BARKER, J.U.; ASHLEY-ROSS, M.A.*: The effect of fiber type heterogeneity on work and power output of hindlimb muscles of the tiger salamander

Most vertebrate muscles are composed of a mixture of fiber types. However, study of the mechanical performance of muscle has concentrated on homogeneous bundles of muscle fibers. Muscles in the hindlimb of the tiger salamander, Ambystoma tigrinum, present an excellent opportunity to explore the consequences of fiber heterogeneity on work and power output. Isometric twitches and work loops were performed in vitro on two muscles, the M. iliotibialis pars posterior (ILTP; heterogeneous, containing SO, FOG and FG fibers) and the M. iliofibularis (ILFB; homogeneous, containing FOG fibers only). Maximal isometric twitch force in the ILTP was over twice as great as in the ILFB; however, when adjusted for cross-sectional area, mean isometric stress was statistically indistinguishable (ILTP: 46.2 +/- 21.4 kN/m²; ILFB: 40.1 +/- 21.5 kN/m²). Work loops were performed over a range of frequencies (0.5 � 3.0 Hz) that encompassed the observed range of in vivo limb cycling frequencies. The homogeneous ILFB showed a steady decline in work per cycle from 1.5 � 3.0 Hz, while power output over those frequencies remained nearly constant. The heterogeneous ILTP showed a different pattern, with a tri-modal distribution for work per cycle with peaks at 0.5, 1.5, and 2.5 Hz, and maximal power output at 2.5 Hz. The differences in mechanical performance between the ILTP and ILFB may relate to the muscles’ function during locomotion. The ILFB is active at the beginning of the swing phase, and serves to lift the hindlimb from the ground. Steady power output may be required for this ballistic-like movement, as the weight of the limb is unchanging. In contrast, the ILTP functions to extend the knee, and thus can contribute to forward propulsion. High power output at high cycling frequencies may indicate the ILTP assists fast locomotion.