Eur J Appl Physiol (2002) 88: 146–151
DOI 10.1007/s00421-002-0706-1
O R I GI N A L A R T IC L E
G. Brickley Æ J. Doust Æ C.A. Williams
Physiological responses during exercise to exhaustion at critical power
Accepted: 24 July 2002 / Published online: 10 September 2002
Springer-Verlag 2002
Abstract Critical power (CP) is a theoretical construct
derived from a series of constant load tests to failure.
Many studies have examined the methodological limitations of deriving CP, but few studies have examined
the responses to exercise at CP in well-trained individuals. The purpose of the present study was to examine
the physiological responses to exercise at CP. Seven male
subjects [mean (SD) body mass 75.6 (6.4) kg, maximum
oxygen uptake 4.6 (0.7) l min–1] performed three constant load tests to derive CP. Subjects then exercised at
CP until volitional exhaustion. Heart rate, oxygen consumption and blood lactate concentration were measured throughout. Repeated measures analysis of
variance revealed significant differences over time in
heart rate 118 (24) to 177(5) beats min–1, oxygen consumption 3.7 (0.6) to 4.1 (0.5) l min–1 and blood lactate
concentration 4.3 (1.8) to 6.5 (2.0) mM. All seven subjects completed 20 min of exercise with the range of time
to failure at CP from 20 min 1 s to 40 min 37 s. Time to
failure and maximum oxygen consumption were significantly correlated (r=0.779, P<0.05). We conclude,
therefore, that CP does not represent a sustainable
steady-state intensity of exercise.
Keywords Critical power Æ Physiological response Æ
Exhaustion
G. Brickley (&)
Department of Sport and Exercise Science,
University of Brighton, Eastbourne, BN20 7SP, UK
E-mail: [email protected]
J. Doust
Department of Sport and Exercise Science,
University of Wales Aberystwyth, Ceredigion, SY23 3DA, UK
C.A. Williams
Department of Exercise and Sport Sciences,
University of Exeter, Exeter, EX1 2LU, UK
Introduction
Theoretical analysis of dynamic work capacity led
Monod and Scherrer (1965) to define the critical power
(CP) of a group of muscles or an individual muscle
rather vaguely as representing the maximum rate ‘‘it can
keep up for a very long time without fatigue’’ (p. 329).
Moritani et al. (1981) expanded the work of Monod and
Scherrer (1965) and applied it to cycle ergometry. The
examination of the boundaries between fatiguing and
non-fatiguing whole-body work has been subject to
extensive research since these early investigations. The
CP concept has been reviewed from a methodological
standpoint (Hill 1993) and with reference to whole-body
fatigue, with Walsh (2000) arguing that each tissue system has its own CP.
Few studies have analysed physiological responses at
CP. The time to exhaustion at CP has been reported to
range from 10–60 min, dependent in part on the mode of
exercise and the subject sample. Jenkins and Quigley
(1990) followed up their evaluation of CP with a time-tofailure test and had to manipulate the workload in order
for the individuals to sustain 30 min of exercise. Poole
et al. (1988) hypothesised that CP represented a
‘‘threshold’’ intensity that was slightly above steady state
and would cause a gradual rise in aerobic strain to the
level of maximum oxygen consumption (V_ O2max).
However, the authors found this not to be the case and
power needed to be increased by approximately 10% to
elicit V_ O2max. Recently, Bull et al. (2000) examined the
time to exhaustion and the heart rate responses at the
lowest estimate of CP based upon five different models.
They found that CP overestimates the power output that
can be maintained for over 60 min and found that in
those subjects who could sustain the exercise, heart rate
rose to 92% of maximum.
Housh et al. (1991) found that whilst there was a
relationship between CP and the onset of blood lactate
accumulation (OBLA) the mechanisms that determine CP
and OBLA were not identical. Jenkins and Quigley (1990)
147
found that when exercising at CP subjects could tolerate
blood lactate concentrations greater than 4 mM.
Identification of a meaningful marker of the intensity
at which exercise is sustainable is useful for training prescription or assessing change following an intervention. In
many training studies an improvement in V_ O2max has
been considered as the index of an improvement. However, in performance terms maintenance of a high percentage of maximal aerobic power may be more crucial.
Investigators have used a percentage of V_ O2max or indeed
a velocity at V_ O2max (vV_ O2max) (Billat et al. 1994) as a
performance measure. Thereafter, time to failure or
volitional exhaustion has been deemed to be a relevant
indicator of an athlete’s ability to sustain exercise.
If the CP concept has physiological meaning then it is
essential to examine the physiological responses of exercise at CP and a steady state would be expected. This study
tested the hypothesis that there would be no increase in:
blood lactate concentration, oxygen uptake and heart rate
during exercise at CP. Moreover it was hypothesised that
exercising at CP could be sustained until exogenous fuel
and fluid supply became critical, i.e. at least 60 min.
Methods
Subjects
Seven trained males (mean age 23.4 (3.1) years) familiar with cycle
ergometry volunteered as subjects for this study (Table 1). The
study was approved by the university ethics committee. All subjects
gave their written informed consent to the experimental procedures
after having the possible risks and benefits of participation explained to them. All subjects completed a pre-test health questionnaire.
Equipment
All exercise tests took place on a modified Monark 814E cycle
ergometer. The modifications included the addition of a fully
adjustable racing saddle. The bottom bracket was changed to
accommodate a conventional spline Shimano type bottom bracket.
To this bracket SRM power measuring cranks (SRM training
system, Schroberer Rad Messtechnik; Jülich, Germany) were fitted.
A sensor was mounted to the frame of the ergometer close to the
measuring device. This was connected to an SRM power control
device that was mounted on the handlebars of the ergometer.
Subjects used their own cycling shoes and cleats.
Heart rate was determined in all experiments using a telemetric
system (Sports Tester, Polar Electro Oy, Kempele, Finland). Heart
Table 1. Descriptive data from
the incremental exercise to
determine maximum minute
power (Pmax ) and maximum
oxygen uptake (V_ O2max), and
the cadence used during all tests
rate was monitored every second, and recorded on the SRM
Powercontrol III (SRM training system) data logger. Data were
downloaded at a later stage to a computer via an interface lead for
further analysis.
Open circuit spirometry was used to measure calorimetric
variables. During exercise subjects wore a nose clip and a large,
broad flanged rubber mouthpiece (Collins, Mass., USA) fitted to a
low-resistance breathing valve (University of Brighton, England).
The valve was of negligible volume (90 ml), of a T-shaped lightweight Perspex construction into which were mounted two rubber
flap one-way low-resistance valves (Mine Safety Appliances Ltd.).
It was connected to a 200-l Douglas bag from the expired side via a
0.5-m length of lightweight Falconia tubing of 3 cm bore (Baxter
Woodhouse and Taylor Ltd.). Subjects were not required to wear
the gas collection assembly at all times whilst exercising. The
breathing apparatus was always in place for at least 30 s prior to a
collection period to allow for familiarity, acclimation and dead
space wash-out. The subjects wore the breathing apparatus from
the 3rd min of exercise to the 5th min of exercise, and every 5 min
thereafter. The expired gas was analysed using a previously calibrated paramagnetic oxygen transducer analyser (series 1100)
(Servomex, Crowborough, England) and an infra-red carbon
dioxide analyser (series 1490) (Servomex). The volume of expired
air was measured using a dry gas volume meter (Harvard
Apparatus Ltd., Edenbridge, England) previously calibrated
against a Tissot spirometer. The meter was checked for linearity
throughout the complete volume range using a 7L calibration
syringe (Hans Rudolph Inc., Kansas City, Mo, USA). The coefficient of variation of this system is 1.4% (James and Doust 1997).
A 25-ll sample of arterialised capillary blood was collected in a
300-ll Microvette (Sarstedt) from a digit puncture. The sample of
whole blood was analysed using a YSI 2300 STAT PLUS glucose
and lactate analyser (Yellow Springs Instruments, Yellow Springs,
Ohio, USA).
Protocol
Subjects completed five exercise tests: an incremental ramp protocol test to determine V_ O2max; three constant loads tests to determine CP; and a final test to exhaustion at the CP intensity. At least
24 h rest was given between tests and the entire battery was completed within 14 days. Subjects were instructed to maintain the
cadence and remain seated throughout each test.
Measurement of V_ O2max; and maximum minute power
Following a 5-min warm-up at 75 W, a ramp protocol
(25 W.min–1) to exhaustion was completed to establish V_ O2max;
and maximum minute power (Pmax). In the latter part of the test
expired air was collected in Douglas bags sequentially for timed
periods of approximately 45 s. V_ O2max; was determined as a
difference of less than 0.15 l min–1 (<2 ml.kg–1.min–1) between
the two last Douglas bags and an RER >1.15. Maximum minute
power was derived from the SRM data logger as the highest
averaged power over 1 min.
Subject no.
Mass (kg)
Pmax (W)
V_ O2max
(l min–1)
Heart rate max
(beats min–1)
Cadence
(rev min–1)
1
2
3
4
5
6
7
Mean
SD
76.5
73.0
71.7
67.3
76.0
84.5
70.5
75.6
6.4
435
460
330
407
349
480
375
410
60
4.6
5.3
3.9
4.7
3.8
5.5
4.5
4.6
0.7
192
199
172
203
191
179
194
190
10.9
90
90
80
100
90
100
95
90
5
148
CP measurement
Results
The CP was determined using three tests to exhaustion. All tests
were performed on separate days. Intensity was assigned based on
the individuals’ Pmax from the ramp test. Pmax is defined as the
maximum minute power derived from the incremental ramp protocol. The tests were carried out at approximately 120%, 100% and
95% Pmax, and designed to fatigue the subject within the 1- to
10-min range (Poole 1986).
Subjects completed a 5-to 10-min warm-up, at the same cadence
to be used in the tests to exhaustion, at a power output of no
greater than 100 W. The cradle was then lifted briefly whilst the
mass was adjusted to the level required for the chosen power output. The subject continued pedalling at the required cadence. The
cradle was carefully lowered. Data were continuously logged on the
SRM data logger. Subjects were able to see the data recorded
(power, heart rate, cadence and total time) and were verbally
encouraged throughout to continue the test to exhaustion. A
constant cadence was required to be maintained throughout the
test. Once the cadence fell below the pre-determined cadence, the
subjects were further encouraged to attempt to increase their
cadence back to the required level. Exhaustion was defined as an
inability to maintain <5 rev min–1 below the pre-set cadence for
5 s or more. At exhaustion the load was removed and the subject
encouraged to continue to pedal to warm down.
Tests to exhaustion at CP
On a separate day, following a 5-min warm-up, subjects were encouraged to cycle for as long as possible at their individually determined CP. Expired air and a fingertip capillary blood sample
were collected every 5 min. Heart rate, power and cadence were
logged continuously. Subjects were cooled using a cooling fan and
were free to drink water ad libitium. The test was terminated when
the subject could no longer maintain the pre-determined cadence.
Subjects were encouraged to attempt to increase their cadence back
to the required level. Data were subsequently analysed to determine
the exact point of failure, defined as an inability to maintain
<5 rev min–1 below the pre-set cadence for 5 s or more. The time
to failure was described to the nearest second.
Table 1 shows the descriptive data for the subjects with
a mean (SD) V_ O2max of 4.6 (0.7) l min–1 [61 (9) ml kg–1
min–1] and a maximum minute power of 410 (60) W at a
cadence of 90 (5) rev min–1.
Data obtained during the three tests to determine CP
were highly linear (mean R2=0.985) and CP was 273
(38) W (Table 2). When exercising at CP, time to failure
ranged widely from 20 min 1 s to 40 min 37 s. Four
subjects completed over 25 min and three subjects over
30 min. Only one subject sustained exercise beyond
40 min.
The individual physiological responses during exercise at CP are shown in Table 3.
Oxygen consumption increased from a mean value of
3.7 l min–1 up to 4.13 l min–1 after 20 min (Fig. 1).
ANOVA revealed a significant difference over time for
oxygen uptake, F(3,6)=8.76, P=0.001.
Heart rate increased from a mean value of 120 beats
min–1 at the start of exercise to a mean value of
178 beats min–1 after 20 min (Fig. 2). There was a significant difference over time for heart rate, F(3,6)=27.85,
P<0.001. Heart rate at exhaustion averaged 186 beats
min–1.
Blood lactate concentration increased to a mean
value of 6.5 mmol l–1 in all subjects after 20 min (Fig. 3).
There was a significant difference over time for blood
lactate concentration, F(3,6)=16.0, P<0.001.
Time to failure was significantly correlated with final
oxygen uptake (r=0.69, P<0.05), as was critical power
(r=0.92, P<0.05). V_ O2max was significantly correlated
with time to exhaustion (r=0.78, P<0.05).
Statistical analysis
Statistical analysis was carried out using SPSS version 10 (SPSS
Inc., USA). Linear regression was used to determine the slope and
intercept of the power–time–1 model. Statistical comparisons over
time were made using a one-way repeated measures analysis of
variance (ANOVA). An a level of P<0.05 was accepted for
establishing statistical significance. Pearson-product moment
correlation coefficients were also calculated to determine relationships between variables.
Table 2. Physiological responses (oxygen uptake, blood lactate
concentration and critical power) and total time to failure for seven
individuals exercising at critical power (CP). V_ O2 4–5 min – Oxygen
uptake determined between 4 and 5 min after the start of exercise
Subject
%V_ O2max
1
2
3
4
5
6
7
Mean
SD
75
75
81
72
84
85
85
80
6
4–5 min
Discussion
Numerous studies examining the critical power concept
have studied the time to failure at CP but very few have
examined the physiological responses to exercise at CP.
The oxygen uptake response to exercise at CP has rarely
been reported within the literature. It is clear that
at CP expressed as a percentage of maximum oxygen uptake; V_ O2
– end oxygen uptake determined as a percentage of maximum
oxygen uptake; [La]b,max – highest blood lactate concentration
%V_ O2end (%)
[La]b,
92
91
93
88
93
91
92
91
2
10
5.3
6.2
6.9
6.6
7.4
9
7.3
1.6
max
(mM)
CP (W)
Time to failure at CP
(min:s)
249
312
240
267
236
338
270
273
38
28:39
36:41
23:32
40:37
20:01
36:23
21:10
29:34
8:22
149
Table 3. Individual physiological responses to exercise at critical power. Heart rate (HR, beats min–1); lactate concentration ([La]b, mM),
oxygen uptake (V_ O2, l min–1)
Subject
HR
5 min
HR
10 min
HR
15 min
HR
20 min
[La]b
5 min
[La]b
10 min
[La]b
15 min
[La]b
20 min
VO2
5 min
VO2
10 min
VO2
15 min
VO2
20 min
1
2
3
4
5
6
7
144
102
148
125
115
120
161
177
164
155
161
171
170
173
179
169
166
173
178
174
176
185
179
170
176
181
175
179
2.54
7.30
4.02
4.20
2.88
3.00
6.35
5.39
7.60
4.07
5.10
3.34
4.00
8.29
5.79
8.50
4.76
5.40
3.58
5.00
9.01
6.21
9.30
5.35
5.80
3.49
6.60
8.58
3.43
3.83
3.18
4.91
3.38
3.99
3.19
3.42
4.07
3.33
4.77
3.87
4.20
3.38
3.63
4.14
3.63
4.68
3.73
4.27
3.51
4.24
4.14
3.62
4.91
4.00
4.50
3.48
Fig. 1. Oxygen consumption (mean, SE) during cycling at critical
power (n=7)
Fig. 2. Heart rate (mean±SE) during exercise at critical power
(n=7)
exercise performed at CP is both non-sustainable and
non-steady state. Data from Table 2 reveal that oxygen
uptake increased from a mean of 79.5% to 91% V_ O2max
for the seven subjects studied between the 5th min and
the end of exercise (the mean duration of the exercise
was 29 min 34 s). Heart rate increased significantly over
time. The data from Table 3 compare favourably with
Fig. 3. Blood lactate concentration (mean±SE) during exercise at
critical power (n=7)
those of Jenkins and Quigley (1990), who demonstrated
that athletes cycling at CP experience high blood lactate
concentrations. The minor energetic contribution of
lactate during exercise at CP is noted (di Prampero and
Ferriti 1999). However the average increase in blood
lactate concentration between minutes 15 and 20 was
0.47 mM = 0.094 mM min–1. Since the increase of
1 mM lactate in the blood yields an energy equivalent of
approximately 3 ml O2/kg body mass the O2 equivalent
would be 0.094·3·75.6=21.3 ml O2 min–1, a rather
small contribution in terms of the total aerobic contribution during this period.
Numerous assumptions have to be considered when
deriving CP. For the purposes of the present study the
assumptions made by Wilkie (1980) are made where the
anaerobic contribution to the three constant load tests is
considered to be constant. The difficulty in measuring
the anaerobic contribution to exercise have been highlighted by di Prampero and Ferriti (1999).
The derivation of CP typically uses bouts of exercise
above V_ O2max. In this study one trial was conducted just
below V_ O2max in order to ensure that a large range of
times to exhaustion were completed. Jenkins et al. (1998)
demonstrated that the duration of the predicting trials
significantly influenced the time to fatigue at CP; however, the variation in the physiological responses was not
examined. Smith and Jones (2001), on using exhaustive
150
bouts above V_ O2max to derive critical velocity, found
that the physiological responses at critical velocity were
around anaerobic threshold and not V_ O2max.
The power and %max at CP which we report here are
higher than those reported for the various markers of
‘‘anaerobic threshold’’ (such as lactate threshold and
ventilatory threshold) which are typically found at 60–
75%max and demark the boundary between moderate
and heavy exercise (Jones and Doust 2001). At a slightly
higher intensity of exercise oxygen uptake and blood
lactate concentration are elevated to a level that is higher
than expected but a steady state is still attained. This
separates the boundary between heavy and severe exercise (Jones and Doust 2001) and is traditionally most
validly determined using a maximal lactate steady-state
test (Beneke and Von Duvillard 1996; Jones and Doust
1998). In the present study, oxygen uptake and blood
lactate concentration were elevated but a steady state
was not achieved. This emphasises that CP reflects an
intensity of exercise which is above the intensity traditionally associated with sustainability and that is greater
than the moderate/heavy and heavy/severe boundaries.
Hill and Smith (1999) found that CP was approximately 80% V_ O2max in an investigation where individuals could maintain CP for close to 60 min. Poole et al.
(1990) found CP values ranging from 60% to 90% of
V_ O2max, overestimating by approximately 17% the total
work that can be maintained for 60 min or greater.
Vandewalle et al. (1997) suggested that CP occurs within
the range of approximately 69–79% V_ O2max depending
on whether shorter or longer (up to 35 min) trials are
used in its determination. It would appear that there is
an element of protocol dependence in the estimation of
CP with significant consequences for whether or not
exercise at CP is sustainable and steady-state. In the
present study, CP was determined from three bouts of
exercise of 1 to 10 minutes and was clearly nonsustainable.
It has been observed that during constant intensity
exercise above lactate threshold V_ O2 will rise above the
level expected and at higher, but still submaximal
intensities, may continue to rise and reach maximum
(Casaburi et al. 1989; Whipp 1994). The mechanistic
basis for this slow component is not yet identified but
the recruitment of fast-twitch fibres offers the most likely
explanation (Barstow et al. 1996; Carter et al. 2000). The
slow component does not appear to be accompanied by
changes in muscle EMG patterns during repeated bouts
of heavy exercise in humans (Scheuermann et al. 2001).
In the present study, exercise at CP did not result in an
increase in oxygen consumption to the maximal level
attained during the incremental ramp protocol (see
Fig. 1) . It is speculated that the attainment of the
V_ O2max does not occur when exercising at CP since the
rate of increase in V_ O2 (400 ml min–1 over 20 min) is
slow and exercise is terminated due to other factors
before the V_ O2max level is reached.
The present findings would endorse continuous
monitoring of V_ O2 where power–V_ O2 relationships are
used to extrapolate non-steady-state exercise data at a
percentage V_ O2max.The drift in V_ O2 has been shown
here to be as much as 16% V_ O2max.
The widespread use of heart rate monitors as a means
of controlling training intensity is most apparent in the
physiological monitoring of cyclists (Boulay 1995).
Variation between subjects and environmental conditions in this measure provide obvious limitations. In the
present study, as well as the drift in oxygen consumption, upward drifts were also noted in heart rate. Heart
rate increased to approximately 180 beats min–1 by the
cessation of exercise but did not quite reach the maximum heart rate recorded on the incremental test.
The secondary finding from this study was the wide
range in time to failure (20 min 1 s to 40 min 37 s). This
refutes the early proposal that power production during
cycling exercise could be sustained for ‘‘a very long
time’’ (Monod and Scherrer 1965) and concurs with the
studies of Jenkins and Quigley (1990) that exercise at CP
can only be maintained for approximately 30 min. The
relationship between oxygen uptake and time to
exhaustion (r=0.78, P<0.05) may suggest that the fitter
subjects can sustain exercise at CP for longer than the
less aerobically trained individuals.
Time to exhaustion at the lowest velocity at which
V_ O2max can be achieved has been used by Billat et al.
(1994) as a practical means for setting training velocities.
However the time to failure at indices such as V_ O2max is
variable between individuals and is mode dependent.
The sustainable time at CP has been shown to have large
variability but we did not compare it directly to indices
such as lactate threshold, maximal lactate steady state or
ventilatory threshold.
Surprisingly within the current literature, characterising the fatiguing mechanisms during exercise at CP
has been given little attention. A range of time to failure
tests at CP have been expressed with some authors
reducing the workload to sustain a certain pre-selected
time period (Jenkins and Quigley 1990). Hill and Smith
(1999) have suggested that time to exhaustion at CP may
be increased by 27% (51.3 min to 65 min) following
familiarisation in a heterogeneous group of recreational
athletes. It would appear from the data of Hill and
Smith (1999) that individuals unfamiliar with exercise at
an estimated CP demonstrate a practice effect. All subjects in the present study were well-trained habituated
subjects from a homogenous group of male cyclists and
triathletes.
In summary, this is the first study to comprehensively
examine the physiological responses of exercise at an
individual’s ‘‘critical power’’. The sustainability of
exercise at CP has been challenged. Typically the work
rate when exercising at CP is non-steady state, approximately at 80% V_ O2max, and a physiological steady state
is not attained. There is an increase over time in oxygen
uptake, blood lactate concentration and heart rate.
There is considerable inter-individual variability in the
time for which exercise can be sustained. A more
appropriate definition for CP will be ‘‘the highest,
151
non-steady-state intensity that can be maintained for a
period in excess of 20 min, but generally no longer than
40 min’’.’
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