THE STEADY-STATE MODEL OF BIOENERGETICS FAILS TO ACCU- RATELY DESCRIBE THE METABO- LISM FOR HIGH-INTENSITY POWER. THIS ARTICLE REEXAMINES THE ROLE OF PHOSPHOCREATINE, LACTATE PRODUCTION, AND THE IMPORTANCE OF AEROBIC METAB- OLISM DURING SHORT-TERM HIGH- INTENSITY POWER PERFORMANCE. METABOLIC AND MECHANICAL TESTS OF HIGH-INTENSITY POWER HAVE EVOLVED IN THE PAST 40 YEARS. THE AUTHORS COMPARED THE MAXIMAL ACCUMULATED OXYGEN-DEFICIT MODEL VERSUS THE CRITICAL POWER MODEL AND SUMMARIZED THE RECENTLY DEVELOPED 3-MINUTE ALL-OUT EXERCISE TEST (3 MT). THE 3 MT OFFERS THE STRENGTH AND CONDITIONING PROFESSIONAL A SIMPLE METHOD OF ESTIMATING AN ATHLETE’S TOLERANCE TO HIGH-INTENSITY POWER EXERCISE.

1. High-Intensity Exercise
Tolerance: An Update
on Bioenergetics and
Assessment
Robert W. Pettitt, PhD, ATC, CSCS*D and Ida E. Clark, MS
Viola Holbrook Human Performance Laboratory, Minnesota State University, Mankato, Minnesota
A B S T R A C T
THE STEADY-STATE MODEL OF
BIOENERGETICS FAILS TO ACCU-
RATELY DESCRIBE THE METABO-
LISM FOR HIGH-INTENSITY POWER.
THIS ARTICLE REEXAMINES THE
ROLE OF PHOSPHOCREATINE,
LACTATE PRODUCTION, AND THE
IMPORTANCE OF AEROBIC METAB-
OLISM DURING SHORT-TERM HIGH-
INTENSITY POWER PERFORMANCE.
METABOLIC AND MECHANICAL
TESTS OF HIGH-INTENSITY POWER
HAVE EVOLVED IN THE PAST 40
YEARS. THE AUTHORS COMPARED
THE MAXIMAL ACCUMULATED
OXYGEN-DEFICIT MODEL VERSUS
THE CRITICAL POWER MODEL AND
SUMMARIZED THE RECENTLY
DEVELOPED 3-MINUTE ALL-OUT
EXERCISE TEST (3 MT). THE 3 MT
OFFERS THE STRENGTH AND
CONDITIONING PROFESSIONAL A
SIMPLE METHOD OF ESTIMATING
AN ATHLETE’S TOLERANCE TO
HIGH-INTENSITY POWER EXERCISE.
INTRODUCTION
I
n sport performance, power is
defined by the total work performed
relative to time, whereby more work
performed in less time is characterized
by greater power. Bioenergetics involves
studying the underlying chemical pro-
cesses responsible for mechanical work
(9). Exercise physiology textbooks often
apply the steady-state model of bioener-
getics to sustained high-intensity power
performances. In reality, the steady-state
model does not reflect the bioenergetics
of high-intensity power accurately (12).
The steady-state model presumes that
the body will meet energy demand aer-
obically within 2–3 minutes. Specifi-
cally, oxygen uptake (V̇O2) will rise
and reach a steady state. As early as
1972, however, researchers determined
that V̇O2 does not achieve steady state at
higher intensities (43). Indeed, the time
course of V̇O2, as it pertains to different
exercising intensities, sheds new light on
the complex topic of anaerobic capacity
and short-term high-intensity power
(7). Moreover, the methods for evaluat-
ing physiological parameters contribut-
ing to short-term high-intensity power
have evolved. The purpose of this article
is to discuss new insights of the bioen-
ergetics for short-term high-intensity
power, metabolic and mechanical mea-
sures of short-term power, and applica-
tions for those measurements.
NEW INSIGHTS ON THE
BIOENERGETICS OF
HIGH-INTENSITY EXERCISE
The bioenergetics of short-term high-
intensity exercise can be divided into 3
systems: the phosphocreatine (PCr)
system, the fast glycolytic or lactate
formation system, and the slow glyco-
lytic or aerobic system. Time ranges of
0–10, 10–30, and beyond 30 seconds
are viewed typically for the utilization
of PCr, fast glycolytic, and slow glyco-
lytic systems, respectively (9). These
time ranges are in no way appropriate
for high-intensity power exercise.
The first primary substrate for muscle
adenosine triphosphate (ATP) produc-
tion is PCr. The steady-state model
presumes that PCr utilization is limited
approximately to the initial 10 seconds
of exercise. For exercise performances
beyond 10 seconds, the fast and slow
glycolytic pathways are viewed as
exclusive mechanisms for supplying
ATP (9). The findings by Jones et al.
(24) indicate that PCr can contribute
progressively to ATP production during
sustained high-intensity exercise. Their
results indicated that PCr was used well
beyond 5 and up to 18-minutes of exer-
cise. Such an observation is explained
by spatial recruitment and the size
principle (15). The size principle states
that when smaller low-threshold motor
K E Y W O R D S :
anaerobic capacity; critical power;
lactate; oxygen uptake;
phosphocreatine
Copyright Ó National Strength and Conditioning Association Strength and Conditioning Journal | www.nsca-scj.com 11

2. units fail to meet power demands, lar-
ger higher-threshold motor units are
recruited.
Housh et al. (19) determined that con-
stant power output high-intensity exer-
cise evoked greater, time-dependent
motor-unit activation in comparison to
lower intensities. Progressive recruit-
ment of myofibers would help explain
why PCr utilization can occur well
beyond 10 seconds during high-intensity
exercise. Specifically, an initial motor-
unit pool will expend PCr within 10
seconds. Because additional fibers are
recruited to meet the constant power
output demands, those fibers will supply
ATP from PCr, resulting in a small but
continual depletion of PCr within the
whole muscle over a span of several mi-
nutes. Such a view of metabolism is dis-
tinct from the notion that the whole
muscle expends PCr within the first
10 seconds of a high-intensity bout.
The accumulation of blood lactate
during exercise has been viewed as
a byproduct associated with hydrogen
ion production (4). However, some lac-
tate yielded by fast glycolysis can be
oxidized readily within type I muscle
fibers (3). The prevailing view on lac-
tate production is that hydrogen ions
are sequestered rather than accumu-
lated, which serves to increase rather
than decrease muscle pH (9,36). In
other words, the acidosis or “burn”
associated with fatigue during high-
intensity exercise has nothing to do
with the formation of lactate (3). A
more likely mechanism for the declin-
ing pH during high-intensity exercise is
the hydrolysis of ATP (36), expressed
as follows:
H2O þ ATP/Hþ
þ ADP þ Pi þ E;
where the addition of water (H2O) sep-
arates ATP, H+
ion accumulation is
associated with a declining pH, ADP
is adenosine diphosphate, Pi is inorganic
phosphate, and E represents energy.
Rather than being viewed as a byproduct,
lactate is better viewed as an interme-
diate sugar, produced by fast glycolysis
(which yields 2 ATP) that can
subsequently enter mitochondria
through a monocarboxyl transporter
(MCT) and be oxidized in the Krebs’
cycle (which yields 36 ATP) (29).
Indeed, endurance athletes can oxidize
lactate at a higher percentage of V̇O2max
because they have more MCTreceptors
and oxidative enzymes (30). The lactate
threshold is therefore not predicated
on the absence or presence of sufficient
oxygen supply within muscle (5).
Rather, the accumulation of intramus-
cular lactate is predicated on the fail-
ure of mitochondria to supply ATP at
appropriate rates.
The steady-state model presumes that
V̇O2 will reach a constant level within
2–3 minutes of exercise onset. For inte-
nsities exceeding the lactate or gas
exchange threshold, V̇O2 exhibits a con-
tinual rise termed the V̇O2 slow compo-
nent (12). The slow component emerges
between 1.5 and 3 minutes of exercise
onset and primarily represents the
added oxygen costs associated with rec-
ruiting type II muscle fibers (7). High-
intensity power exercise can cause dra-
matic gains in the V̇O2 slow component
(e.g., in excess of 1 L of added oxygen)
(34), and modeling of bioenergetics in-
dicates that the slow glycolysis contrib-
utes more substantially to endeavors
that are deemed as “anaerobic.” For
example, aerobic metabolism could con-
tribute ;60–70% of the total energy for
sprinting 800 m (;2 minutes), because
of how the V̇O2 slow component rises
during intense exercise.
TRADITIONAL ANAEROBIC
CAPACITY TESTING
One of the more popular long-standing
tests of anaerobic capacity is the Wing-
ate anaerobic power test. The Wingate
test is a 30-second all-out exercise test
using a fixed load, typically 7.5–10%
body mass (recreational or athletic sub-
ject, respectively), whereby flywheel
velocity and cycling power are mea-
sured relative to time (20). Most com-
mercial software programs for the
Wingate test calculate a peak power
(Pmax) based on the average of the high-
est 5-second sampling bin along with
a fatigue index of Pmax relative to the
lowest 5-second sampling bin. Concern
on the validity of the fatigue index val-
ues from the Wingate test has been
raised. Specifically, the 30-second dura-
tion of the Wingate test has been
viewed as too short of a duration to
estimate depletion of anaerobic capac-
ity, where 1 group (11) has reported
that 90 seconds (i.e., 3 times the dura-
tion of the Wingate test) was insufficient
to effectively deplete the anaerobic
capacity. Thus, using the fatigue index
from the Wingate test to rank-order the
anaerobic capacities for a group of ath-
letes is an inaccurate practice.
In the late 1980s, the maximal accu-
mulated oxygen deficit (MAOD) test
emerged as an index of anaerobic
capacity (25). The test involved extrap-
olating expected V̇O2 demands for
a supramaximal bout, or bouts, based
on a series of preliminary constant-load
submaximal bouts (e.g., 40, 50, and 60%
of power evoking V̇O2max in a graded
exercise test). Subjects were asked to
subsequently perform one or more
exhaustive supramaximal bouts (37).
Hypothetically, an identical MAOD
value would be yielded, regardless of
the power output and time to exhaus-
tion (Figure 1) (40). The MOAD
method was advocated to distinguish
the high-power capacity of untrained,
sprint-trained, and endurance-trained
subjects (14).
The MAOD protocol was attractive, in
principle, but failed to gain traction in
the sport science community (28). The
shortcomings of the MOAD protocol
were as follows: First, energy demand
for the MOAD protocol is influenced by
the fact that the V̇O2-power relationship
below and above the gas exchange
threshold is not consistent. Thus, the
regression equation for estimating V̇O2
demand for supramaximal bouts is
dependent on the number and intensity
of bouts (i.e., the more bouts above gas
exchange threshold used in the equa-
tion would inflate the extrapolated
V̇O2 demand) (1). Indeed, 1 group re-
ported that a minimum of 10 steady-
state bouts are needed to perform the
MOAD test validly (28). Clearly, having
to complete 10 steady-state bouts per
athlete is a time-consuming task.
High-Intensity Exercise Tolerance
VOLUME 35 | NUMBER 2 | APRIL 201312

3. Second, the time limit for constant-load
bouts carried out to exhaustion (Tlim) is
an unreliable measurement (42). Third,
the MOAD test yields units of measure-
ment for anaerobic capacity, in absolute
values (L) or values relative to body
mass (in milliliters per kilogram) that
can conceivably rank-order athletes
(37); however, the MOAD metric can-
not predict Tlim at given power outputs
or be used to prescribe exercise. Thus,
a mechanical measure of anaerobic
capacity is preferred.
MECHANICAL MEASURES OF
HIGH-INTENSITY POWER AND
CAPACITY
For nearly a century, we have appreci-
ated that the power and the Tlim (P-Tlim)
relationship is proportional such that
fatigue occurs earlier at higher intensi-
ties (16). Monod and Scherrer (26) are
credited as the first to link the P-Tlim
relationship to a finite limit of stored
energy within the muscle (i.e., anaerobic
capacity). The classic method of deter-
mining anaerobic capacity and critical
power (CP) was to conduct a series of
exhaustive bouts at different high-power
intensities (26,27). With power output
and Tlim data at 3 and preferably 4 dif-
ferent intensities known (17), CP and
the curvature constant abbreviated as
W9 (pronounced W-prime) could be
determined. Specifically, the P-Tlim rela-
tionship was modeled mathematically
as a regression of total work (y-axis)
and Tlim (x-axis) to yield:
Total work 5 ðCP$TlimÞ þ W 9; (1)
where total work is in joules, CP is in
watts and represents the slope, Tlim is
in seconds, and W9 is in joules and
represents the y-intercept (Linear-W
model). Whipp et al. (41) introduced
a subsequent iteration for the linear
model, whereby power output and
the inverse of Tlim were interpolated
to solve the W9 as the slope and the
CP as the y-intercept (Linear-P model).
With the Linear-P model, the equation
can be expressed as:
Power 5 ½W 9$ð1=TlimÞ þ CPŠ; (2)
The Linear-P model also can be
transformed to:
Power 5 ðW 9=TlimÞ þ CP: (3)
Finally, the Tlim for a given power
output can be derived using:
Tlim 5 W 9=ðP 2 CPÞ: (4)
Figure 2 provides sample calculations
for a representative subject. Take notice
that the same power-Tlim data points
from Figure 1 were used to assist with
comparing the MOAD and CP models,
respectively. Directions for how to
construct the CP model using a Micro-
soft Excel spreadsheet (Microsoft Cor-
poration, Remond, WA) are published
elsewhere (31).
In accordance to Newton’s work-energy
theorem, the assessment of work, in
essence, is a measure of kinetic energy
required to complete that work. Thus,
W9 is better viewed as an energy reser-
voir and work performed at intensities
exceeding CP would deplete the W9 res-
ervoir in a time-dependent manner (31).
Concurrently, exercising above CP
evokes a time-dependent rate of metab-
olite accumulation associated with
fatigue (e.g., hydrogen ions) (22). Indeed,
the higher the intensity relative to CP,
the more rapid these metabolites accu-
mulate (24). Such mechanisms explain
Figure 1. An example of an individual’s maximal accumulated oxygen deficit (MAOD). Where an individual’s at a V̇O2max of 3.8 L min21
was 300 W (Wpeak), linear regression from V̇O2-power time points (open circles, panel A) were used to calculate supramaximal
V̇O2 demands, that is, demand at 105% Wpeak (315 W) is 4.0 L min21
5 (315 3 0.0113) + 0.4585 (panel B) and demand at
110% Wpeak (330 W) is 4.2 L min21
(panel C). The extrapolated accumulated V̇O2 demand for 3 minutes at 315 W is 12.0 L
and 2 minutes at 330 W is 8.4 L. If accumulated V̇O2 at 315 W was 10 L and at 330 W was 6.4 L, the O2 deficit would be
equivalent at 2 L (i.e., MAOD 5 2 L, a volume of energy met by the anaerobic capacity). Note that the measured V̇O2 during
the square-wave bouts at 105 and 110% Wpeak reached the V̇O2max of 3.8 L min21
.
Strength and Conditioning Journal | www.nsca-scj.com 13

4. the hyperbolic nature of the P-Tlim rela-
tionship (see Figure 2, right panel).
3-MINUTE ALL-OUT EXERCISE
TEST
Some studies (6,39) have emerged
demonstrating that a 3-minute all-out
exercise test (3 MT) can predict CP
and W9. The 3 MT is quite similar pro-
cedurally to the more familiar 30-second
Wingate test (20); however, the load
for the 3 MT is lower (;3–5% body
mass) in comparison to the Wingate
test (2,8). Early attempts of the 90-sec-
ond all-out exercise duration resulted
in inflated estimates of CP (11); how-
ever, 180 seconds was reported as
a suitable duration to identify CP, as
determined by evaluating V̇O2 and
blood lactate below and above CP
(6). Indeed, the first 150 seconds also
was sufficient to estimate the finite
capacity for work above CP or W9 (39).
Using a prescribed load for resistance
(i.e., 3–5% body mass dependent on
fitness level) (8), the subject pedals
all-out for a 3-minute duration. Data
are subsequently retrieved from the
ergometer, and the average power for
the last 30 seconds is calculated to
arrive at CP. Figure 3 illustrates a rep-
resentative subject’s data from the 3
MT. Take notice of how the values
derived from the 3 MT correspond to
the traditional method of estimating
CP, as shown in Figure 2.
The ability to obtain measures of CP
and W9 from a single all-out exercise test
has been projected to advance our
understanding of short-term high-inten-
sity power (33). From a practical stand-
point, the 3 MT offers better direction
for assessing and prescribing exercise for
short-term high-intensity power athletes
than methods such as measuring target
heart rates or relying on ratings of the
perceived exertion. Moreover, findings
Figure 2. An example of an individual’s power-time limit (P-Tlim) relationship as modeled from the following 4 exhaustive square-
wave bouts in the severe exercise domain: 303 W 5 300 seconds, 307 W 5 240 seconds, 315 W 5 180 seconds, and 330 W 5
120 seconds (Note: an example data point is shown on each graph). Tlim for each bout is inverted (1/time in seconds) and
plotted relative to power (panel A). Linear regression of the data reveals critical power (CP 5 285 W, the y-intercept) and
anaerobic capacity (W9 5 5,400 joules, the slope). Panel B illustrates the hyperbolic P-Tlim relationship where the y-axis
(power) can be solved using power 5 (W9/Tlim) + CP and the x-axis (Tlim) can be solved using Tlim 5 1/[(Power 2 CP)/W9].
Figure 3. An example of subject’s
3-minute all-out exercise test
(3 MT). Notice that all-out
power declines and levels out
at 285 W, identified by the
average power between 150
and 180 seconds, or critical
power (CP). The average
power for 150 seconds (P150s)
was 321 W, or +36 W relative
to CP, where +36 W 3 150
seconds 5 5,400 joules—
representing the maximal
amount of work above CP
that is supported by the
anaerobic capacity (W9).
High-Intensity Exercise Tolerance
VOLUME 35 | NUMBER 2 | APRIL 201314

5. from our laboratory indicate that 3 MT
may be a better method than the “gold
standard,” Linear-W or Linear-P techni-
ques, which are more labor- and time-
intensive and rely on a series of exhaus-
tive bouts of known variable Tlim meas-
ures (21). The 3 MT also removes the
debate between which equations to use
(i.e., the Linear-W or Linear W-models)
because estimates from the 3 MT, using
equations 1–4, are the same.
Our laboratory also validated the 3 MT
for running (32). The athlete simply
runs all-out for a period of 3 minutes,
and their displacement in relation to
time is monitored using global position-
ing sensor data or video technology.
The technique estimates the critical
speed and D9, running analogues to
CP and W9, and Tlim estimates can be
established for sustaining given running
speeds or distances [see reference (31)
for summary of equations]. The 3 MT
for running is actually easier to imple-
ment because the resistance load (mass
and gravitational force) is removed from
the computation. Moreover, athletes
can be tested in batches and entire
teams of athletes can be assessed within
a short period of time (e.g., ;dozen
athletes within an hour) (32).
When applying the CP model using the
3 MT for training purposes, we recom-
mend keeping a few issues in mind.
First, the parameter of CP or critical
speed has better test-retest reliability,
whereas W9 or D9 can be quite variable
(13,21). The CP and CV measure can
represent a good index of an athlete’s
maximal steady state (22). Second,
short-term high-intensity power perfor-
mance is a compilation of W9and more
importantly the athlete’s CP (31). Thus,
the total power measures from the 3
MT (i.e., average power for 150 seconds
and average power for 180 seconds) has
better test-retest reliability and is a better
metric for monitoring improved short-
term high-intensity power, than either
CP or W9 individually (21).
PRACTICAL APPLICATIONS FOR
THE CRITICAL POWER MODEL
With a better understanding of the pa-
rameters influencing short-term high-
intensity power, that is W9 and CP, we
can discuss the utility of the CP model
for training different types of athletes.
Take for instance a wrestler and a
5,000 m runner. The wrestler should
be focusing on W9 with less regard for
CP, using short more intense workouts
(e.g., 30 seconds–2 minutes). Conversely,
the 5,000 m runner should spend more
time training for longer durations to
stress both W9 and CP. Daniels (10)
recommended 2–5 minutes for the
interval training of distance runners.
The elegance of the 3 MT is that the test
is sensitive for detecting training-induced
adaptations in CP (38). An athlete spend-
ing more time training in the extreme
domain should expand the anaerobic
capacity resulting in an improved all-
out power performance along with
higher constant-power capacities for
shorter durations (i.e., increased W9).
Conversely, the athlete spending more
time training in the severe domain will
increase CP and either maintain or
slightly decrease W9 (35,38).
CONCLUSIONS
When exercise is exhaustive, from
either all-out effort or constant-load
bouts in the severe domain, additional
type II muscle fiber recruitment evokes
a continual expenditure of PCr (23),
a disproportionate rise in lactate from
stored muscle glycogen (34), and a con-
tinual rise of V̇O2 toward maximum
(18). Training with short-term high-
intensity power exercise stresses the
energy systems responsible for anaero-
bic capacity, represented by the
mechanical measure W9; but, such
intensities are too short in duration
for evoking attainment of V̇O2max
(10). Conversely, training between 2
and 5 minutes, in the severe domain,
can evoke improvements in CP or the
maximal steady state for lactate and
V̇O2. As such, endurance-trained ath-
letes can oxidize lactate at higher per-
centages of V̇O2max (30). The 3 MT is
recommended to strength and condi-
tioning professionals as an efficient
method for measuring and monitoring
bioenergetic adaptations to short-term
high-intensity power exercise (22).
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
Robert W.
Pettitt serves as
coordinator of the
exercise science
and exercise
physiology
programs at
Minnesota State
University.
Ida E. Clark
serves as an
assistant profes-
sor of exercise
science at Min-
nesota State
University.
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High-Intensity Exercise Tolerance
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