Abstract
An important and unresolved issue is the mechanism(s) that lead to task failure during intense aerobic exercise. Marcora and Staiano (2010) have recently reinvestigated this problem using human subjects on a bicycle ergometer. Subjects were asked to exercise at 80% of their maximal aerobic power (242 W) and failed (unable to maintain the pedal frequency above 60 min despite encouragement) after 10.5 min. Under control conditions, their peak power output measured over 5 s and at a pedal frequency of 137 min was 1,075 W. When peak power was measured as soon as possible after the fatiguing event, it was 731 W. Because peak power (731 W) was so much greater than the power which the subjects failed to achieve at the end of the exercise period (242 W) Marcora and Staiano argue that muscle performance was not the main problem, instead the subjects stopped because their ‘perception of effort’ was greater than they were prepared to tolerate. One interpretation problem with these experiments has already been raised by Burnley (2010) who is concerned that the difference in pedal frequency at exhaustion (\60 min) as compared to the peak power (137 min) may compromise the comparison. Here, we raise a second interpretation issue with potential impact on Marcora and Staiano’s conclusion. Our concern is based on the fact that a small degree of muscle recovery can have a large effect on the power output of fatigued muscles. Because of the need to change the mode of the ergometer between the exercise bout and the peak power determination, there was a delay of 3–4 s between these activities in Marcora and Staiano’s study. It is conceivable that this short period was sufficient for a substantial recovery of power between the two activities. Marcora and Staiano argue against this possibility on the grounds that recovery of peak power after high-intensity exercise is relatively slow with a half time of 32 s (Sargeant and Dolan 1987). Whilst this is correct, it is also the case that half the recovery to 100% occurred at 5 s (Fig. 6, Sargeant and Dolan 1987). Thus, we suggest that the subjects’ muscles may have been severely fatigued and that much of the observed difference between the fatigued performance and the peak power may simply be rapidly occurring recovery processes. In further support of our interpretation, we point to experiments on isolated muscle in which substantial recovery from fatigue occurs in a few seconds. For instance, in mouse muscle fibres fatigued by repeated isometric tetani, it is possible for force to recover by 100% in 10 s (Fig. 5, Westerblad and Allen 1991). This large effect is related to the fact that the slope of the force–Ca relation is very steep in fatigued muscles so a small recovery of sarcoplasmic reticulum (SR) Ca release and/or myofibrillar Ca sensitivity leads to a large recovery of force. For instance, with the force–Ca relation being described by a Hill equation with a Hill coefficient of 5, a 10% increase in either tetanic Ca or myofibrillar Ca sensitivity results in a force increase of *40%. In addition, the loss of power during exercise involves a decrease in shortening velocity (Jones et al. 2006), and experiments on single mouse muscle fibres show that this slowing Communicated by Susan Ward.
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