The mechanistic bases of the power-time relationship: muscle metabolic responses and relationships to muscle fibre type

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 (T-lim), slightly < CP for 24 min and until the > CP T-lim 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 similar to 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 T-lim (740 +/- 186 s) than during the < CP trial. The CP (234 +/- 53W) 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 amechanistic link between the bioenergetic characteristics of differentmuscle 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 and not above CP.