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

<jats:sec><jats:title>Key points</jats:title><jats:p><jats:list list-type="bullet"> <jats:list-item><jats:p>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 (<jats:italic>W</jats:italic>′) indicates a fixed capacity for work &gt;CP that is related to a loss of muscular efficiency.</jats:p></jats:list-item> <jats:list-item><jats:p>The present study reports novel evidence on the muscle metabolic underpinnings of CP and <jats:italic>W</jats:italic>′ during whole‐body exercise and their relationships to muscle fibre type.</jats:p></jats:list-item> <jats:list-item><jats:p>We show that the <jats:italic>W</jats:italic>′ is not correlated with muscle fibre type distribution and that it represents an elevated energy contribution from both oxidative and glycolytic/glycogenolytic metabolism.</jats:p></jats:list-item> <jats:list-item><jats:p>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 &lt;CP but not &gt;CP.</jats:p></jats:list-item> <jats:list-item><jats:p>Our findings indicate a mechanistic link between the bioenergetic characteristics of muscle fibre types and the power–time relationship for high‐intensity exercise.</jats:p></jats:list-item> </jats:list></jats:p></jats:sec><jats:sec><jats:title>Abstract</jats:title><jats:p>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 (<jats:italic>W</jats:italic>′) 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 &gt;CP until exhaustion (<jats:italic>T</jats:italic><jats:sub>lim</jats:sub>), slightly &lt;CP for 24 min and until the &gt;CP <jats:italic>T</jats:italic><jats:sub>lim</jats:sub> 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 <jats:italic>vs</jats:italic>. 49.4 ± 6.9 mmol (kg d.w.)<jats:sup>−1</jats:sup>], [La<jats:sup>−</jats:sup>] [34.8 ± 12.6 <jats:italic>vs</jats:italic>. 35.5 ± 13.2 mmol (kg d.w.)<jats:sup>−1</jats:sup>] and pH (7.11 ± 0.08 <jats:italic>vs</jats:italic>. 7.10 ± 0.11) remained stable between ∼12 and 24 min (<jats:italic>P</jats:italic> &gt; 0.05 for all), whereas these variables changed with time &gt;CP such that they were greater [[La<jats:sup>−</jats:sup>] 95.6 ± 14.1 mmol (kg d.w.)<jats:sup>−1</jats:sup>] and lower [[PCr] 24.2 ± 3.9 mmol (kg d.w.)<jats:sup>−1</jats:sup>; pH 6.84 ± 0.06] (<jats:italic>P</jats:italic> &lt; 0.05) at <jats:italic>T</jats:italic><jats:sub>lim</jats:sub> (740 ± 186 s) than during the &lt;CP trial. The CP (234 ± 53 W) was correlated with muscle type I (<jats:italic>r</jats:italic> = 0.67, <jats:italic>P</jats:italic> = 0.025) and inversely correlated with muscle type IIx fibre proportion (<jats:italic>r</jats:italic> = −0.76, <jats:italic>P</jats:italic> = 0.01). There was no relationship between <jats:italic>W</jats:italic>′ (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 and not above CP.</jats:p></jats:sec>