Force-velocity properties of human skeletal muscle fibres: Myosin heavy chain isoform and temperature dependence

1. A large population (n = 151) of human skinned skeletal muscle fibres has been studied. Force-velocity curves of sixty-seven fibres were obtained by load-clamp manoeuvres at 12°C. In each fibre maximum shortening velocity (V(max)), maximum power output (Ẇ(max)), optimal velocity (velocity at which Ẇ(max), is developed, V(max)), optimal force (force at which is developed, P(opt)), specific tension (P₀/CSA, isometric tension/cross-sectional area) were assessed. Unloaded shortening velocity (V₀) was also determined at 12°C in a different group (n = 57) of fibres by slack-test procedure. 2. All fibres used for mechanical experiments were characterized on the basis of the myosin heavy chain (MHC) isoform composition by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis and divided into five types: type I (or slow), types IIA and IIB (or fast), and types I-IIA and IIA-IIB (or mixed types). 3. V(max), Ẇ(max), V(opt), P(opt), V(opt)/V(max) ratio, P₀/CSA and V₀ were found to depend on MHC isoform composition. All parameters were significantly lower in type I than in the fast (type IIA and IIB) fibres. Among fast fibres, V(max), Ẇ(max), V(opt) and were significantly lower in type IIA than in IIB fibres, whereas P(opt), P₀/CSA and V(opt)/V(max) were similar. 4. The temperature dependence of V₀ and P₀/CSA was assessed in a group of twenty-one fibres in the range 12-22°C. In a set of six fibres temperature dependence of V(max) was also studied. The Q₁₀ (5.88) and activation energy E (125 kJ mol^-1) values for maximum shortening velocity calculated from Arrhenins plots pointed to a very high temperature sensitivity. P₀/CSA was very temperature dependent in the 12-17°C range, but less dependent between 17 and 22°C.