MVS 110
Exercise Physiology
Page 1
READING #6
EVALUATING ENERGY-GENERATING CAPACITIES
Introduction
We all possess the capability for anaerobic and aerobic energy metabolism, although the capacity for each
varies considerably among individuals. These differences underlie the concept of individual differences. A
person’s capacity for energy transfer depends largely on the exercise mode used for training and evaluation.
A high aerobic power in running does not assure a similar aerobic power when activating different muscle
groups as in swimming and rowing. This disparity represents an example of specificity of metabolic capacity.
On the other hand, some individuals with high aerobic power in one form of exercise also possess an above
average aerobic power in other diverse activities. This illustrates the generality of metabolic capacity. For the
most part, more specificity exists than generality in metabolic and physiologic function.
Lecture Objectives
 Explain specificity as it applies to exercise.
 Describe procedures to administer two practical “field tests” to evaluate power output capacity of the
high-energy intramuscular phosphates (immediate energy system).
 Describe a commonly used test to evaluate the power output capacity of the short-term energy system.
 Define maximal oxygen uptake (VO2max), including the physiological significance of this measure.
 Describe a graded exercise test.
 List criteria that indicate when a person reaches a “true” VO2max during a graded exercise test.
 Explain how each of the following affect maximal oxygen uptake: (1) mode of exercise, (2) heredity, (3)
state of training, (4) gender, (5) body composition, and (6) age.
Overview of Energy Transfer Capacity During Exercise
The immediate and short-term energy
systems mainly power all-out exercise for
up to 2-minutes duration. Both systems
operate anaerobically because their
transfer of chemical energy does not
require oxygen (refer to lecture 6.)
Generally, fast movements at a given
speed place great reliance on anaerobic
energy transfer. Figure 1 shows the
involvement of anaerobic and aerobic
energy transfer systems for different
durations of all-out exercise.
At the initiation of either high- or lowspeed movements, intramuscular ATP,
phosphagens, and PCr provide
nonaerobic energy for muscle action.
After the first few seconds of movement,
the glycolytic energy system provides an
increasingly greater proportion of the
Figure 1. Three energy systems and their percentage contribution (Yaxis) to total energy output during all-out exercise of different durations
(X-axis).
MVS 110
Exercise Physiology
Page 2
total energy. For exercise to continue (at a lower intensity), a progressively greater demand is placed on the
aerobic metabolic pathways of ATP resynthesis.
Some activities require the capacity of more than one energy transfer system, whereas other activities rely
predominately on a single system. However, all activities activate each energy system to some degree,
depending on exercise intensity and duration. Of course, the greater demand for anaerobic energy transfer
occurs for higher intensity, shorter duration activities.
Anaerobic Energy: The Immediate and Short-Term Energy Systems
Evaluation of the Immediate Energy System
Performance tests that rely on maximal activation of the intramuscular ATP-PCr energy reserves have
been developed as “field tests” to evaluate the immediate energy transfer system. These maximal effort
performances, generally referred to as power tests, evaluate the time-rate of doing work (i.e., work
accomplished per unit time). The following formula (where, F equals force generated, D equals distance
through which the force moves, and T equals exercise duration) computes power output:
Watts represents a common expression of power
One watt equals 0.73756 ft-lb•sec–1 or 6.12 kg-m•min–1
STAIR-SPRINTING POWER TESTS
Researchers have measured short-term power by sprinting up a flight of stairs. Figure 2 shows a subject
running up a staircase as fast as possible taking three steps at a time. The external work performed equals the
total vertical distance the body rises up the stairs; for six stairs this distance usually equals about 1.05 meters.
Figure 2. Stair-sprinting power test. The subject begins at point A and runs as fast as
possible up a flight of stairs, taking 3 steps at a time. Electric switch mats placed on the
steps record the time to cover the distance between stair 3 and 9 to the nearest 0.01 s.
Power output equals the product of the subject’s mass (F) and vertical distance covered
(D), divided by the time (T).
The power output for a 65-kg woman who traverses six steps in 0.52 seconds computes as follows:
MVS 110
Exercise Physiology
Page 3
F = 65 kg; D = 1.05 m; T = 0.52 s
Power = [65 kg x 1.05 m] ÷ 52 s
Power = 131.3 kg-m•s-1 (1287 watts)
Because body mass greatly influences the power-output score in stair sprinting, a heavier person
necessarily generates greater power at the same speed as a lighter person who covers the same vertical
distance. Because of the influence of body mass, use caution in interpreting differences in stair-sprinting
power scores and making inferences about individual differences in ATP-PCr energy capacity. The test may
be better suited for evaluating individuals of similar body mass, or the same people before and after a specific
training regimen.
FOR YOUR INFORMATION
INTERCHANGEABLE EXPRESSIONS FOR ENERGY AND WORK
1 foot-pound (ft-lb) = 0.13825 kilogram-meters (kg-m)
1 kg-m = 7.233 ft-lb = 9.8066 joules
1 kilocalorie (kcal) = 3.0874 ft-lb = 426.85 kg-m = 4.186 kilojoules (kJ)
1 Joule (J) = 1 Newton-meter (Nm)
1 kilojoule (kJ) = 1000 J = 0.23889 kcal
Jumping-Power Tests
For years, physical fitness test batteries have included jumping tests such as the jump-and-reach test or a
standing broad jump. The jump-and-reach test score equals the difference between a person’s standing reach
and the maximum jump-and-touch height. For the broad jump, the score represents the horizontal distance
covered in a leap from a semicrouched position. Although both tests purport to measure leg power, they
probably fail to achieve this goal. For one thing, with jump tests, power generated in propelling the body from
the crouched position occurs only in the time the feet contact the floor's surface. This brief period cannot
sufficiently evaluate a person’s ATP and PCr power capacity.
OTHER POWER TESTS
A 6 to 8-second performance involving all-out exercise measures the person’s capacity for immediate
power from the intramuscular high-energy phosphates (refer to Figure 1). Examples of other such tests
include sprint running or cycling, shuttle runs and more localized movements such as arm cranking or
simulated stair climbing, rowing, or skiing. In the popular Quebec 10-second test of leg cycling power, the
subject performs two all-out 10-second rides at a frictional resistance equal to 0.09 kg per kg of body mass,
with 10- minutes rest between exercise bouts. Exercise begins by pedaling as fast as possible as the friction
load is applied and continues all-out for 10 seconds. Performance represents the average of the two tests
reported in peak joules per kg of body weight, and total joules per kg of body weight.
Power tests may be used to show changes in an athlete’s performance with specific training. Such tests also
serve as an excellent means for self-testing and motivation, and provide the actual movement-specific exercise
for training the immediate energy system. Many football teams, for example, routinely use the 40-yard dash
as a criterion to evaluate a player’s speed. Although many types of “speed” need to be evaluated in football,
the 40-yard scores may provide useful information for player evaluation. It should be emphasized that
research needs to establish how 40-yard speed in a straight line relates to overall football ability for players at
MVS 110
Exercise Physiology
Page 4
similar positions. A run test of shorter duration (up to 20 yd), or one with frequent changes in direction may
be an equal or more suitable performance measure.
Several physiologic and biochemical measures, in addition to exercise performance, can estimate the
energy-generating capacity of the immediate energy system. These include:
 Size of the intramuscular ATP-PCr pool
 ATP and PCr depletion rates from all-out exercise of short duration
 Magnitude of the oxygen deficit calculated from the oxygen uptake curve
 Magnitude of the alactic (fast component) portion of recovery oxygen uptake
Evaluation of the Short-Term Energy System
As displayed in Figure 1, the anaerobic reactions of glycolysis (short-term energy system) generate
increasingly greater energy for ATP resynthesis when all-out exercise continues longer than a few seconds.
This does not mean that aerobic metabolism remains unimportant at this stage of exercise, or that the oxygenconsuming reactions have not been “switched-on.” To the contrary, Figure 2 reveals an increase occurs in
aerobic energy contribution very early in exercise. However, the energy requirement in all-out exercise
significantly exceeds energy generated by hydrogen's oxidation in the respiratory chain. This means that the
anaerobic reactions of glycolysis predominate, with large quantities of lactic acid accumulating within the
active muscle and ultimately appearing in the blood.
Unlike tests for maximal oxygen uptake, no specific criteria exist to indicate that a person has reached a
maximal anaerobic effort. In fact, one's level of self-motivation, including external factors in the test
environment, likely influences the test score. Researchers most commonly use the level of blood lactate to
indicate the degree of activation of the short-term energy system.
Performance Tests of Glycolytic Power
Activities requiring substantial activation of the short-term energy system demand maximal work for up
to three minutes. All-out runs and cycling exercise have usually been used, although weight lifting of a certain
percentage of maximum and shuttle runs are also used. Because age, sex, skill, motivation, and body size
affect maximal performance,
Figure 3. Contribution of each of the energy systems to the total work accomplished
difficulty exists selecting a
in three tests of short-duration.
suitable criterion test for
developing standards for
glycolytic energy capacity. A
test that maximally uses only leg
muscles cannot adequately
assess short-term anaerobic
capacity for upper-body exercise
like rowing. Thus, the
performance test must be
similar to the activity for which
the energy capacity is being
evaluated. Thus, the actual
activity serves as the test.
Figure 3 presents the percent
contribution of each metabolic
pathway during three different
all-out cycle ergometer tests.
The results are shown as a
10 sec
30 sec
Short-duration tests
90 sec
MVS 110
Exercise Physiology
Page 5
percent of the total work output. Note the progressive change in the percent contribution of each of the
energy systems to the total work output as duration increases.
Blood Lactate Levels
Blood lactate levels remain relatively low during steady-rate exercise up to about 55% of the VO2max.
Thereafter, blood lactate begins to accumulate, with a precipitous increase noted in the region of the VO2max.
Glycogen Depletion
Because the short-term energy system largely depends on glycogen stored in the specific muscles activated
by exercise, these muscles' pattern of glycogen depletion provides an indication of the contribution of
glycolysis to exercise.
With steady-rate exercise at about 30% of VO2max, a considerable reserve of muscle glycogen remains, even
after cycling for 180 minutes. Because relatively light exercise relies mainly on a low level of aerobic
metabolism, large quantities of fatty acids provide energy with only moderate use of stored glycogen. The
most rapid and pronounced glycogen depletion occurs at the two heaviest workloads. This makes sense from
a metabolic standpoint because glycogen provides the only stored nutrient for anaerobic ATP resynthesis.
Thus, glycogen has high priority in the “metabolic mill” during strenuous exercise.
Changes in total muscle glycogen may not give a precise indication of the degree of glycogen breakdown
in specific muscle fibers, however. Depending on exercise intensity, glycogen depletion occurs selectively in
either fast- or slow-twitch fibers. For example, during all-out one-minute sprints on a bicycle ergometer,
activation of the fast-twitch fibers provides the predominant power for the exercise. Glycogen content in these
fibers becomes almost totally depleted because of the sprint's anaerobic nature. In contrast, slow-twitch fibers
become glycogen-depleted early during moderate to heavy prolonged aerobic exercise. Glycogen utilization
(and depletion) mainly in specific muscle type fibers makes it difficult to evaluate the degree of glycolytic
activation from changes in a muscle’s total glycogen content before and after exercise.
Anaerobic Energy Transfer Capacity
Differences in training level, capacity to buffer acid metabolites produced in heavy exercise, and
motivation contribute to individual differences in capacity to generate short-term anaerobic energy.
Effects of Training
Short-term supermaximal exercise on a bicycle ergometer in trained subjects always produces higher
levels of blood and muscle lactic acid, and greater muscle glycogen depletion. For all subjects, better
performances usually associate with higher blood lactate levels. These results support the belief that training
for brief, all-out exercise enhances the glycolytic system's capacity to generate energy.
Buffering of Acid Metabolites
Lactic acid accumulates when anaerobic energy transfer predominates. This causes an increase in the
muscle's acidity, negatively affecting the intracellular environment. The deleterious intracellular alterations
during anaerobic exercise have caused speculation that anaerobic training might enhance short-term energy
capacity by increasing the body’s buffering reserve to enable greater lactic acid production through more
effective buffering.
MVS 110
Exercise Physiology
Page 6
Motivation
Individuals with greater “pain tolerance,” “toughness,” or ability to “push” beyond the discomforts of
fatiguing exercise definitely accomplish more anaerobic work. These people usually generate greater levels of
blood lactate and glycogen depletion; they also score higher on tests of short-term energy capacity. Although
difficult to categorize and quantify, motivation plays a key role in superior performance at all levels of
competition.
Aerobic Energy: The Long-Term Energy System
The data in Figure 4 illustrate that persons who engage in sports that require sustained, high-intensity
exercise (i.e., endurance) generally possess a large aerobic energy transfer capacity. Men and women who
compete in distance running, swimming, bicycling, and cross-country skiing generally record the highest
maximal oxygen uptakes. These athletes have almost twice the aerobic capacity as sedentary individuals. This
does not mean that only VO2max determines endurance exercise capacity. Other factors, especially those at the
muscle level such as capillary density, enzymes, and fiber type, strongly influence the capacity to sustain a
high percentage of VO2max. However, the VO2max does provide useful information about the capacity of the
long-term energy system. Attainment of VO2max requires integration of ventilatory, cardiovascular, and
neuromuscular systems; this
gives significant physiologic
“meaning” to this metabolic
measure. For these reasons,
VO2max represents a fundamental
measure in exercise physiology
and often serves as the standard
against which to compare
performance estimates of aerobic
capacity and endurance fitness.
Measurement of
Maximal Oxygen
Uptake
Tests for VO2max use exercise
tasks that activate large muscle
groups with sufficient intensity
and duration to engage maximal
aerobic energy transfer. Exercise
includes treadmill walking or
running, bench stepping, or
cycling. VO2max has also been
measured during free, tethered,
and flume swimming and swimbench ergometry, and simulated
rowing, skiing, stair climbing, as
well as ice skating and arm-crank
exercise. Considerable research
effort has been directed toward
(1) development and
standardization of tests for
VO2max, and (2) establishment of
Figure 4. Maximal oxygen uptake of male and female Olympic-caliber athletes in
different sport categories compared to healthy sedentary subjects.
MVS 110
Exercise Physiology
Page 7
norms related to age, sex, state of training, and body composition.
Criteria for VO2max
A leveling-off, or peaking-over, in oxygen uptake during increasing exercise intensity signifies attainment
of maximum capacity for aerobic metabolism (i.e., a “true” VO2max). When this generally accepted criterion is
not met, or local muscle fatigue in the arms or legs rather than central circulatory dynamics limits test
performance, the term “peak oxygen uptake” (VO2peak) usually describes the highest oxygen uptake value
during the test.
Tests of Aerobic Power
Numerous tests have been devised and standardized to measure VO2max. These test performances should
be independent of muscle strength, speed, body size, and skill, with the exception of specialized swimming,
rowing, and ice skating tests.
The VO2max test may require a continuous 3- to 5-minute “supermaximal” effort, but it usually consists of
increments in exercise intensity (referred to as a graded exercise test or GXT) until the subject stops. Some
researchers have imprecisely termed this end point “exhaustion,” but it should be kept in mind that the
subject terminates the test (for whatever reason). A variety of psychological or motivational factors can
influence this decision, and it may not reflect true physiologic exhaustion. It can take considerable urging and
prodding to get subjects to the point of acceptable criteria for VO2max, particularly individuals unaccustomed
to producing maximal exercise. Children and adults encounter particular difficulty if they have had little
prior experience performing strenuous exercise. Practical experience has shown that attaining a plateau in
oxygen uptake during the VO2max test requires high motivation and a relatively large anaerobic component.
Factors That Affect Maximal Oxygen Uptake
Of the many factors influencing VO2max, the most important include mode of exercise and the person’s
heredity, training state, sex, body composition, and age.
Mode of Exercise
Variations in VO2max during different modes of exercise reflect the quantity of muscle mass activated
during the performance. In experiments that determined VO2max on the same subjects during exercise,
treadmill exercise produced the highest values. Bench stepping, however, has generated VO2max scores
identical to treadmill values and significantly higher than values on a bicycle ergometer. With arm-crank
exercise, aerobic capacity reaches only about 70% of one’s treadmill VO2max.
The treadmill represents the laboratory apparatus of choice for determining VO2max in healthy subjects.
The treadmill provides easy quantification and regulation of exercise intensity. Compared with other forms of
exercise, subjects achieve one or more of the criteria for establishing VO2max more easily on the treadmill.
Bench stepping or bicycle exercise is suitable alternatives under non-laboratory “field” conditions.
Heredity
A question frequently raised concerns the relative contribution of natural endowment to physiologic
function and exercise performance. For example, to what extent does heredity determine extremely high
aerobic capacities of the endurance athletes? Do these exceptionally high levels of functional capacity reflect
more than the training effect? Although the answer remains incomplete, some researchers have focused on
the question of how genetic variability accounts for differences between individuals in physiologic and
metabolic capacity.
MVS 110
Exercise Physiology
Page 8
Studies were made of 15 pairs of identical twins (with the same heredity since they came from the same
fertilized egg) and 15 pairs of fraternal twins (who do not differ from ordinary siblings because they result
from separate fertilization of two eggs) raised in the same city by parents with similar socioeconomic
backgrounds. The researchers concluded that heredity alone accounted for up to 93% of the observed
differences in aerobic capacity as measured by the VO2max. In addition, genetic determination accounted for
81% of the capacity of the short-term glycolytic energy system and 86% of maximum heart rate. Subsequent
investigations of larger groups of brothers, fraternal twins, and identical twins indicate a significant but much
smaller effect (20–30% for VO2max, 50% for maximum heart rate, and 70% for physical working capacity) of
inherited factors on aerobic capacity and endurance performance. Genetic makeup plays such a prominent
role in determining training response that it is nearly impossible to predict a specific individual’s response to
a given training stimulus.
Training State
VO2max scores must be evaluated relative to the person’s state of training at the time of measurement.
Improvements in aerobic capacity with training generally range between 6 and 20%, although increases have
been reported as high as 50% above pretraining levels.
Gender
VO2max values (mL•kg-1•min-1) for women typically average 15 to 30% below scores for men. Even among
trained athletes, this difference ranges between 15 and 20%. Such differences increase considerably when
expressing the VO2max as an absolute value (L•min–1) rather than relative to body mass (mL•kg-1•min-1).
Between world-class male and female cross-country skiers, for example, a 43% lower VO2max value for women
(6.54 vs. 3.75 L•min–1) decreased to 15% (83.8 v 71.2 mL•kg -1•min-1) using the athletes' body mass in the ratio
expression of VO2max.
Sex difference in VO2max has generally been attributed to differences in body composition and hemoglobin
content. Untrained young adult women generally possess about 25% body fat, whereas the corresponding
value for men averages 15%. Although trained athletes have a lower percentage of fat, trained women still
possess significantly more body fat than male counterparts. Thus, the male generally generates more total
aerobic energy simply because he possesses a relatively large muscle mass and less fat than the female.
Probably due to their higher level of testosterone, men show a 10 to 14% greater concentration of
hemoglobin. This difference in the blood's oxygen-carrying capacity potentially enables males to circulate
more oxygen during exercise and gives them a slight edge in aerobic capacity.
Although lower body fat and higher hemoglobin provide the male with some advantage in aerobic power,
we must look for other factors to fully explain the disparity between the sexes. Differences in normal physical
activity level between an “average” male and “average” female provide a possible explanation. Perhaps
considerably less opportunities exist for women to become as physically active as men due to social structure
and constraints. In fact, even among prepubertal children, boys become more active in daily life than their
female counterparts.
Age
Changes in VO2max relate to chronological age.
Although limitations exist in drawing inferences
form cross-sectional studies of different people at
different ages, the available data provide insight
into the possible effects of aging on physiologic
function. Figure 5 shows the VO2max as a
function of age. Note the dramatic increases
Figure 5. General trend for maximal oxygen uptake with age
and level of activity in males and females.
MVS 110
Exercise Physiology
Page 9
during the growth years. Longitudinal studies (measuring the same people over a prolonged period) of
children’s aerobic capacity show that absolute VO2max increases from about 1.0 L•min-1 at age 6 years to 3.2
L•min-1 at age 16 years. VO2max in girls peaks at about age 14 and declines thereafter. At age 14, the
differences in VO2max between boys and girls approximate 25%, with the spread reaching 50% by age 16. Note
also the decline in VO2max with increasing age. Beyond age 25, VO2max declines steadily at about 1% per year,
so that by age 55 it averages 27% below values reported for 20 year olds.
One’s habitual level of physical activity through middle age determines changes in aerobic capacity to a
greater extent than chronological age.
Body Composition
Differences in body mass explain roughly 70% of the differences in VO2max scores among individuals.
Thus, meaningful comparisons of exercise performance or the absolute value (L•min-1) for VO2max become
difficult among individuals who differ in body size or body composition. This has led to the common practice
of expressing oxygen uptake in terms of these components – either related to body surface area (BSA), body
mass, fat-free body mass (FFM), or limb volume.