Variation in the determinants of power of chemically skinned type I rat soleus muscle fibres

We explored to which extent maximal velocity of shortening (Vmax), force per cross-sectional area (specific tension, Po) and curvature of the force–velocity relationship (a/Po in the Hill equation) contribute to differences in peak power of single, chemically skinned rat type I fibres. Force–velocity relationships were determined from isotonic contractions of 94 maximally activated fibres. Peak power (±SD) was 3.50 ± 1.64 W L⁻¹. There was a tenfold range of peak power and five-, six- and fourfold ranges for Po, Vmax and a/Po, respectively. None of the differences between fibres was explicable by differences in myosin heavy or light chain composition. The inverse relationship between a/Po and Vmax suggests a similar underlying cause. Fitting the data to the Huxley (Progr Biophys Biophys Chem 7:255–318, 1957) cross-bridge model showed that the rate constant g ₂ and the sum of the rate constants (f + g ₁) co-varied, both being low in the slowest fibres. Approximately 16% of the variation in Po could be explained by variation in the proportion of attached cycling cross-bridges (f/(f + g ₁)), but the origin of most of the variance in Po remains unknown.