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ing. Sechenov-Physiological Journal of the USSR Journal of Public Health 1959;49:1630-1639.
1901.47:56-59. .

57
CHAPTER 3
PHysiOLOGIC RESPONSES AND LONG-TERM
ADAPTATIONS TO EXERCISE

Contents
IntroductiON oo... ee ee EEE eee Eee 61
Physiologic Responses to Episodes of Exercise .... 0.6... 6.0 eee eee eee 61
Cardiovascular and Respiratory Systems ....... 0.00002 eee es 61
Cardiovascular Responses to Exercise ....-. 0... 0-2 eee eet eee eee 62
Cardiac Output 0. ne ett 62
Blood Flow 1.0.0.0... 0c ee ttt eee 63
Blood Pressure ...... cece nee eb bbe teen e eee e eee eee 63
Oxygen Extraction 2.12.6... teen eee 63
Coronary Circulation ........... 00620 e eee ee ee teens 63
Respiratory Responses to Exercise ... 6.62... ec tte eee 64
Resistance Exercise ....... esses. 65
Skeletal Muscle 000 en nee ee eee eee 65
Skeletal Muscle Energy Metabolism ... 0.0... 006000 e ete e eee eeeeeeees 65
Energy Systems 2.2.0.0... 0.02 e tet 65
Metabolic Rate... eee eee eee es 66
Maximal Oxygen Uptake .......... 000 eee eee tee eens 66
Lactate Threshold .. 00.0... ce eee eee e eee 66
Hormonal Responses to Exercise 2.2.00... ts 66
Immune Responses to Exercise .... 0... 0 eect te eee eee 67
i ong-Term Adaptations to Exercise Training ..... 22-6... e eee ee cee eee 67
Adaptations of Skeletal Muscle and Bone ............. 00000 e eee eee teen eens 67
Metabolic Adaptations... 0.000000 cc eee tenes s+. 69
Cardiovascular and Respiratory Adaptations ............ 000 eee eter eee eee ees 71

Long-Term Cardiovascular Adaptations ............-.-. 22 e cece ee eee eee 71
Respiratory Adaptations
Physical Activity and Health

Contents, continued

Maintenance, Detraining, and Prolonged Inactivity ......-- +--+ e esse eee eet eres 71
Maintaining Fitness and Muscular Strength .........- 0200+ - sere etter ert: 72
Detraining 2.2.0.0... ee tee tt eee tenes eee eee 72
Prolonged Inactivity 0.0.2.0... eee cence eee teen ttre ener nes 72

Special Considerations ........-. 60.0 cece eee ener ester ttt ernest sree 73
Disability 0.0.0.0... eee eee teeter eerie asses 73
Environmental Conditions ............2-0 eee etree tet eeetee ae. 73
Effects of Age 0.0... eee cnet r ttt eee eerste ress r trees 75
Differences by Sex .........----- Lee eee bebe eee eee enter eens 76

Conclusions ..... 0.00. ccc cece eee ee nee eens 77
Research Needs ........ 1.000 c cee ect en eet e teen eee teen eens 77

References 0.00... eee eee enter eee ene 77

60
CHAPTER 3

PHYSIOLOGIC RESPONSES AND LONG-TERM

Introduction

hen challenged with any physical task, the

human body responds through a series of
integrated changes in function that involve most, if
not all, of its physiologic systems. Movement re-
quires activation and control of the musculoskeletal
system; the cardiovascular and respiratory systems
provide the ability to sustain this movement over
extended periods. When the body engages in exer-
cise training several times a week or more frequently,
each of these physiologic systems undergoes specific
adaptations that increase the body’s efficiency and
capacity. The magnitude of these changes depends
largely on the intensity and duration of the training
sessions, the force or load used in training, and the
body’s initial level of fitness. Removal of the train-
ing stimulus, however, will result in loss of the
efficiency and capacity that was gained through
these training-induced adaptations; this loss is a
process called detraining.

This chapter provides an overview of how the
body responds to an episode of exercise and adapts
to exercise training and detraining. The discussion
focuses on aerobic or cardiorespiratory endurance
exercise (e.g., walking, jogging, running, cycling,
swimming, dancing, and in-line skating) and resis-
tance exercise (e.g., strength-developing exercises).
It does not address training for speed, agility, and
flexibility. In discussing the multiple effects of
exercise, this overview will orient the reader to the
physiologic basis for the relationship of physical
activity and health. Physiologic information perti-
nent to specific diseases is presented in the next
‘chapter. For additional information, the reader is

referred to the selected textbooks shown in the
sidebar.

ADAPTATIONS TO EXERCISE

 

Selected Textbooks on Exercise Physiology

Astrand PO, Rodahl K. Textbook of work physiology.
3rd edition. New York: McGraw-Hill Book Company,
1986.

Brooks GA, Fahey TD, White TP. Exercise physiology:
human bioenergetics and its applications. 2nd edition.
Mountain View, CA: Mayfield Publishing Company,
1996.

Fox E, Bowers R, Foss M. The physiological basis for
exercise and sport. 5th edition. Madison, WI: Brown
and Benchmark, 1993.

McArdle WD, Katch Fi, Katch VL. Essentials of

exercise physiology. Philadelphia, PA: Lea and
Febiger, 1994.

Powers SK, Howley ET. Exercise physiology: theory
and application to fitness and performance. Dubuque,
1A: William C. Brown, 1990.

Wilmore JH, Costill DL. Physiology of sport and

 

 

exercise. Champaign, IL: Human Kinetics, 1994.

Physiologic Responses to Episodes
of Exercise

The body’s physiologic responses to episodes of
aerobic and resistance exercise occur in the muscu-
loskeletal, cardiovascular, respiratory, endocrine,
and immune systems. These responses have been
studied in controlled laboratory settings, where ex-
ercise stress can be precisely regulated and physi-
ologic responses carefully observed.

Cardiovascular and Respiratory Systems

The primary functions of the cardiovascular and
respiratory systems are to provide the body with
Physical Activity and Health

oxygen (O,) and nutrients, to rid the body of carbon
dioxide (CO,) and metabolic waste products, to
maintain body temperature and acid-base balance,
and to transport hormones from the endocrine
glands to their target organs (Wilmore and Costill
1994). To be effective and efficient, the cardiovascu-
lar system should be able to respond to increased
skeletal muscle activity. Low rates of work, such as
walking at 4 kilometers per hour (2.5 miles per
hour), place relatively small demands on the cardio-
vascular and respiratory systems. However, as the
rate of muscular work increases, these two systems
will eventually reach their maximum capacities and
will no longer be able to meet the body’s demands.

Cardiovascular Responses to Exercise

The cardiovascular system, composed of the heart,
blood vessels, and blood, responds predictably to
the increased demands of exercise. With few excep-
tions, the cardiovascular response to exercise is
directly proportional to the skeletal muscle oxygen
demands for any given rate of work, and oxygen
uptake (VO,) increases linearly with increasing rates
of work.

Cardiac Output
Cardiac output Q) is the total volume of blood
pumped by the left ventricle of the heart per minute.
It is the product of heart rate (HR, number of beats
per minute) and stroke volume (SV, volume of blood
pumped per beat). The arterial-mixed venous oxygen
(A-VO,) difference is the difference between the oxy-
gen content of the arterial and mixed venous blood. A
person’s maximum oxygen uptake V O, max) is a
function of cardiac output Q) multiplied by the
AO, difference. Cardiac output thus plays an im-
portant role in meeting the oxygen demands for
work. As the rate of work increases, the cardiac
output increases in a nearly linear manner to meet
the increasing oxygen demand, but only up to the
point where it reaches its maximal capacity Q max).
To visualize how cardiac output, heart rate, and
stroke volume change with increasing rates of work,
consider a person exercising on a cycle ergometer,
starting at 50 watts and increasing 50 watts every 2
minutes up to a maximal rate of work (Figure 3-1 A,
B, and C). In this scenario, cardiac output and heart
rate increase over the entire range of work, whereas
stroke volume only increases up to approximately 40

62

 

Figure 3-1. Changes in cardiac output (A), heart rate
(B), and stroke volume (C) with increasing
rates of work on the cycle ergometer

 

 

 

 

 

 

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Physiologic Responses and Long-Term Adaptations to Exercise

to 60 percent of the person’s maximal oxygen uptake
(VO, max), after which it reaches a plateau. Recent
studies have suggested that stroke volume in highly
trained persons can continue to increase up to near

maximal rates of work (Scruggs et al. 1991; Gledhill,
Cox, Jamnik 1994).

Blood Flow

The pattern of blood flow changes dramatically when
a person goes from resting to exercising. At rest, the
skin and skeletal muscles receive about 20 percent of

the cardiac output. During exercise, more blood is’

sent to the active skeletal muscles, and, as body
temperature increases, more blood is sent to the skin.
This process is accomplished both by the increase in
cardiac output and by the redistribution of blood flow
away from areas of low demand, such as the splanch-
nic organs. This process allows about 80 percent of the
cardiac output to go to active skeletal muscles and
skin at maximal rates of work (Rowell 1986). With
exercise of longer duration, particularly in a hot and
humid environment, progressively more of the car-
diac output will be redistributed to the skin to counter
the increasing body temperature, thus limiting both
the amount going to skeletal muscle and the exercise
endurance (Rowell 1986).

Blood Pressure

Mean arterial blood pressure increases in response to
dynamic exercise, largely owing to an increase in
systolic blood pressure, because diastolic blood pres-
sure remains at near-resting levels. Systolic blood
pressure increases linearly with increasing rates of
work, reaching peak values of between 200 and 240
millimeters of mercury in normotensive persons. Be-
cause mean arterial pressure is equal to cardiac output
times total peripheral resistance, the observed increase
in mean arterial pressure results from an increase in
cardiac output that outweighs a concomitant decrease
in total peripheral resistance. This increase in mean
arterial pressure is a normal and desirable response, the
result of a resetting of the arterial baroreflex toa higher
pressure. Without such a resetting, the body would
experience severe arterial hypotension during intense
" activity (Rowell 1993). Hypertensive patients typically
reach much higher systolic blood pressures for a given
rate of work, and they can also experience increases in
diastolic blood pressure. Thus, mean arterial pressure

63

is generally much higher in these patients, likely owing
to a lesser reduction in total peripheral resistance.

For the first 2 to 3 hours following exercise,
blood pressure drops below preexercise resting lev-
els, a phenomenon referred to as postexercise hy-
potension (Isea etal. 1994). The specific mechanisms
underlying this response have not been established.
The acute changes in blood pressure after an episode |
of exercise may be an important aspect of the role of
physical activity in helping control blood pressure in
hypertensive patients.

Oxygen Extraction ;

The A-VO, difference increases with increasing rates
of work (Figure 3-2) and results from increased
oxygen extraction from arterial blood as it passes
through exercising muscle. At rest, the A-VO, differ-
ence is approximately 4 to 5 ml of O, for every 100 ml
of blood (ml/100 ml); as the rate of work approaches
maximal levels, the A¥O, difference reaches 15 to 16
ml/100 ml of blood.

Coronary Circulation

The coronary arteries supply the myocardium with
blood and nutrients. The right and left coronary
arteries curve around the external surface of the heart,
then branch and penetrate the myocardial muscle
bed, dividing and subdividing like branches of a tree
to form a dense vascular and capillary network to
supply each myocardial muscle fiber. Generally one
capillary supplies each myocardial fiber in adult hu-
mans and animals; however, evidence suggests that
the capillary density of the ventricular myocardium
can be increased by endurance exercise training.

At rest and during exercise, myocardial oxygen
demand and coronary blood flow are closely linked.
This coupling is necessary because the myocardium
depends almost completely on aerobic metabolism
and therefore requires a constant oxygen supply.
Even at rest, the myocardium’s oxygen use is high
relative to the blood flow. About 70 to 80 percent of
the oxygen is extracted from each unit of blood
crossing the myocardial capillaries; by comparison,
only about 25 percent is extracted from each unit
crossing skeletal muscle at rest. In the healthy heart,
a linear relationship exists between myocardial oxy-
gen demands, consumption, and coronary blood
flow, and adjustments are made on a beat-to-beat
Physical Activity and Health

 

Figure 3-2. Changes in arterial and mixed venous oxygen content with increasing rates of work on the

cycle ergometer

 

 

 

 

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basis. The three major determinants of myocardial
oxygen consumption are heart rate, myocardial
contractility, and wall stress (Marcus 1983;
Jorgensen et al. 1977). Acute increases in arterial
pressure increase left ventricular pressure and wall
stress. Asa result, the rate of myocardial metabolism
increases, necessitating an increased coronary blood
flow. A very high correlation exists between both
myocardial oxygen consumption and coronary blood
flow and the product of heart rate and systolic blood
pressure (SBP) (Jorgensen et al. 1977). This so-
called double product (HR ¢ SBP) is generally used
to estimate myocardial oxygen and coronary blood
flow requirements. During vigorous exercise, all
three major determinants of myocardial oxygen re-
quirements increase above their resting levels.

The increase in coronary blood flow during exer-
cise results from an increase in perfusion pressure of
the coronary artery and from coronary vasodilation.
Most important, an increase in sympathetic nervous
system stimulation leads to an increase in circulating
catecholamines. This response triggers metabolic pro-
cesses that increase both perfusion pressure of the

64

coronary artery and coronary vasodilation to meet the
increased need for blood flow required by the increase
in myocardial oxygen use.

Respiratory Responses to Exercise

The respiratory system also responds when chal-
lenged with the stress of exercise. Pulmonary ven-
tilation increases almost immediately, largely
through stimulation of the respiratory centers in
the brain stem from the motor cortex and through
feedback from the proprioceptors in the muscles
and joints of the active limbs. During prolonged
exercise, or at higher rates of work, increases in CO,
production, hydrogen ions (H*), and body and
blood temperatures stimulate further increases in
pulmonary ventilation. At low work intensities, the
increase in ventilation is mostly the result of in-
creases in tidal volume. At higher intensities, the
respiratory rate also increases. In normal-sized,
untrained adults, pulmonary ventilation rates can
vary from about 10 liters per minute at rest to more
than 100 liters per minute at maximal rates of work,
in large, highly trained male athletes, pulmonary
Physiologic Responses and Long-Term Adaptations to Exercise

ventilation rates can reach more than 200 liters per
minute at maximal rates of work.

Resistance Exercise

The cardiovascular and respiratory responses to
episodes of resistance exercise are mostly similar to
those associated with endurance exercise. One no-
table exception is the exaggerated blood pressure
response that occurs during resistance exercise. Part
of this response can be explained by the fact that
resistance exercise usually involves muscle mass
that develops considerable force. Such high, isolated
force leads to compression of the smaller arteries and
results in substantial increases in total peripheral
resistance (Coyle 1991). Although high-intensity
resistance training poses a potential risk to hyperten-
sive patients and to those with cardiovascular dis-
ease, research data suggest that the risk is relatively
low (Gordon et al. 1995) and that hypertensive
persons may benefit from resistance training (Tipton
1991, American College of Sports Medicine 1993).

Skeletal Muscle

The primary purpose of the musculoskeletal system is
to define and move the body. To provide efficient and
effective force, muscle adapts to demands. In response
to demand, it changes its ability to extract oxygen,
choose energy sources, and rid itself of waste prod-
ucts. The body contains three types of muscle tissue:
skeletal (voluntary) muscle, cardiac muscle or myo-
cardium, and smooth (autonomic) muscle. This sec-
tion focuses solely on skeletal muscle.

Skeletal muscle is composed of two basic types of
muscle fibers distinguished by their speed of con-
traction—slow-twitch and fast-twitch—a character-
istic that is largely dictated by different forms of the
enzyme myosin adenosinetriphosphatase (ATPase).
Slow-twitch fibers, which have relatively slow con-
tractile speed, have high oxidative capacity and fa-
tigue resistance, low glycolytic capacity, relatively
high blood flow capacity, high capillary density, and
high mitochondrial content (Terjung 1995). Fast-
twitch muscle fibers have fast contractile speed and
are classified into two subtypes, fast-twitch type “a”
(FT,) and fast-twitch type “b” (FT,). FT, fibers have
moderately high oxidative capacity, are relatively
fatigue resistant, and have high glycolytic capacity,
relatively high blood flow capacity, high capillary

65

density, and high mitochondrial content (Terjung
1995). FT, fibers have low oxidative capacity, low
fatigue resistance, high glycolytic capacity, and fast
contractile speed. Further, they have relatively low
blood flow capacity, capillary density, and mito-
chondrial content (Terjung 1995).

There is a direct relationship between predomi-
nant fiber type and performance in certain sports.
For example, in most marathon runners, slow-twitch
fibers account for up to or more than 90 percent of
the total fibers in the leg muscles. On the other hand,
the leg muscles in sprinters are often more than 80
percent composed of fast-twitch fibers. Although the
issue is not totally resolved, muscle fiber type ap-
pears to be genetically determined; researchers have
shown that several years of either high-intensity
sprint training or high-intensity endurance training
do not significantly alter the percentage of the two
major types of fibers (Jolesz and Sreter 1981).

Skeletal Muscle Energy Metabolism

Metabolic processes are responsible for generating
adenosine triphosphate (ATP), the body’s energy
source for all muscle action. ATP is generated by three
basic energy systems: the ATP-phosphocreatine
(ATP-PCr) system, the glycolytic system, and the
oxidative system. Each system contributes to energy
production in nearly every type of exercise. The
relative contribution of each will depend on factors
such as the intensity of work rate at the onset of
exercise and the availability of oxygen in the muscle.

Energy Systems

The ATP-PCr system provides energy from the ATP
stored in all of the body’s cells. PCr, also found in all
cells, is a high-energy phosphate molecule that stores
energy. As ATP concentrations in the cell are reduced
by the breakdown of ATP to adenosine diphosphate
(ADP) to release energy for muscle contraction, PCris
broken down to release both energy and a phosphate
toallow reconstitution of ATP from ADP. This process
describes the primary energy system for short, high-
intensity exercise, such as a 40- to 200-meter sprint;
during such exercise, the system can produce energy
at very high rates, and ATP and PCr stores, which are
depleted in 10-20 seconds, will last just long enough
to complete the exercise.
Physical Activity and Health

At high rates of work, the active muscle cell’s
oxygen demand exceeds its supply. The cell must
then rely on the glycolytic energy system to produce
ATP in the absence of oxygen (i.e., anaerobically).
This system can only use glucose, available in the
blood plasma and stored in both muscle and the liver as
glycogen. The glycolytic energy system is the primary
energy system for all-out bouts of exercise lasting
from 30 seconds to 2 minutes, such as an 800-meter
run. The major limitation of this energy system is
that it produces lactate, which lowers the pH of both
the muscle and blood. Once the pH drops below a
value of 6.4 to 6.6, enzymes critical for producing
energy are no longer able to function, and ATP
production stops (Wilmore and Costill 1994).

The oxidative energy system uses oxygen to
produce ATP within the mitochondria, which are
special cell organelles within muscle. This process
cannot generate ATP ata high enough rate to sustain
an all-out sprint, but it is highly effective at lower
rates of work (e.g., long distance running). ATP can
also be produced from fat and protein metabolism
through the oxidative energy system. Typically, car-
bohydrate and fat provide most of the ATP; under
most conditions, protein contributes only 5 to 10
percent at rest and during exercise.

Metabolic Rate

The rate at which the body uses energy is known as
the metabolic rate. When measured while a person is
at rest, the resulting value represents the lowest (i-e.,
basal) rate of energy expenditure necessary to main-
tain basic body functions. Resting metabolic rate is
measured under highly controlled resting condi-
tions following a 12-hour fast and a good night's
sleep (Turley, McBride, Wilmore 1993). To quantify
the rate of energy expenditure during exercise, the
metabolic rate at rest is defined as 1 metabolic
equivalent (MET); a 4 MET activity thus represents
an activity that requires four times the resting meta-
bolic rate. The use of METs to quantify physical
activity intensity is the basis of the absolute intensity
scale. (See Chapter 2 for further information.)

Maximal Oxygen Uptake

During exercise, VO, increases in direct proportion to
the rate of work. The point at which a person’s VO, is
no longer able to increase is defined as the maximal

oxygen uptake (VO,max) (Figure 3-3). A person’s
VO, max is in part genetically determined; it can be
increased through training until the point that the
genetically possible maximum is reached. VO, max is
considered the best estimate of a person's cardio-

respiratory fitness or aerobic power (Jorgensen etal.
1977).

Lactate Threshold

_ Lactate is the primary by-product of the anaerobic

66

glycolytic energy system. At lower exercise intensi-
ties, when the cardiorespiratory system can meet the
oxygen demands of active muscles, blood lactate
levels remain close to those observed at rest, because
some lactate is used aerobically by muscle and is
removed as fast as it enters the blood from the
muscle. As the intensity of exercise is increased,
however, the rate of lactate entry into the blood from
muscle eventually exceeds its rate of removal from
the blood, and blood lactate concentrations increase
above resting levels. From this point on, lactate
levels continue to increase as the rate of work in-
creases, until the point of exhaustion. The point at
which the concentration of lactate in the blood
begins to increase above resting levels is referred to
as the lactate threshold (Figure 3-3).

Lactate threshold is an important marker for endur-
ance performance, because distance runners set their
race pace at or slightly above the lactate threshold
(Farrell et al. 1979). Further, the lactate thresholds
of highly trained endurance athletes occur at a much
higher percentage of their VO max, and thusat higher
relative workloads, than do the thresholds of un-
trained persons. This key difference is what allows
endurance athletes to perform at a faster pace.

Hormonal Responses to Exercise

The endocrine system, like the nervous system,
integrates physiologic responses and plays an im-
portant role in maintaining homeostatic conditions
at rest and during exercise. This system controls the
release of hormones from specialized glands through-
out the body, and these hormones exert their actions
on targeted organs and cells. In response to an
episode of exercise, many hormones, such as cat-
echolamines, are secreted at an increased rate, though
insulin is secreted at a decreased rate (Table 3-1).
The actions of some of these hormones, as well as
Physiologic Responses and Long-Term Adaptations to Exercise

 

Figure 3-3. Changes in oxygen uptake and blood lactate concentrations with increasing rates of work on the
cycle ergometer’

   

 

 

 

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their specific responses to exercise, are discussed in
more detail in Chapter 4.

Immune Responses to Exercise

The immune system is a complex adaptive system
that provides surveillance against foreign proteins,
viruses, and bacteria by using the unique functions of
cells produced by the bone marrow and the thymus
gland. By interacting with neural and endocrine
factors, the immune system influences the body’s
overall response to exercise (Reichlin 1992). A grow-
ing body of literature indicates that the incidence of
some infections may be influenced by the exercise
history of the individual (Nieman 1994; Hoffman-
Goetz and Pedersen 1994).

Moderate exercise has been shown to bolster the
function of certain components of the human immune
system—such as natural killer cells, circulating T- and
B-lymphocytes, and cells of the monocyte-macroph-
age system—thereby possibly decreasing the inci-
dence of some infections (Keast, Cameron, Morton
1988; Pedersen and Ullum 1994; Woods and Davis

1994) and perhaps of certain types of cancer (Shephard
and Shek 1995).

67

Exercise of high intensity and long duration or
exercise that involves excessive training may have
adverse effects on immune function. In general, a
high-intensity, single episode of exercise results in a
marked decline in the functioning of all major cells of
the immune system (Newsholme and Parry-Billings
1994; Shephard and Shek 1995). In addition, over-
training may reduce the response of T-lymphocytes to
mutagenic stimulation, decrease antibody synthesis
and plasma level ofimmunoglobins and complement,
and impair macrophage phagocytosis. The reduced
plasma glutamine levels that occur with high-intensity
exercise or excessive training are postulated to con-
tribute to these adverse effects on the immune system
(Newsholme and Parry-Billings 1994).

Long-Term Adaptations to
Exercise Training

Adaptations of Skeletal Muscle and Bone

Skeletal muscle adapts to endurance training chiefly
through a small increase in the cross-sectional area
of slow-twitch fibers, because low- to moderate-
Physical Activity and Health

 

 

Table 3-1. Asummary of hormonal changes during an episode of exercise
Exercise
Hormone response _ Special relationships Probable importance
Catecholamines Increases Greater increase with intense Increased blood glucose;
exercise; norepinephrine > increased skeletal muscle and
epinephrine; increases less after liver glycogenolysis; increased
training lipolysis
Growth hormone (GH) Increases Increases more in untrained
persons; declines faster in trained =Unknown
persons
Adrenocorticotropic Increases Greater increase with intense Increased gluconeogenesis in
hormone (ACTH)-cortisol exercise; increases less liver; increased mobilization of
after training with fatty acids
submaximal exercise
Thyroid-stimulating Increases Increased thyroxine turnover
hormone (TSH)-thyroxine with training but no toxic effects Unknown
are evident
Luteinizing hormone (LH) No change None None
Testosterone Increases None Unknown
Estradiol-progesterone Increases Increases during luteal phase Unknown
of the menstrual cycle
Insulin Decreases Decreases less after training Decreased stimulus to use blood
, glucose
Glucagon Increases Increases less after training Increased blood glucose via
glycogenolysis and
gluconeogenesis
Renin-angiotensin- Increases Same increase after training Sodium retention to maintain
aldosterone in rats plasma volume
Antidiuretic hormone Expected None Water retention to maintain
(ADH) increase plasma volume
Parathormone Unknown None Needed to establish proper
(PTH)-calcitonin bone development
Erythropoietin Unknown None Would be important to increase
erythropoiesis
Prostaglandins May May increase in response to May be local vasodilators
increase sustained isometric contractions;

may need ischemic stress

Adapted from Wilmore JH, Costill DL. Physiology of sport and exercise. Champaign, IL: Human Kinetics, 1994, p. 136.

68
Physiologic Responses and Long-Term Adaptations to Exercise

intensity aerobic activity primarily recruits these
fibers (Abernethy, Thayer, Taylor 1990). Prolonged
endurance training (i.e., months to years) can lead to
a transition of FT, fibers to FT, fibers, which have a
higher oxidative capacity (Abernethy, Thayer, Taylor
1990). No substantive evidence indicates that fast-
twitch fibers will convert to slow-twitch fibers under
normal training conditions (JJolesz and Sreter 1981).
Endurance training also increases the number of
capillaries in trained skeletal muscle, thereby allow-
ing a greater capacity for blood flow in the active
muscle (Terjung 1995).

Resistance-trained skeletal muscle exerts con-
siderably more force because of both increased muscle
size (hypertrophy) and increased muscle fiber re-
cruitment. Fiber hypertrophy is the result of in-
creases in both the size and number of myofibrils in
both fast-twitch and slow-twitch muscle fibers
(Kannus etal. 1992). Hyperplasia, or increased fiber
number, has been reported in animal studies, where
the number of individual muscle fibers can be counted
(Gonyea etal. 1986), and has been indirectly demon-
strated during autopsies on humans by using direct
fiber counts to compare dominant and nondominant
paired muscles (Sjéstrém et al. 1991).

During both aerobic and resistance exercise,
active muscles can undergo changes that lead to
muscle soreness. Some soreness is felt immediately
after exercise, and some can even occur during exer-
cise. This muscle soreness is generally not physically
limiting and dissipates rapidly. A more limiting sore-
ness, however, may occur 24 to 48 hours following
exercise. This delayed-onset muscle soreness is pri-
marily associated with eccentric-type muscle action,
during which the muscle exerts force while lengthen-
ing, as can happen when a person runs down a steep
hill or lowers a weight from a fully flexed to a fully
extended position (e.g., the two-arm curl). Delayed-
onset muscle soreness is the result of structural dam-
age to the muscle; in its most severe form, this damage
may include rupture of the cell membrane and disrup-
tion of the contractile elements of individual muscle
fibers (Armstrong, Warren, Warren 1991). Such dam-
age appears to result in an inflammatory response
(MacIntyre, Reid, McKenzie 1995).

Total inactivity results in. muscle atrophy and
loss of bone mineral and mass. Persons who are
sedentary generally have less bone mass than those
who exercise, but the increases in bone mineral and

69

mass that result from either endurance or resistance
training are relatively small (Chesnut 1993). The
role of resistance training in increasing or maintain-
ing bone mass is not well characterized. Endurance
training has little demonstrated positive effect on
bone mineral and mass. Nonetheless, even small
increases in bone mass gained from endurance or
resistance training can help prevent or delay the
process of osteoporosis (Drinkwater 1994). (See
Chapter 4 for further information on the effects of
exercise on bone.)

The musculoskeletal system cannot function with-
out connective tissue linking bones to bones (liga-
ments) and muscles to bones (tendons). Extensive
animal studies indicate that ligaments and tendons
become stronger with prolonged and high-intensity
exercise. This effect is the result of an increase in the
strength of insertion sites between ligaments, ten-
dons, and bones, as well as an increase in the cross-
sectional areas of ligaments and tendons. These
structures also become weaker and smaller with sev-
eral weeks of immobilization (Tipton and Vailas 1990),
which can have important implications for muscu-
loskeletal performance and risk of injury.

Metabolic Adaptations

Significant metabolic adaptations occur in skeletal
muscle in response to endurance training. First, both
the size and number of mitochondria increase sub-
stantially, as does the activity of oxidative enzymes.
Myoglobin content in the muscle can also be aug-
mented, increasing the amount of oxygen stored in
individual muscle fibers (Hickson 1981), but this
effect is variable (Svedenhag, Henriksson, Sylvén
1983). Such adaptations, combined with the increase
in capillaries and muscle blood flow in the trained
muscles (noted ina previous section), greatly enhance
the oxidative capacity of the endurance-trained muscle.

Endurance training also increases the capacity of
skeletal muscle to store glycogen (Kiens et al. 1993).
The ability of trained muscles to use fat as an energy
source is also improved, and this greater reliance on
fat spares glycogen stores (Kiens et al. 1993). The
increased capacity to use fat following endurance
training results from an enhanced ability to mobilize
free-fatty acids from fat depots and an improved
capacity to oxidize fat consequent to the increase in
the muscle enzymes responsible for fat oxidation
(Wilmore and Costill 1994).
Physical Activity and Health

These changes in muscle and in cardiorespi-
ratory function are responsible for increases in
both VO, max and lactate threshold. The endurance-
trained person can thus perform atconsiderably higher
rates of work than the untrained person. Increases in

VO,max generally range from 15 to 20 percent follow-
ing a 6-month training period (Wilmore and Costill
1994). However, individual variations in this response
are considerable. In one study of 60- to 71-year-old
men and women who endurance trained for 9 to 12
months, the improvement in VO,max varied from 0
to 43 percent; the mean increase was 24 percent

(Kohrt et al. 1991). This variation in response may
be due in part to genetic factors and to initial levels
of fitness. To illustrate the changes that can be
expected with endurance training, a hypothetical
sedentary man’s pretraining values have been com-
pared with his values after a 6-month period of
endurance training and with the values of a typical
elite endurance runner (Table 3-2).

Responses to endurance training are similar for
men and women. At all ages, women and men show
similar gains in strength from resistance training

_ (Rogers and Evans 1993; Holloway and Baechle 1990)

 

 

 

 

Table 3-2. A hypothetical example of alterations in selected physiological variables consequent to a 6-month
endurance training program in a previously sedentary man compared with those of a typical elite
endurance runner

Sedentary man
Variable Pretraining Posttraining Runner
Cardiovascular
HR at rest (beats ¢ min’) 71 59 36
HR max (beats © min') 185 183 174
SV rest (ml) 65 80 125
SV max (ml) 120 140 200
Q rest (L © min") 4.6 4.7 4.5
Q max (L ¢ min") 22.2 25.6 32.5
Heart volume (ml) 750 820 1,200
Blood volume (L) 4.7 5.1 6.0
Systolic BP rest (mmHg) 135 130 120
Systolic BP max (mmHg) 210 205 210
Diastolic BP rest (mmHg) 78 76 65
Diastolic BP max (mmHg) 82 80 65
Respiratory
V, rest (L © min’) 7 6 6
V, rest (L © min’) 110 135 195
TV rest (L) 0.5 0.5 0.5
TV max (L) 2.75 3.0 3.9
RR rest (breaths * min”) 14 12 12
RR max (breaths ¢ min‘') 40 45 50
Metabolic
AWO, diff rest (ml * 100 ml") 6.0 6.0 6.0
ANO, diff max (ml © 100 ml") 14.5 15.0 16.0
VO, rest (ml ¢ kg! © min") 3.5 3.5 3.5
VO, max (ml © kg! ¢ min’) 40.5 49.8 76.5
Blood lactate rest (mmol ¢ L") 1.0 1.0 1.0
Blood lactate max (mmol ¢ L"') 7.5 8.5 9.0

 

Adapted from Wilmore JH, Costill DL. Physiology of sport and exercise. Champaign, iL: Human Kinetics, 1994, p. 230.

HR = heart rate; max = maximal; SV = stroke volume; Q = cardiac output; BP = blood pressure; V, = ventilatory volume; TV = tidal volume;
RR = respiration rate; A-vO, diff = arterial-mixed venous oxygen difference; VO, = oxygen consumption.
Physiologic Responses and Long-Term Adaptations to Exercise

and similar gains in VO,max from aerobic‘endurance
training (Kohrrt et al. 1991; Mitchell et al. 1992).

Cardiovascular and Respiratory Adaptations

Endurance training leads to significant cardiovascu-
lar and respiratory changes at rest and during steady-
state exercise at both submaximal and maximal rates
of work. The magnitude of these adaptations largely
depends on the person's initial fitness level; on mode,
intensity, duration, and frequency of exercise; and
on the length of training (e.g., weeks, months, years).

Long-Term Cardiovascular Adaptations

Cardiac outputat rest and during submaximal exer-
cise is essentially unchanged following an endur-
ance training program. At or near maximal rates of
work, however, cardiac output is increased sub-
stantially, up to 30 percent or more (Saltin and
Rowell 1980). There are important differences in
the responses of stroke volume and heart rate to
training. After training, stroke volume is increased
at rest, during submaximal exercise, and during
maximal exercise; conversely, posttraining heart
rate is decreased at rest and during submaximal
exercise and is usually unchanged at maximal rates
of work. The increase in stroke volume appears to
be the dominant change and explains most of the
changes observed in cardiac output.

Several factors contribute to the increase in
stroke volume from endurance training. Endurance
training increases plasma volume by approximately
the same percentage that it increases stroke volume
(Green, Jones, Painter 1990). An increased plasma
volume increases the volume of blood available to
return to the right heart and, subsequently, to the
left ventricle. There is also an increase in the end-
diastolic volume (the volume of blood in the heart
at the end of the diastolic filling period) because of
increased amount of blood and increased return of
blood to the ventricle during exercise (Seals et al.
1994). This acute increase in. the left ventricle’s
end-diastolic volume stretches its walls, resulting
in a more elastic recoil.

Endurance training also results in long-term
changes in the structure of the heart that augment
stroke volume. Short-term adaptive responses in-
clude ventricular dilatation; this increase in the vol-
ume of the ventricles allows end-diastolic volume to

7\

increase without excessive stress on the ventricular
walls. Long-term adaptive responses include hyper-
trophy of the cardiac muscle fibers (i.e., increases in
the size of each fiber). This hypertrophy increases
the muscle mass of the ventricles, permitting greater
force to be exerted with each beat of the heart.
Increases in the thickness of the posterior and septal
walls of the left ventricle can lead to a more forceful
contraction of the left ventricle, thus emptying more
of the blood from the left ventricle (George, Wolfe,
Burggraf 1991).

Endurance training increases the number of cap-
illaries in trained skeletal muscle, thereby allowing a
greater capacity for blood flow in the active muscle
(Terjung 1995). This enhanced capacity for blood
flow is associated with a reduction in total peripheral
resistance; thus, the left ventricle can exert a more
forceful contraction against a lower resistance to
flow out of the ventricle (Blomqvist and Saltin 1983).

Arterial blood pressure at rest, blood pressure
during submaximal exercise, and peak blood pres-
sure all show a slight decline asa result of endurance
training in normotensive individuals (Fagard and
Tipton 1994). However, decreases are greater in
persons with high blood pressure. After endurance
training, resting blood pressure (systolic/diastolic)
will decrease on average -3/-3 mmHg in persons with
normal blood pressure; in borderline hypertensive
persons, the decrease will be -6/-7 mmHg; and in
hypertensive persons, the decrease will be -10/-8
mmHg (Fagard and Tipton 1994). (See Chapter 4 for
further information. )

Respiratory Adaptations

The major changes in the respiratory system from en-
durance training are an increase in the maximal rate of
pulmonary ventilation, which is the result of increases
in both tidal volume and respiration rate, and an
increase in pulmonary diffusion at maximal rates of
work, primarily due to increases in pulmonary blood
flow, particularly to the upper regions of the lung.

Maintenance, Detraining, and
Prolonged Inactivity

Most adaptations that result from both endurance
and resistance training will be reversed if a person
stops or reduces training. The greatest deterioration
Physical Activity and Health

in physiologic function occurs during prolonged bed
rest and immobilization by casts. A basic mainte-

nance training program is necessary to prevent these
losses in function.

Maintaining Fitness and Muscular Strength

Muscle strength and cardiorespiratory capacity are
dependent on separate aspects of exercise. After a per-
son has obtained gains in VO,max by performing
cardiorespiratory exercise six times per week, two
to four times per week’is the optimal frequency of
training to maintain those gains (Hickson and
Rosenkoetter 1981). Further, a substantial part of
the gain can be retained when the duration of each
session is reduced by as much as two-thirds, but
only if the intensity during these abbreviated ses-
sions is maintained at >70 percent of VO,max
(Hickson et al. 1985). If training intensity is reduced
by as little as one-third, however, a substantial
reduction in VO,max can be expected over the next
15 weeks (Hickson et al. 1985).

In previously untrained persons, gains in mus-
cular strength can be sustained by as little as a single
session per week of resistance training, but only if
the intensity is not reduced (Graves et al. 1988).

Detraining

With complete cessation of exercise training, a sig-
nificant reduction in VO,max and a decrease in
plasma volume occur within 2 weeks; all prior func-
tional gains are dissipated within 2 to 8 months, even
if routine low- to moderate-intensity physical activ-
ity has taken the place of training (Shephard 1994).
Muscular strength and power are reduced ata much
slower rate than VO,max, particularly during the
first few months after an athlete discontinues resis-
tance training (Fleck and Kraemer 1987). In fact, no
decrement in either strength or power may occur for
the first 4 to 6 weeks after training ends (Neufer etal.
1987). After 12 months, almost half of the strength
gained might still be retained if the athlete remains
moderately active (Wilmore and Costill 1994).

Prolonged Inactivity

The effects of prolonged inactivity have been studied
by placing healthy young male athletes and sedentary
volunteers in bed for up to 3 weeks after a control
period during which baseline measurements were

72

made. The'resulting detrimental changes in physi-
ologic function and performance are similar to those
resulting from reduced gravitational forces during
space flight and are more dramatic than those result-
ing from detraining studies in which routine daily
activities in the upright position (e.g., walking, stair
climbing, lifting, and carrying) are not restricted.
Results of bed rest studies show numerous physi-
ologic changes, such as profound decrements in
cardiorespiratory function proportional to the dura-

_ tion of bed rest (Shephard 1994; Saltin et al. 1968).

Metabolic disturbances evident within a few days of
bed rest include reversible glucose intolerance and
hyperinsulinemia in response to a standard glucose
load, reflecting cell insulin resistance (Lipman et al.
1972); reduced total energy expenditure; negative
nitrogen balance, reflecting loss of muscle protein;
and negative calcium balance, reflecting loss of bone
mass (Bloomfield and Coyle 1993). There is also a
substantial decrease in plasma volume, which affects
aerobic power.

From one study, a decline in VO,max of 15 per-
cent was evident within 10 days of bed rest and
progressed to 27 percent in 3 weeks; the rate of loss was
approximately 0.8 percent per day of bed rest
(Bloomfieldand Coyle 1993). ThedecrementinVO,max
from bed rest and reduced activity results from a
decrease in maximal cardiac output, consequent toa
reduced stroke volume. This, in turn, reflects the
decrease in end-diastolic volume resulting from a
reduction in total blood and plasma volume, and
probably also from a decrease in myocardial contrac-
tility (Bloomfield and Coyle 1993). Maximal heart
rate and A-VO, difference remain unchanged
(Bloomfield and Coyle 1993). Resting heart rate
remains essentially unchanged or is slightly in-
creased, whereas resting stroke volume and cardiac
output remain unchanged orare slightly decreased.
However, the heart rate for submaximal exertion is
generally increased to compensate for the sizable
reduction in stroke volume.

Physical inactivity associated with bed rest or
prolonged weightlessness also results in a progres-
sive decrement in skeletal muscle mass (disuse
atrophy) and strength, as well as an associated
reduction in bone mineral density that is approxi-
mately proportional to the duration of immobiliza-
tion or weightlessness (Bloomfield and Coyle 1993).
The loss of muscle mass is not as great as that which
Physiologic Responses and Long-Term Adaptations to Exercise

occurs with immobilization of a limb by a plaster
cast, but it exceeds that associated with cessation of
resistance exercise training. The muscle groups
most affected by prolonged immobilization are the
antigravity postural muscles of the lower extremi-
ties (Bloomfield and Coyle 1993). The loss of nor-
mal mechanical strain patterns from contraction of
these muscles results in a corresponding loss of
density in the bones of the lower extremity, particu-
larly the heel and the spine (Bloomfield and Coyle
1993), Muscles atrophy faster than bones lose their
density. For example, 1 month of bed rest by healthy
voung men resulted in a 10 to 20 percent decrease
in muscle fiber cross-sectional area and a 2] percent
reduction in peak isokinetic torque of knee exten-
sors (Bloomfield and Coyle 1993), whereas a simi-
lar period of bed rest resulted in a reduction in bone
mineral density of only 0.3 to 3 percent for the
lumbar spine and 1.5 percent for the heel.

Quantitative histologic examination of muscle
Inopsies of the vastus lateralis of the leg following
immobilization shows reduced cross-sectional area
tor both slow-twitch and fast-twitch fibers, actual
necrotic changes in affected fibers, loss of capillary
density, and a decline in aerobic enzyme activity,
creatinine phosphate, and glycogen stores (Bloomfield
and Coyle 1993). On resuming normal activity,
reversibility of these decrements in cardiorespiratory,
metabolic, and muscle function is fairly rapid (within
avs. to weeks) (Bloomfield and Coyle 1993). By
vantrast, the reversal of the decrement of bone min-
vral density requires weeks to months.

Special Considerations

the physiologic responses to exercise and physi-
“logic adaptations to training and detraining, re-
\iewed in the preceding sections, can be influenced
x tnumber of factors, including physical disability,
“nvironmental conditions, age, and sex.

Disability

Although there is a paucity of information about
physiologic responses to exercise among persons
with disabilities, existing information supports the
‘Aotion that the capacity of these persons to adapt to
‘ncreased levels of physical activity is similar to that
ol persons without disabilities. Many of the acute

73

effects of physical activity on the cardiovascular,
respiratory, endocrine, and musculoskeletal systems
have been demonstrated to be similar among persons
with disabilities, depending on the specific nature of
the disability. For example, physiologic responses to
exercise have been studied among persons with
paraplegia (Davis 1993), quadriplegia (Figoni 1993),
mental retardation (Fernhall 1993), multiple sclero-
sis (Ponichtera-Mulcare 1993), and postpolio syn-
drome (Birk 1993).

Environmental Conditions

The basic physiologic responses to an episode of
exercise vary considerably with changes in environ-
mental conditions. As environmental temperature
and humidity increase, the body is challenged to
maintain its core temperature. Generally, as the
body’s core temperature increases during exercise,
blood vessels in the skin begin to dilate, diverting
more blood to the body’s surface, where body heat
can be passed on to the environment (unless envi-
ronmental temperature exceeds body temperature).
Evaporation of water from the skin’s surface signifi-
cantly aids in heat loss; however, as humidity in-
creases, evaporation becomes limited.

These methods for cooling can compromise car-
diovascular function during exercise. Increasing
blood flow to the skin creates competition with the
active muscles for a large percentage of the cardiac
output. When a person is exercising for prolonged
periods in the heat, stroke volume will generally
decline over time consequent to dehydration and
increased blood flow in the skin (Rowell 1993;
Montain and Coyle 1992). Heart rate increases sub-
stantially to try to maintain cardiac output to com-
pensate for the reduced stroke volume.

High air temperature is not the only factor that
stresses the body’s ability to cool itself in the heat.
High humidity, low air velocity, and the radiant heat
from the sun and reflective surfaces also contribute
to the total effect. For example, exercising under
conditions of 32°C (90°F) air temperature, 20 per-
cent relative humidity, 3.0 kilometers per hour (4.8
miles per hour) air velocity, and cloud cover is much
more comfortable and less stressful to the body than
the same exercise under conditions of 24°C (75°F)
air temperature, 90 percent relative humidity, no air
movement, and direct exposure to the sun.
Physical Activity and Health

Children respond differently to heat than adults
do. Children have a higher body surface area to body
mass ratio (surface area/mass), which facilitates heat
loss when environmental temperatures are below
skin temperature. When environmental tempera-
ture exceeds skin temperature, children are at an
even greater disadvantage because these mecha-
nisms then become avenues of heat gain. Children
also have a lower rate of sweat production, even
though they have more heat-activated sweat glands,
each gland produces considerably less sweat than
that of an adult (Bar-Or 1983).

The inability to maintain core temperature can
lead to heat-related injuries. Heat cramps, character-
ized by severe cramping of the active skeletal muscles,
is the least severe of three primary heat disorders.
Heat exhaustion results when the demand for blood
exceeds what is available, leading to competition for
the body’s limited blood supply. Heat exhaustion is
accompanied by symptoms including extreme fa-
tigue, breathlessness, dizziness, vomiting, fainting,
cold and clammy or hot and dry skin, hypotension,
and a weak, rapid pulse (Wilmore and Costill 1994).
Heatstroke, the most extreme of the three heat disor-
ders, is characterized by a core temperature of 40°C
(104°F) or higher, cessation of sweating, hot and dry
skin, rapid pulse and respiration, hypertension, and
confusion or unconsciousness. Ifleft untreated, heat-
stroke can lead to coma, then death. People experi-
encing symptoms of heat-related injury should be
taken to a shady area, cooled with by whatever means
available, and if conscious given nonalcoholic bever-
ages to drink. Medical assistance should be sought.
To reduce the risk of developing heat disorders, a
person should drink enough fluid to try to match
that which is lost through sweating, avoid extreme
heat, and reduce the intensity of activity in hot
weather. Because children are less resistant to the
adverse effects of heat during exercise, special atten-
tion should be given to protect them when they
exercise in the heat and to provide them with extra
fluids to drink. .
Stresses associated with exercising in the ex-
treme cold are generally less severe. For most situa-
tions, the problems associated with cold stress can be
eliminated by adequate clothing. Still, cold stress can
induce a number of changes in the physiologic re-
sponse to exercise (Doubt 1991; Jacobs, Martineau,

74

Vallerand 1994; Shephard 1993). These include the
increased generation of body heat by vigorous activ-
ity and shivering, increased production of catechola-
mines, vasoconstriction in both the cutaneous and
nonactive skeletal muscle beds to provide insulation
for the body’s core, increased lactate production, and
a higher oxygen uptake for the same work (Doubt
1991). For the same absolute temperature, exposure
to cold water is substantially more stressful than
exposure to cold air because the heat transfer in
water is about 25 times greater than in air (Toner and
McArdle 1988). Because the ratio of surface area to
mass is higher in children than in adults, children
lose heat at a faster rate when exposed to the same
cold stress. The elderly tend to have a reduced
response of generating body heat and are thus more
susceptible to cold stress.

Altitude also affects the body’s physiologic re-
sponses to exercise. As altitude increases, barometric
pressure decreases, and the partial pressure of inhaled
oxygen is decreased proportionally. A decreased par-
tial pressure of oxygen reduces the driving force to
unload oxygen from the air to the blood and from the
blood to the muscle, thereby compromising oxygen
delivery (Fulco and Cymerman 1988). VO,max is
significantly reduced at altitudes greater than 1,500
meters. This effect impairs the performance of endur-
ance activities. The body makes both short-term and
long-term adaptations to altitude exposure that en-
able it to at least partially adapt to this imposed stress.
Because oxygen delivery is the primary concern, the
initial adaptation that occurs within the first 24 hours
of exposure to altitude is an increased cardiac output
both at rest and during submaximal exercise. Ventila-
tory volumesare also increased. An ensuing reduction
in plasma volume increases the concentration of red
blood cells (hemoconcentration), thus providing more
oxygen molecules per unit of blood (Grover, Weil,
Reeves 1986). Over several weeks, the red blood cell
mass is increased through stimulation of the bone
marrow by the hormone erythropoietin.

Exercising vigorously outdoors when air qual-
ity is poor can also produce adverse physiologic
responses. In addition to decreased tolerance for
exercise, direct respiratory effects include increased
airway reactivity and potential exposure to harmful
vapors and airborne dusts, toxins, and pollens
(Wilmore and Costill 1994).
Physiologic Responses and Long-Term Adaptations to Exercise

Effects of Age

When absolute values are scaled to account for
differences in body size, most differences in physi-
ologic function between children and adults dis-
appear. The exceptions are notable. For the same
absolute rate of work on a cycle ergometer, chil-
dren will have approximately the same metabolic
cost, or VO, demands, but they meet those demands
differently. Because children have smaller hearts,
their stroke volume is lower than that for adults for
the same rate of work. Heart rate is increased to
compensate for the lower stroke volume; but be-
cause this increase is generally inadequate, cardiac
output is slightly lower (Bar-Or 1983). The AO,
difference is therefore increased to compensate for
the lower cardiac output to achieve the same VO,,.
The VO,max, expressed in liters per minute, in-
creases during the ages of 6-18 years for boys and
6-14 years for girls (Figure 3-4) before it reaches a
plateau (Krahenbuhl, Skinner, Kohrt 1985). When
expressed relative to body weight (milliliters per
kilogram per minute),VO,max remains fairly stable
for boys from 6-18 years of age but decreases

steadily for girls during those years (Figure 3-4)
(Krahenbuhl, Skinner, Kohrt 1985). Most likely,
different patterns of physical activity contribute to
this variation because the difference in aerobic
capacity between elite female endurance athletes
and elite male endurance athletes is substantially
less than the difference between boys and girls in
general (e.g., 10 percent vs. 25 percent) (Wilmore
and Costill 1994).

The deterioration of physiologic function with
aging is almost identical to the change in function
that generally accompanies inactivity. Maximal heart
rate and maximal stroke volume are decreased in
older adults; maximal cardiac output is thus de-
creased, which results ina VO,max lower than that
of a young adult (Raven and Mitchell 1980). The
decline inVO,max approximates 0.40 to 0.50 milli-
liters per kilogram per minute per year in men,
according to data from cross-sectional studies; this
rate of decline is less in women (Buskirk and
Hodgson 1987). Through training, both older men
and women can increase their VO,max values by
approximately the same percentage as those seen

 

Figure 3-4, Changes in VO, max with increasing age from 6 to 18 years of age in boys and girls*

 

 

 

 

 

4.0 - i 60
_35- Boys, mi/kg ad z
& ead r- 50 =
E - . 7 x,
> 3.0- Girls, ml/kg o” 3
£ ee 2
= o* r~ 40 g
x 2.5 5 on oS
x oc” . &
> 20-7 ee Boys, min 30 ¢
g oor" eseerct7TT 5
= 15- ae eee ee ee orn Girls, L/min &
= i t- 20 3
E 1.0 4pene22--7 =
£ = eS
S . . . 10 2.
> 0.54 Values are expressed in both liters per minute and Ss

relative to body weight (milliliters per kilogram per minute).

0.0 1 1 1 0

6 7 8 9 10 11 12 13 14 15 16 17 18

Age (years)

Data were taken from Krahenbuhl GS, Skinner JS, Kohrt WM 1985

and Bar-Or O 1983.

 

75