BURNLEY, MARK1; DOUST, JONATHAN H.2; VANHATALO, ANNI1
The present study has shown that a 3-min all-out cycle ergometer test against a fixed resistance results in a reproducible power output profile and in the attainment of V·O2peak, which is consistent with our first and second hypotheses (Figs. 1 and 3B). The end-test power output was above that associated with the GET but below the power output achieved at the end of the ramp test (Figs. 2 and 3). Our third hypothesis was that this end-test power output would represent the boundary between the heavy- and severe-exercise intensity domains. Therefore, we predicted that constant-work rate exercise performed below this power output would result in steady-state blood [lactate] and V·O2 responses, whereas exercise above the end-test power output would result in a continued rise in these variables until fatigue ensued. The present study provides some support for this hypothesis: 9 of the 11 subjects were able to complete 30 min of exercise at 15 W below the end-test power, and seven of these met the criteria for a steady-state blood [lactate] profile (Fig. 4). In contrast, none of the subjects completed 30 min of exercise 15 W above the end-test power output, and in all cases blood [lactate] and V·O2 continued to rise until exhaustion, at which point V·O2 did not differ significantly from V·O2peak. These data suggest that it is possible to establish V·O2peak during a 3-min all-out exercise test and that this test also represents a promising method of identifying the maximal steady-state power output in a single test.
All-out exercise tests are typically used for measuring maximum dynamic power output. Consequently, test duration is usually limited to less than 90 s (30). Some previous reports have suggested that all-out tests can be used to establish V·O2peak in adults (12) and adolescents (29), whereas others have not (13,30). The present results show that V·O2peak can be achieved during all-out exercise even when the power output falls considerably below levels associated with the achievement of V·O2peak during ramp exercise (Table 1, Figs. 1 and 3). It is well established that the work rate need not be maximal for subjects to achieve V·O2peak; submaximal constant-work rate exercise performed in the severe-intensity domain results in a V·O2 slow component that drives V·O2 to V·O2peak before fatigue ensues (7,14,23). It is now also clear that all-out exercise lasting 1.5-3 min also yields V·O2peak, with little evidence of V·O2 declining towards the end of the test in adolescents or adults ((29), present study). These data add to the growing body of evidence that V·O2peak can be established using a variety of work rate-forcing functions. Ramp/incremental tests, all-out tests lasting 1.5-3 min, and submaximal constant-work rate tests in the severe-intensity domain performed to volitional exhaustion all result in the same end-point V·O2 (i.e., V·O2peak) (7,29).
A common feature of previous work investigating prolonged all-out exercise is that the power output falls below that associated with the attainment of V·O2peak in a ramp or incremental exercise test (6,8,12,29,30). It was our original contention that if the fall in power was continued until a leveling out could be identified, the end-test power would equal the power output demarcating the heavy- and severe-intensity domains. This contention stems from the fact that the power-duration relationship is hyperbolic (19-21), with the critical power representing the maximal steady-state power output (20) and W′ representing a fixed amount of work that can be performed above critical power (9). We reasoned that if the performance of all-out exercise were continued for long enough to reduce W′ to zero, then the end-test power output would necessarily equal the maximal steady state. We did not, however, establish the parameters of the power-duration relationship in the present study. The definition of critical power requires mathematical extrapolation of the results of a series of exhaustive exercise tests to the asymptote on the power axis (10), which may (20,21) or may not (4,22) yield a maximal steady-state power output. In establishing the critical power, therefore, the physiological response profile above and below the hypothesized heavy-severe boundary (the end-test power) would have remained uncertain. Instead, we chose to directly address the physiological responses to exercise above and below the end-test power output, using a previously established criterion for the achievement of a blood [lactate] steady state (an increase in blood [lactate] of < 1 mM between 10 and 30 min of exercise) (4,15,17). Thus, if the end-test power successfully defined the boundary between heavy- and severe-intensity exercise, a steady-state blood [lactate] response below, but not above, this power would be expected.
Individual responses to 3 min of all-out test
Figure 1: Correlation and Bland-Altman analyses for the difference between ramp-determined V·O2 and V·O2peak measured during all-out exercise (panels A and B) and the end-test power output during the all-out test (panels C and D). In panels A and C, the solid line is the best-fit linear regression, and the dashed line is the line of identity. In panels B and D (Bland-Altman plots), the solid horizontal line represents the mean difference between the two measures, and the dashed lines represent the 95% limits of agreement between the measures.
Figure 2: Group mean power output during the 3-min all-out test (panel A).Dashed lines represent the standard deviation. Panel B shows the group mean power output averaged every 30 s. Asterisks indicate a significant difference in power output from the previous time period. Note that power output reaches a plateau in approximately 120 s in panel A and that end-test power output is not significantly different from the preceding power output in panel B, in contrast to all other time points.
Figure 3: Power output (A) and oxygen uptake response profiles (B) during a 3-min all-out test in a representative subject. Panel A shows that power output fell to values considerably below the highest power output attained at the end of the ramp test but remained substantially higher than the power associated with the gas exchange threshold (GET), with power output changing little in the last 60 s of the test. Panel B shows that V·O2 rapidly increased towards V·O2peak, where it remained for the last 60 s of the test.
Figure 4: Oxygen uptake (panel A) and blood [lactate] (panel C) responses to constant power output exercise 15 W below (closed circles) and above (open circles) the power output attained in the last 30 s of the all-out test. These panels represent the mean (± SD) responses in the nine subjects who completed 30 min of exercise at the lower power output. Panels B and D show these same responses to exercise in the subject presented in Figure 3. Note that V·O2 and blood [lactate] continue to rise until exhaustion at the higher power output (reaching V·O2peak in panel B) but are stable from 10 min onward at the lower power output
At the end of the 3-min all-out test, power output had declined to approximately 70% of the power output measured at the end of the ramp test, and power output showed only a small (and statistically insignificant) decline in the last 60 s of the test (of approximately 5 W). Thus, we were successful in conducting an all-out exercise test in which power output reached a relatively stable level (the end-test power, Figs. 2 and 3). The mean results for the end-test power output in relation to the other parameters of aerobic function shown in Table 1 are remarkably similar to the critical power data presented by Poole et al. (20) (see their Table 2). For example, the end-test power occurred at approximately 43% Δ, whereas Poole et al. (20) reported that critical power occurred at approximately 46% Δ. Other investigators have reported similar findings for the exercise intensity at the maximal steady state (22,25). The present results further demonstrate that exercise above the end-test power is situated in the severe-intensity exercise domain, where V·O2 and blood [lactate] increased until exhaustion ensued. Exercise below the end-test power was, in most subjects, situated in the heavy-intensity domain, where blood [lactate] and V·O2 eventually stabilized. In this intensity domain, exercise can be maintained for a considerable, but finite, period of time, with fatigue likely being mediated by limitations in the rate of or capacity for substrate use and/or hyperthermia (11). These results demonstrate that a 3-min all-out exercise test can be used to estimate a power output at the physiologically important boundary between the heavy- and severe-exercise intensity domains in more than 60% of the subjects sampled. Although by no means perfect, we believe this is a promising result in light of the observation that the end-test power output occurred in the correct region of the exercise intensity spectrum to yield the maximal steady state (or critical power) in all subjects (i.e., approximately halfway between the GET and end-ramp power outputs).
The findings of the present investigation have potentially important implications for the power-duration relationship. Assuming that critical power and the maximal steady state can be used interchangeably ((16,20,25), but see (4,22)), the present results suggest that the current formulation of the power-duration relationship (equations 1 and 2) is fundamentally correct and may be generalized to maximal all-out exercise. The power output during 3 min of all-out exercise declined to, or at least towards, a power output below which a steady-state blood [lactate] and V·O2 response profile could be observed (Figs. 3 and 4). This would be expected if W′ were reduced to zero during the exercise, at which point the highest power that could theoretically be maintained would be the maximal steady state (5,10). There is currently no standard method of establishing W′ during all-out exercise (8). However, the total work done above the end-test power was 14.3 ± 4.7 kJ, which is of the same order of magnitude as W′ estimates presented in the literature (5,8,10,20,21), suggesting that W′ could also be estimated from a 3-min all-out test. Thus, the expenditure of W′ provides the simplest explanation for the attainment of the end-test power, in terms of its proximity to other physiological landmarks (end-ramp and GET power outputs) and the physiological response profiles above and below the end-test power. However, further work is necessary to address this notion quantitatively.
Although the majority of subjects in this study were able to attain a steady state below the end-test power, four subjects did not. We did not undertake further testing to establish the magnitude of the discrepancy between the end-test power and the true heavy-severe domain boundary in these subjects, although it would appear that in two of the four cases the discrepancy would have been small because these subjects completed 30 min of exercise. Therefore, we can only speculate as to why the 3-min test did not establish the domain boundary in these subjects. Firstly, it is important to note that in no instance was the maximal steady state underestimated: the 3-min test either succeeded in identifying the boundary or it overestimated it. Consequently, test failure was not caused by muscle fatigue inducing a continual fall in power output to an end-test power output below the maximal steady state. It is possible that 3 min of all-out exercise is, in some subjects, not long enough to expend the entire W′. However, this is unlikely because Medbø et al. (18) reported that a maximal O2 deficit (theoretically analogous to W′) (11) could be accumulated in approximately 2 min of constant-power exercise, and Gastin and Lawson (13) reported that the accumulated O2 deficit during all-out exercise did not differ with exercise durations of 60 or 90 s. Perhaps more likely, averaging the power output in the last 30 s of exercise, though apparently justified because power output in the last 60 s was not falling significantly, may have resulted in an overestimation of the heavy-severe domain boundary in those subjects in whom power output was still falling during the last 60 s. In these subjects, a longer test duration (e.g., 4 min) or a shorter period of data averaging (15 vs 30 s) may have resulted in successful identification of the domain boundary. It is possible, however, that prolonging the test duration beyond 3 min would result in a continued fall in power output with time, exposing the apparent success of a 3-min test in estimating the maximal steady state as fortuitous, though no less useful. Thus, a 3-min test duration was chosen as a compromise between a test that would be too short for power output to level out and one that might be needlessly long, such as a 5-min test in which subjects might exercise all out with little or no change in power output for the final 3 min. We do not claim, nor do we have any means of showing, that 3 min is the optimal test duration.
The Lode Excalibur Sport ergometer we used in the present study does not operate isokinetically, unlike the ergometers used in recent reports (8,29). Therefore, we set the ergometer's linear factor so that when (if) the subjects reached their preferred cadence, they would be producing a power output halfway between GET and V·O2peak. We adopted this approach in preference to normalizing the resistance to pedaling based on body mass (12,13) because we were attempting to establish the maximal steady state, rather than anaerobic performance. It is not known whether the approach adopted in the present study is an optimal strategy for the achievement of a valid end-test power in all subjects. However, the cadence at the end of the 3-min test was similar to the subjects' preferred cadence of 80-90 rpm, despite the variance in power output across the sample (187-338 W). Had the resistance been equivalent to 7.5% of body mass, which is commonly used to set the resistance on a Monark cycle ergometer for all-out tests (12,13), the subjects would have achieved their end-test power output at approximately 50 rpm, approximately 30-40 rpm lower than their preferred cadence. Whether other resistances (based on physiological status or body mass) or the adoption of isokinetic ergometry (8,29) would influence the outcome of the test requires further work.
In summary, we have presented evidence that the power profile in a 3-min all-out test against a fixed resistance is reproducible and that the test can be used to establish V·O2peak in adults, extending the recent findings of Williams et al. (29) in adolescents. For the first time, the present study has demonstrated that during all-out exercise, power output falls towards, and in more than 60% of cases attains, a power output below which steady-state responses in blood [lactate] and pulmonary V·O2 generally occur, but above which these variables rise inexorably until fatigue ensues. The present work therefore suggests that a 3-min bout of all-out exercise represents a promising method of estimating the maximal steady state, which has previously required repeated bouts of prolonged and/or exhaustive exercise to identify.