Twisting and bending: The functional role of salamander lateral hypaxial musculature during locomotion

The function of the lateral hypaxial muscles during locomotion in tetrapods is controversial. Currently, there are two hypotheses of lateral hypaxial muscle function. The first, supported by electromyographic (EMG) data from a lizard (Iguana iguana) and a salamander (Dicamptodon ensatus), suggests t hat hypaxial muscles function to bend the body during swimming and to resis t long-axis torsion during walking, The second, supported by EMG data from lizards during relatively high-speed locomotion, suggests that these muscle s function primarily to bend the body during locomotion, not to resist tors ional forces. To determine whether the results from D, ensatus hold for ano ther salamander, we recorded lateral hypaxial muscle EMGs synchronized with body and limb kinematics in the tiger salamander Ambystoma tigrinum. In ag reement with results from aquatic locomotion in D, ensatus, all four layers of lateral hypaxial musculature were found to show synchronous EMG activit y during swimming in A. tigrinum. Our findings for terrestrial locomotion a lso agree with previous results from D. ensatus and support the torsion res istance hypothesis for terrestrial locomotion. We observed asynchronous EMG bursts of relatively high intensity in the lateral and medial pairs of hyp axial muscles during walking in tiger salamanders (we call these 'alpha -bu rsts'). We infer from this pattern that the more lateral two layers of obli que hypaxial musculature, Mm. obliquus externus superficialis (OES) and obl iquus externus profundus (OEP), are active on the side towards which the tr unk is bending, while the more medial two layers, Mm. obliquus internus (OI ) and transversus abdominis (TA), are active on the opposite side. This res ult is consistent with the hypothesis proposed for D, ensatus that the OES and OEP generate torsional moments to counteract ground reaction forces gen erated by forelimb support, while the OI and TA generate torsional moments to counteract ground reaction forces from hindlimb support. However, unlike the EMG pattern reported for D, ensatus, a second, lower-intensity burst o f EMG activity ('beta -burst') was sometimes recorded from the lateral hypa xial muscles in A. tigrinum. As seen in other muscle systems, these beta -b ursts of hypaxial muscle coactivation may function to provide fine motor co ntrol during locomotion. The presence of asynchronous, relatively high-inte nsity alpha -bursts indicates that the lateral hypaxial muscles generate to rsional moments during terrestrial locomotion, but it is possible that the balance of forces from both alpha- and beta -bursts may allow the lateral h ypaxial muscles to contribute to lateral bending of the body as well.