We hypothesised 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 W' (curvature constant 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 for 24 min and until the >CP Tlim isotime to test hypothesis 1. Eleven men performed 3-min all-out tests and donated muscle biopsies to test hypothesis 2. Below CP, muscle [PCr] (42.6 ± 7.1 vs 49.4 ± 6.9 mmol/kgDW), [La(-) ] (34.8 ± 12.6 vs 35.5 ± 13.2 mmol/kgDW) 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/kgDW) and lower ([PCr] 24.2 ± 3.9 mmol/kgDW; pH 6.84 ± 0.06) (P<0.05) at Tlim (740 ± 186 s) than during the <CP trial. The CP (234 ± 53 W) was correlated with muscle type I (r = 0.67, P = 0.025) and inversely correlated with muscle type IIx fibre proportion (r = -0.76, P = 0.01). There was no relationship between W' (19.4 ± 6.3 kJ) and muscle fibre type. These data indicate a mechanistic link between the bioenergetic characteristics of different muscle fibre types and the power-duration relationship. The CP reflects the bioenergetic characteristics of highly oxidative type I muscle fibres, such that a muscle metabolic steady-state is attainable below, but not above CP. This article is protected by copyright. All rights reserved.