The motor protein myosin in skeletal muscle is a hexameric polypeptide consisting of two myosin heavy chain (MHC)

polypeptide consisting of two myosin heavy chain (MHC) and four myosin light chain (MLC) subunits. Each MHC is associated with one essential light chain (ELC) and one regulatory light chain (RLC; also called alkali light chain). All vertebrates, including humans, express a family of MHC, ELC and RLC isoforms in their skeletal muscles. In a given species, variable expression of multiple MHC and MLC isoforms results in a diverse population of muscle fibres with a wide range of shortening velocities and power capabilities. Thus, myosin isoforms have a critical impact on functional diversity in muscular systems and influence how muscles are adapted to meet the specific motor requirements of an organism. The influence of MHC and MLC isoforms on the force–velocity relationship remains controversial and may vary among vertebrates (for reviews, see Moss et al., 1995; Schiaffino and Reggiani, 1996; Bottinelli and Reggiani, 2000). Mechanical studies on single skinned fibres established that MHC isoforms influence maximal shortening velocity (Vmax) and maximal power (Sweeney et al., 1988; Larsson and Moss, 1993; Bottinelli et al., 1994a, 1996; Hilber and Galler, 1997). In some studies, Vmax was also affected by the ratio of MLC3/MLC1f (Moss et al., 1982; Sweeney et al., 1988; Bottinelli et al., 1994a). However, Larsson and Moss found no relationship between MLC3/MLC1f and Vmax in human fibres (Larsson and Moss, 1993). The influence of MHC and MLC isoforms on maximal isometric tension also remains the subject of considerable debate (Moss et al., 1995; Schiaffino and Reggiani, 1996; Bottinelli and Reggiani, 2000). Frog muscle is unique in that relatively long, single, intact ‘living’ fibres can be isolated that retain excellent mechanical stability and from which sarcomere length transients can readily be measured by laser diffraction. For this reason, 4237 The Journal of Experimental Biology 204, 4237–4248 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3569