The tolerable duration (tlim) of high intensity exercise is well described by a hyperbolic function of the external power output (P), where only a fixed amount of work (W¢) can be performed above the 'critical' power asymptote (CP) i.e. supra-CP exercise cannot be tolerated once W¢ is fully 'depleted'. Consistent with this notion, Coats et al (J Appl Physiol 95: 483-, 2003) have shown that the ability to continue exercise following intolerance is dependent on P being reduced to a sub-CP level.
PURPOSE: We therefore aimed to determine whether the maximum sustainable P (MSP) following W¢ depletion was equivalent to CP.
METHODS: Six men each performed (using electromagnetically-braked cycle ergometry): 1) Four different high intensity constant-P tests, on separate days and to the tolerable limit, to estimate CP, W¢ and maximum oxygen uptake (VO2); each bout was immediately followed by an 'all-out' effort (3 min; variable-P) to estimate MSP (Burnley et al MSSE 38: 1995-, 2006); 2) A Wingate test to estimate peak P; 3) Constant-P tests for 30 min at CP and MSP to establish whether a steady state was attained. The ergometer provided cadence-dependent and -independent P control where necessary. Pulmonary gas exchange was measured breath-by-breath via mass spectrometry and turbinometry, and lactate:pyruvate ratio was determined from arterialized venous blood samples.
RESULTS: VO2 reached maximum (3.79 ± 0.47 l/min) in all protocol 1 exercise bouts. MSP (234 ± 32 W) was significantly lower (p < 0.05) than peak P (724 ± 102 W), and was not significantly different (p > 0.05) from CP (221 ± 47 W). However, MSP was poorly predicative of CP; individual differences ranged between ± 35 W, with the closest estimates deriving from constant-P tests with the longest tlim. On average the total work done above MSP (15.2 ± 5.9 kJ) was less than (p < 0.05) W¢ (19.6 ± 2.8 kJ) - the 'shortfall' being largely dependent on the overestimation of CP by MSP. Only one subject achieved a steady state in VO2 and pyruvate:lactate at MSP, whereas all subjects attained a steady state at CP.
CONCLUSION: The highest sustainable work rate following W¢ depletion was close to CP, but did not reflect a steady state P. These data suggest that the rate at which intolerance is induced (i.e. W¢ 'depletion' rate) may play an important role in determining the extent of muscle fatigue, and hence MSP.