobic glycolysis) and aerobic sources (aerobic glycolysis and b-oxidation of fatty acids, both
leading to the provision of electrons to the electron transport system in the mitochondria), the
entire human organism and all of its component
cells are fundamentally aerobic. Exercise performed at low enough intensities can be performed entirely with energy from aerobic
metabolism. The provision of significant
amounts of energy for muscular activity by the
anaerobic mechanisms, however, is limited in
amount and therefore in time. Most importantly,
the return to the pre-exercise or resting state
following any amount of anaerobic energy
release is accomplished exclusively by aerobic
metabolism.
Therefore, the essential features in the provision of oxygen for metabolism during aerobic
exercise and recovery following anaerobic exercise become pulmonary ventilation (air movement into and out of the lungs), external
respiration (exchange of O2 and CO2 between
alveoli and pulmonary capillary blood), blood
circulation and internal respiration (exchange of
O2 and CO2 between systemic capillary blood
and interstitial fluid). The essential elements as
regards these processes are cardiac output, blood
volume, blood composition and skeletal muscle
capillarization.
Pulmonary ventilation and external respiration
Movement of air into and out of the lungs is
accomplished by the diaphragm and various
muscles of the neck and trunk. Pulmonary ventilation is usually accomplished as a subconscious
activity under the influence of chemical stimuli
provided by the systemic arterial blood to a
nervous centre in the brain stem. While this
centre serves the sole function of controlling the
minute volume of pulmonary ventilation (by
interaction of frequency of ventilation and magnitude of tidal volume), it is interesting to note
that it is identified anatomically and physiologically as the ‘respiratory centre.’
For continuous aerobic activity that would
involve attainment of a ‘steady state’ of oxygen
uptake (and carbon dioxide elimination) via
the lungs, pulmonary ventilation corresponds
directly to oxygen uptake by an approximate
20 : 1 ratio (litres per minute are used in the
presentation of both variables). Starting at rest,
an 80-kg athlete would expect the values presented in Table 1.2 for oxygen uptake and pulmonary ventilation.
The increase in the ratio for the highest level of
activity reflects the increased acidity of the blood
due to the production in the muscle and appear-
Table 1.2 Representative data for steady-state oxygen
uptake and ventilatory minute volume at rest and
during various intensities of constant-intensity
exercise (for an 80-kg athlete). The maximum aerobic
power is 4.5 l · min-1.
Rest
---Intense aerobic exercise
Intense aerobic exercise with
anaerobic contribution
.
V o2
(l · min-1)
.
VE
(l · min-1)
0.25
1.00
2.00
3.00
3.50
4.00
5
20
40
60
70
100
basic exercise physiology
ance in the blood of lactic acid, as related to the
anaerobic metabolism.
For the athlete performing aerobic exercise
under most conditions, it is considered that the
individual’s capacity for ventilation is adequate
to provide O2 from the atmosphere to the alveoli
and to carry CO2 from the alveoli to the atmosphere. In elite endurance athletes who are highly
conditioned for aerobic metabolism and are performing near their capacities for aerobic power
production, it can be frequently observed that
blood leaving the lungs via the pulmonary veins
is not as saturated with oxygen as it is under the
conditions of rest and submaximal aerobic exercise. It can thus be concluded that, under the very
special conditions where a very highly conditioned athlete is performing high-intensity
aerobic exercise, pulmonary ventilation serves as
a limiting factor for external respiration and
therefore oxygen uptake.
Circulation
For the delivery of oxygen, the removal of carbon
dioxide and the transport of anabolites and
catabolites to and from the body cells, the organism is dependent upon the circulation of the
blood. With regard to the aerobic metabolism
related to exercise and recovery, the most important factors are: the oxygen-carrying capacity of
the blood, the blood volume available, the ability
of the heart to pump blood (cardiac output) and
the capillarization of the skeletal muscles.
11
The term cardiac output can actually refer to the
amount of blood ejected through the aorta or the
pulmonary arteries per minute (‘minute volume’
.
or Q) or the amount of blood ejected per systole (‘stroke volume’ or SV). The relationship
between minute volume and stroke volume
includes the contraction frequency of the heart
.
(fH) as follows: Q = fH · SV.
The relationship of these variables with
oxygen uptake includes the unloading factor of
oxygen in the tissues as determined from the
content of oxygen in systemic arterial blood
(Cao2) and the content in systemic mixed venous
blood (Cvo2). It is:
.
Vo2 = fH · SV · a-vo2diff.
Representative values for an 80-kg athlete are
presented in Table 1.3. It can be observed that the
relationship between aerobic power (oxygen
uptake) and heart rate is essentially rectilinear.
Stroke volume increases from a resting value of
104 ml to near maximum values even during
low-intensity aerobic exercise. The increase in
cardiac minute volume as higher levels of
oxygen uptake are attained is accounted for by
the increase in heart rate.
Meanwhile, the arteriovenous oxygen difference continues to increase due solely to the
lowered concentration of oxygen in systemic
mixed venous blood leaving the active tissues.
The arterial concentration remains constant at a
value of approximately 20 ml · l–1 blood, indicating that pulmonary capillary blood becomes
Table 1.3 Representative data for steady-state oxygen uptake and circulatory variables at rest and during various
intensities of constant-intensity exercise (for an 80-kg athlete). The maximum aerobic power for the athlete is
4.5 l · min-1 and maximum fH is 195.
Rest
---Intense aerobic exercise
Intense aerobic exercise with
anaerobic contribution
.
V o2
(l · min-1)
.
Q
(l · min-1)
fH
(beats · min-1)
SV (ml)
a-vo2diff.
(ml O2 · l-1)
0.25
1.00
2.00
3.00
3.50
4.00
6.4
12.3
14.8
17.2
19.7
22.1
60
100
120
140
160
180
104
123
123
123
123
123
40
81
136
174
178
180
12
nutrition and exercise
completely saturated with O2 obtained from
the alveoli. It should be noted that, at the highest levels of oxygen uptake, highly trained
endurance athletes (e.g. distance runners and
cross-country skiers) show a lowered oxygen
saturation in arterial blood. This is taken to indicate that the blood flow through the lungs during
such intense aerobic exercise for these athletes
exceeds the capacity of the ventilatory system to
provide oxygen to the lungs.
As will be discussed, maximum aerobic power
.
(Vo2max.) can be increased mainly by increasing
the stroke volume capability of the heart,
which increases the minute volume capability.
Maximum heart rate does not increase with
aerobic conditioning but, actually, it either
remains the same or decreases.
.
The Vo2max. of 4.5 l · min–1 corresponds to a
metabolic power production of 1500 W. As the
athlete in the example is capable of power production for short periods (e.g. 1–20 s) in the range
of 3000–6000 W, the question could be raised as to
what values for the circulatory variables would
be expected during such exercise performance.
The answer is that these values, if measured,
would be irrelevant. The athlete would be performing in the range of power production where
oxidative (aerobic) metabolism contributes little
or no energy and the muscles will rely on ATP,
PCr and anaerobic glycolysis.
1.5). For events lasting between 1 and 3 min,
aerobic conditioning is important but anaerobic
sources of energy for the power demands
become more important the higher the exercise
intensity and the shorter the accompanying performance time. It also makes a great difference
whether or not the athlete performs to exhaustion (such as in the 10-km run) or is involved in
one half of a soccer match (45 min) which
involves a wide range of aerobic/anaerobic
intensities and intermittent activity. Also, skill
may be more important than any other performance consideration.
If an increase in aerobic power is required, the
athlete must follow a programme designed to
increase the cardiac output capability (SV and
Adaptations to conditioning
The adaptations of the human organism to programmes of exercise conditioning are highly specific to the exercise programme (i.e. the stimulus)
provided. Adaptations to resistance training for
strength, to anaerobic training (as in sprinting)
and to endurance (aerobic) training are very different and, if used inappropriately, can actually
serve to be counterproductive.
Aerobic conditioning
For athletes engaged in events lasting approximately 3 min or longer, aerobic conditioning is a
crucial factor in preparing for competition (Fig.
Fig. 1.5 A sport such as road cycling depends
predominantly upon aerobic metabolism. Photo ©
Allsport / M. Powell.
basic exercise physiology
.
Q), the total circulating haemoglobin and the
capillarization of the skeletal muscles that are
involved. Such conditioning also serves to
enhance the aerobic metabolic capacities of the
skeletal muscle cells including both type I and
type II fibres.
The programme would consist of a combination of interval training and some extended
bouts of exercise (e.g. 10–60 min) consistent with
the particular competitive event. Depending
upon the individual athlete and the point in time
relative to the competitive season, the athlete will
train vigorously three to seven times per week.
It is important to note that such aerobic conditioning can adversely affect the particular skeletal muscles involved as regards the ability for the
generation of high power and the explosive
effort involved in activities such as jumping and
throwing. The adaptation of the systems of the
body and, in particular, the skeletal muscles will
be specific to the conditioning stimulus or, in
other words, the conditioning programme.
Anaerobic conditioning
For events lasting less than 10 min, energy
obtained from anaerobic glycolysis is an important factor; the shorter the event the greater is the
contribution of this source. There is an obvious
overlapping with oxidative metabolism the
longer the duration of the activity.
With a programme of conditioning that combines a considerable amount of strength training
with very high but continuous exercise intensity
that mimics the event (e.g. the 100-m run, the
100-m swim, wrestling), the emphasis is on an
appropriate increase in the size of type II muscle
cells, enhancing the capability of the cells for
anaerobic glycolysis, and increasing the concentrations of ATP and PCr. Most, if not all, type IIb
fibres that exist at the initiation of such conditioning convert to type IIa. Except for the shortest
lasting performances (weightlifting, high jump,
pole vault, discus, shot-put, javelin), maintenance of high concentrations of glycogen in
the muscle cells through proper nutrition is
important.
13
Strength conditioning
Increases in maximum force production
(strength) and maximal power of the muscles are
brought about through exercise programmes of
very high opposing force (routinely termed
‘resistance’) that limits repetitions to approximately 20 or fewer and therefore a duration of
less than 30 s. Exercise programmes based on
higher repetitions (e.g. 30–50 repetitions leading
to exhaustion) develop local muscular endurance but are not conducive to strength development. Exercise involving many repetitions
in a bout (e.g. 400–1000 repetitions) brings
about physiological adaptations that result in
enhanced aerobic performance that can be especially counterproductive to power development
and, to a lesser extent, on the performance of
strength tests.
‘Resistance training’ is performed with a
variety of exercise machines, free weights or
even the use of gravity acting upon the athlete’s
body mass. Most resistance training (strength)
programmes are based on a system of exercise to
a repetition maximum (RM) as presented in the
mid-1940s by DeLorme (1945). Every time the
athlete performs a particular exercise, the bout is
performed for the maximum number of repetitions, or RM, possible and this number is
recorded along with the mass lifted or opposing
force imposed by an exercise machine. Repeated
testing at increasingly higher opposing force will
eventually lead to the determination of a 1RM, in
which the athlete can perform the movement but
once and not repeat it. In this system, the mass
lifted or opposing force is described as the
athlete’s strength at that particular point in time
and for the particular movement.
Bouts of strength exercise and the daily programme can be based on percentages of a 1 RM,
preferably, within heavy (3–5), medium (9–10)
and light (15–18) RM zones (Fleck & Kraemer
1997). The number of bouts performed in a set,
the number of sets performed per day and the
number of daily workouts per week are then prescribed for each movement or muscle group as
based on the point in time in the competitive
14
nutrition and exercise
season, the physical condition of the athlete, programme variation for both physiological and
psychological considerations and programme
objectives.
The principal adaptation of the athlete’s body
is the increase in size (commonly termed hypertrophy) of type II muscle cells. It is generally held
that no interchange takes place between type I
and type II fibres as the result of specific conditioning programmes.
As the force development capability of a
muscle is directly related to its physiological
cross-section, the increase in size of the muscle
cells is the principal reason for increased force
development. The energy requirement for performance of a 1 RM is quite small, as is the performance of any bout of exercise from the 1 RM to a
20 RM. However, the total energy requirement of
performing multiple bouts of exercise for each of
a number of movements or exercises (e.g. 10) in a
daily workout is large. This deserves careful consideration from the standpoint of the athlete’s
nutrition, both in terms of quantity and content.
In addition to the total energy balance and
accompanying maintenance of appropriate body
mass, consideration must be given to suitable
protein intake.
Adaptations of skeletal muscle
Muscle cells and the structure of an individual
muscle, in general, respond in very different
ways to the unique exercise stimulus that is provided. The muscles respond to the acute stimulus
by providing the forces and power demanded by
such widely diverse performances as weightlifting, high jumping, 100-m sprinting and
running at marathon pace. Following a single
bout of exercise or a single day’s conditioning
session, however, the individual muscle fibres
and the total muscle recover to a physiological
state with little or no measurable change.
Repeated workouts over weeks and months
elicit adaptations, and these structural and
functional changes are highly specific to the
conditioning programme (i.e. the stimulus)
as appropriate to the competitive event for
which the athlete is preparing (Fig. 1.6). A highresistance (strength) programme which results
in significant muscle hypertrophy could be detrimental to distance running performance. A
conditioning programme for distance running
would definitely be detrimental to weightlifting,
high jumping and sprint performance.
Strength training results in an increase in size
(girth and therefore cross-sectional area) of type
II muscle fibres and the muscles themselves.
Capillarization can evidence either no change or
a ‘dilution effect’, where the hypertrophy of
muscle cells spreads out the existing capillaries,
with the result that an individual capillary serves
a larger cross-sectional area of muscle.
Fig. 1.6 Many team sports such as
international football (soccer)
require combinations of aerobic
power, anaerobic power and
strength, as well as a wide variety
of skills. Photo © Allsport / S.
Bruty.
basic exercise physiology
A combination of strength and anaerobic
conditioning, as appropriate to sprinters, results
in some hypertrophy and an increase in the
anaerobic metabolic capabilities of type II fibres.
The resting concentrations of ATP and PCr
increase as well as the capability of the cells to
produce force and power with energy from
anaerobic glycolysis.
In both strength conditioning and combination
strength/anaerobic conditioning, there is little or
no adaptation of the cardiovascular system in
terms of stroke volume, minute volume or blood
composition.
Highly aerobic training involving a large
number of movement repetitions (e.g. 500–2000)
results in adaptations to both muscle cells and to
the cardiovascular system. The aerobic metabolic
capacities of type I fibres is greatly enhanced, as
is, to a lesser extent, the aerobic capacity of type II
fibres. This includes increases in mitochondrial
count, myoglobin content and glycogen storage.
An increase in capillarization provides enhanced
capability for oxygen and substrate delivery and
for carbon dioxide and catabolite removal. The
abilities of the muscle for high force and power
development diminish.
Nutrition of an athlete
All of the factors involving muscle, ventilation/
respiration and circulation are important in
determining the success of a particular individual in competing in a particular sport. Additional
factors involve coordination (skilled movement),
body size and motivation. However, energy is
needed for the performance of short-term explosive events, long-term endurance events and the
many sport activities that involve the development of varying amounts of power during the
course of a contest. Therefore, proper nutrition
must be considered to be a key element to success
in a wide variety of competitive sports.
Frequently overlooked by athletes when considering the nutrition of sport is the tremendous
time and energy involved in the conditioning
programme between competitions and/or
leading up to a competitive season. Performance
15
of a throwing event in track and field or of
Olympic weightlifting events takes but a few
seconds of time, but preparation involves many
hours of skill practice and conditioning.
The nutrition of an athlete is a 12-months-ofthe-year consideration. Too often, the focus of
attention is placed on the days or even hours
leading up to a competitive event. While preevent food ingestion is of great importance,
optimal health and optimal performance are
dependent on year-around planning. Under
certain circumstances, nutrition during an event
and/or immediately following an event also
carry great importance.
Each athlete must perform at an appropriate
body weight. In addition to the total mass
involved, the relative contribution to total
mass by muscle, fat and bone is of importance.
Optimal values for the various constituents are
best developed through a combination of proper diet and appropriate conditioning that is
continuous.
The moment a competitive event begins, the
athlete should be at appropriate body mass, sufficiently hydrated, possess proper amounts of
vitamins and minerals, and be nourished with
sufficient carbohydrate that an appropriate
balance of carbohydrate and fat metabolism will
provide the energy for the ensuing muscular
activity.
Nutritional and energetic limits
to performance
It can be generally accepted that each athlete
enters his/her event with fat stores in excess of
what will be utilized during the course of a competition. It is well known, however, that the
higher the intensity of the muscular activity, the
greater the proportion of energy that the muscles
will obtain from carbohydrate (glucose and
glycogen) compared with that obtained from fat
(fatty acids).
Herein lies a major challenge to athletes
competing in a wide range of sports involving
moderate intensity and long duration, that of
ensuring that the carbohydrate stores in the
16
nutrition and exercise
skeletal muscles and liver are optimal as the
event starts. Skeletal muscle cells will depend
both on endogenous glycogen stores as well as
carbohydrate delivered by the blood as glucose.
The nervous system depends totally on glucose
obtained from the blood for its completely
aerobic metabolism. Insufficient glucose for the
nervous system results in loss of control and
coordination of the muscles and the movements.
There is a small amount of glucose circulating in
the blood as an event starts, but the blood
glucose level must be maintained from glycogen
stored in the liver. Low glycogen concentrations
in the skeletal muscle cells reduce an athlete’s
capacity for power production. Low blood
glucose can therefore adversely affect both
nervous system function and muscle function.
The athlete’s conditioning programme must
be planned with great care and appreciation for
the specific demands of each event or sport activity. The force, power, metabolic and associated
nutritional demands of both competition and the
conditioning programmes involve great differences among such varied activities as Olympic
weightlifting, high jumping, 100-m running,
400-m swimming, tennis, field hockey, basketball, road cycling, cross-country skiing and
marathon running.
References
Åstrand, P.-O. & Rodahl, K. (1986) Textbook of Work
Physiology. McGraw-Hill, New York.
Bureau International des Poids et Mésures (1977) Le
Système International d’Unités (SI), 3rd edn. Sèvres,
France.
Cavanagh, P.R. (1988) On ‘muscle action’ vs. ‘muscle
contraction.’ Journal of Biomechanics 22, 69.
DeLorme, T.L. (1945) Restoration of muscle power by
heavy resistance exercises. Journal of Bone and Joint
Surgery 27, 645–667.
Fleck, S.J. & Kraemer, W.J. (1997) Designing Resistance
Training Programs. Human Kinetics, Champaign, IL.
Knuttgen, H.G. & Kraemer, W.J. (1987) Terminology
and measurement in exercise performance. Journal of
Applied Sports Science Research 1, 1–10.
Komi, P.V. (ed.) (1992) Strength and Power in Sport.
Blackwell Scientific Publications, Oxford.
Further reading
Dirix, A., Knuttgen, H.G. & Tittel, K. (eds) (1992) The
Olympic Book of Sports Medicine. Blackwell Scientific
Publications, Oxford.
Komi, P.V. & Knuttgen, H.G. (1996) Sport science and
modern training. In Sports Science Studies, Vol. 8, pp.
44–62. Verlag Karl Hofmann, Schorndorf.
Shephard, R.J. & Åstrand, P.-O. (eds) (1992) Endurance
in Sport. Blackwell Scientific Publications, Oxford.