Quantitative analysis of muscle fibre type and myosin heavy chain distribution in the frog hindlimb: Implications for locomotory design

To investigate the design of the frog muscular system for jumping, fibre type distribution and myosin heavy chain (MHC) isoform composition were quantified in the hindlimb muscles of Rana pipiens. Muscles were divided into two groups: five large extensor muscles which were predicted to shorten and produce mechanical power during jumping (JP), and four much smaller muscles commonly used in muscle physiology studies, but that do not shorten or produce power during jumping (NJP). Fibres were classified as one of four different types (type 1, 2, 3 or tonic) or an intermediate type (type 1-2) based on their relative myosin-ATPase reactivity and MHC immunoreactivity in muscle cross-sections according to previous nomenclature established for amphibian skeletal muscle. Type 1 fibres correspond to the fastest and most powerful of the twitch fibres, and type 3 fibres are the slowest and least powerful. Myosin-ATPase histochemistry revealed that the JP muscles were composed primarily of type 1 fibres (89%) with a small percentage of type 2 (7%) and intermediate type 1-2 fibres (4%). The fibre type composition of NJP muscles was more evenly distributed between type 1 (29%), type 2 (46%) and type 1-2 (24%) fibres. Tonic fibres comprised less than 2% of the muscle cross-section in both JP and NJP groups. Similarly, MHC composition determined by quantitative SDS-PAGE revealed that JP muscles were composed predominantly of type 1 MHC (86%), with a balance of type 2 MHC (14%). The opposite pattern was found for MHC composition in the NJP muscles: type 1 (28%), type 2 (66%) and type 3 (6%). These results demonstrate that the large extensor muscles that produce the power required for jumping have a fibre type distribution that enables them to generate high levels of mechanical power, with the type 1 isoform accounting for 85-90% of the total MHC content.