Morphology and Function of Lateral Hypaxial Musculature in Salamanders1

Abstract Synopsis. 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 in myomeres. These layers produce three primary mechanical actions: they bend the body, pressurize the body, and produce or resist torsion about the long axis of the body. The optimum muscle fiber angle for forceful bending is 0° to the long axis, the optimum angle for pressurization is 90°, and the optimum angle for torsion is 45°. For generating bending and torsional moments, lateral (superficial) muscle layers have greater mechanical advantage than medial (deep) layers. For increasing body pressure, by contrast, medial layers have greater mechanical advantage. A comparison of muscle fiber angles in aquatic and terrestrial salamanders reveals that some aquatic salamanders have one muscle layer with a low fiber angle which may represent a specialization for swimming. Overall, however, the fiber angles in the LHM of terrestrial and aquatic salamanders are surprisingly similar. In contrast, the pattern of fiber angles in caecilians is different, suggesting that these amphibians use their LHM differently. The fiber angle models and morphological observations presented here form a framework which may be useful in future studies of lateral hypaxial musculature.