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The mechanistic bases of the power-time relationship: muscle metabolic responses and relationships to muscle fibre type.
Vanhatalo, Anni; Black, Matthew I.; DiMenna, Fred J.; Blackwell, Jamie R.; Schmidt, Jakob Friis; Thompson, Christopher; Wylie, Lee J.; Mohr, Magni; Bangsbo, Jens; Krustrup, Peter; Jones, Andrew M.
[Article] Journal of Physiology. 594(15):4407-4423, August 1, 2016.

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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.

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

Key points:

* 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.