WHOLE-MUSCLE AND SINGLE-FIBER CONTRACTILE PROPERTIES AND MYOSIN HEAVY-CHAIN ISOFORMS IN HUMANS

The contractile characteristics of three human muscle groups (triceps surae, quadriceps femoris and triceps brachii) of seven young male sub jects were examined. The contractile properties were determined from e lectrically evoked isometric responses and compared with fibre type co mposition determined from needle biopsy samples. Fibre types were iden tified using myosin heavy chain (MHC) isoforms as molecular markers wi th gel electrophoresis (SDS-PAGE) and histochemical ATPase staining. F our contractile parameters (twitch time to peak torque, the maximal ra te of torque development, frequency response and fatiguability) were f ound to be related to fibre type composition. From the biopsy samples, single muscle fibres were isolated and chemically skinned. Isometric tension (P-o) unloaded shortening velocity (V-o) and rate of tension r ise (dP/dt) were determined. Each fibre was classified on the basis of its MHC isoform composition determined by SDS-PAGE. Fibres belonging to the same type showed identical contractile parameters regardless of the muscle of origin, except minor differences in P-o of the fast fib res and dP/dt of slow fibres. The results are in favour of the conclus ion that fibre type composition, determined using MHC isoforms as mark ers, is the major determinant of the diversity of contractile properti es among human muscle groups.