The effect of fiber-type heterogeneity on optimized work and power output of hindlimb muscles of the salamander Ambystoma tigrinum

Most vertebrate muscles are composed of a mixture of fiber types. However, studies of muscle mechanics have concentrated on homogeneous bundles of fibers. Hindlimb muscles of the tiger salamander, Ambystoma tigrinum, present an excellent system to explore the consequences of fiber heterogeneity. Isometric twitches and work loops were obtained in vitro from two muscles, the m. iliotibialis pars posterior (heterogeneous, containing types I, IIa and IIb fibers) and the m. iliofibularis (nearly homogeneous for type IIa fibers). Maximal isometric twitch and tetanic stresses in m. iliotibialis posterior were significantly greater than in iliofibularis. Work loops were obtained over a range of frequencies (0.5-3.0 Hz) and strains (2-6% muscle length) that encompassed the observed ranges in vivo. Work per cycle from the homogeneous iliofibularis declined from 1.5-3.0 Hz, while that from the heterogeneous m. iliotibialis posterior increased from 0.5 Hz to 2.5 Hz and declined at 3.0 Hz. Power output from the iliofibularis rose with frequency to at least 3 Hz; power from the iliotibialis posterior rose with frequency to 2.5 Hz and declined thereafter. Mass-specific work per cycle and power output were higher in iliofibularis than iliotibialis posterior over most frequencies and strains tested.