Activity of the hypaxial muscles during walking in the lizard Iguana iguana.

The role that the hypaxial muscles play in locomotion has been largely ignored by biologists. In tetrapods, there are at least three possibilities. First, the hypaxial muscles might bend the trunk laterally to increase stride length. Second, they might stabilize the trunk against the horizontal, lateral and vertical components of the propulsive force. Alternatively, they might not be involved in locomotion. This study evaluated these three hypotheses by analyzing the activity of the hypaxial muscles of green iguanas (Iguana iguana). During walking, the rectus abdominis, obliquus externus superficialis and profundus, intercostales externi, and ventral portion of the intercostales interni on one side of the trunk acted synergistically with the lateral portion of the intercostales interni and obliquus internus on the other side of the trunk. This pattern supports the hypothesis that the hypaxial muscles act to stabilize the trunk during locomotion. Specifically, the longitudinally oriented rectus abdominis, obliquus externus profundus and ventral portion of the intercostales interni appear to stabilize the trunk against the horizontal and lateral components of the propulsive force, which tend to rotate the girdles in the horizontal plane. The obliquely oriented obliquus externus superficialis, intercostales externi, lateral portion of the intercostales interni and obliquus internus appear to stabilize the trunk against the vertical component, which induces long-axis torsion in the trunk. Thus, the demands of locomotion may provide a functional explanation for the basic organization of the hypaxial muscles of tetrapods.