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Key points The power-asymptote (critical power; CP) of the hyperbolic power-time relationship for high-intensity exercise defines a threshold between steady-state and non-steady-state exercise intensities and the curvature constant ( W′) indicates a fixed capacity for work >CP that is related to a loss of muscular efficiency., The present study reports novel evidence on the muscle metabolic underpinnings of CP and W′ during whole-body exercise and their relationships to muscle fibre type., We show that the W′ is not correlated with muscle fibre type distribution and that it represents an elevated energy contribution from both oxidative and glycolytic/glycogenolytic metabolism., We show that there is a positive correlation between CP and highly oxidative type I muscle fibres and that muscle metabolic steady-state is attainable CP., Our findings indicate a mechanistic link between the bioenergetic characteristics of muscle fibre types and the power-time relationship for high-intensity exercise., Abstract We hypothesized that: (1) the critical power (CP) will represent a boundary separating steady-state from non-steady-state muscle metabolic responses during whole-body exercise and (2) that the CP and the curvature constant ( W′) of the power-time relationship for high-intensity exercise will be correlated with type I and type IIx muscle fibre distributions, respectively. Four men and four women performed a 3 min all-out cycling test for the estimation of CP and constant work rate (CWR) tests slightly >CP until exhaustion ( Tlim), slightly CP Tlim isotime to test the first hypothesis. Eleven men performed 3 min all-out tests and donated muscle biopsies to test the second hypothesis. Below CP, muscle [PCr] [42.6 ± 7.1 vs. 49.4 ± 6.9 mmol (kg d.w.)−1], [La−] [34.8 ± 12.6 vs. 35.5 ± 13.2 mmol (kg d.w.)−1] and pH (7.11 ± 0.08 vs. 7.10 ± 0.11) remained stable between ∼12 and 24 min ( P > 0.05 for all), whereas these variables changed with time >CP such that they were greater [[La−] 95.6 ± 14.1 mmol (kg d.w.)−1] and lower [[PCr] 24.2 ± 3.9 mmol (kg d.w.)−1; pH 6.84 ± 0.06] ( P < 0.05) at Tlim (740 ± 186 s) than during the

It has been shown that the critical power (CP) in cycling estimated using a novel 3‐min all‐out protocol is reliable and closely matches the CP derived from conventional procedures. The purpose of this study was to assess the predictive validity of the all‐out test CP estimate. We hypothesised that the all‐out test CP would be significantly correlated with 16.1‐km road time‐trial (TT) performance and more strongly correlated with performance than the gas exchange threshold (GET), respiratory compensation point (RCP) and V̇O2 max. Ten club‐level male cyclists (mean±SD: age 33.8±8.2 y, body mass 73.8±4.3 kg, V̇O2 max 60±4 ml·kg−1·min−1) performed a 10‐mile road TT, a ramp incremental test to exhaustion, and two 3‐min all‐out tests, the first of which served as familiarisation. The 16.1‐km TT performance (27.1±1.2 min) was significantly correlated with the CP (309±34 W; r=−0.83, P<0.01) and total work done during the all‐out test (70.9±6.5 kJ; r=−0.86, P<0.01), the ramp incremental test peak power (433±30 W; r=−0.75, P<0.05) and the RCP (315±29 W; r=−0.68, P<0.05), but not with GET (151±32 W; r=−0.21) or the V̇O2 max (4.41±0.25 L·min−1; r=−0.60). These data provide evidence for the predictive validity and practical performance relevance of the 3‐min all‐out test. The 3‐min all‐out test CP may represent a useful addition to the battery of tests employed by applied sport physiologists or coaches to track fitness and predict performance in atheletes. [ABSTRACT FROM AUTHOR]