Morphology and function of lateral hypaxial musculature in salamanders

The lateral hypaxial musculature (LHM) of salamanders may serve as a useful model for understanding the functions of LHM in tetrapods more generally. Salamanders have between two and four layers of LHM, arranged segmentally i n myomeres, These layers produce three primary mechanical actions: they ben d the body, pressurize the body, and produce or resist torsion about the lo ng axis of the body, The optimum muscle fiber angle for forceful bending is 0 degrees to the long axis, the optimum angle for pressurization is 90 deg rees, and the optimum angle for torsion is 45 degrees, For generating bendi ng and torsional moments, lateral (superficial) muscle layers have greater mechanical advantage than medial (deep) layers. For increasing body pressur e, by contrast, medial layers have greater mechanical advantage, A comparis on of muscle fiber angles in aquatic and terrestrial salamanders reveals th at some aquatic salamanders have one muscle layer with a low fiber angle wh ich may represent a specialization for swimming. Overall, however, the fibe r angles in the LHM of terrestrial and aquatic salamanders are surprisingly similar. hi contrast, the pattern of fiber angles In caecilians is differe nt, suggesting that these amphibians use their LHM differently, The fiber a ngle models and morphological observations presented here form a framework which may be useful in future studies of lateral hypaxial musculature.