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 degrees C. In each fibre maximum shortening velocity (V-max), maximum power output (W-max), optimal velocity (velocity at which W-max is developed, V-opt), optimal force (force at which W-max is developed, P-opt), specific tension (P-o/CSA, isometric tension/cross-sectional area) were assessed. Unloaded shortening velocity (V-o) was also determined at 12 OC 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, W-max V-opt, P-opt, V-opt/V-max ratio, P-o/CSA and V-o 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, W-max, V-opt and V-o were significantly lower in type II A than in IIB fibres, whereas P-opt, P-o/CSA and V-opt/V-max were similar. 4. The temperature dependence of V-o and P-o/CSA was assessed in a group of twenty-one fibres in the range 12-22 degrees C. In a set of six fibres temperature dependence of V-max was also studied. The Q(10) (5.88) and activation energy E (125 kJ mol(-1)) values for maximum shortening velocity calculated from Arrhenius plots pointed to a very high temperature sensitivity. P-o/CSA was very temperature dependent in the 12-17 degrees C range, but less dependent between 17 and 22 degrees C.