Crawling without wiggling: muscular mechanisms and kinematics of rectilinear locomotion in boa constrictors.

A central issue for understanding locomotion of vertebrates is how muscle activity and movements of their segmented axial structures are coordinated, and snakes have a longitudinal uniformity of body segments and diverse locomotor behaviors that are well suited for studying the neural control of rhythmic axial movements. Unlike all other major modes of snake locomotion, rectilinear locomotion does not involve axial bending, and the mechanisms of propulsion and modulating speed are not well understood. We integrated electromyograms and kinematics of boa constrictors to test Lissmann's decades-old hypotheses of activity of the costocutaneous superior (CCS) and inferior (CCI) muscles and the intrinsic cutaneous interscutalis (IS) muscle during rectilinear locomotion. The CCI was active during static contact with the ground as it shortened and pulled the axial skeleton forward relative to both the ventral skin and the ground during the propulsive phase. The CCS was active during sliding contact with the ground as it shortened and pulled the skin forward relative to both the skeleton and the ground during the recovery phase. The IS shortened the ventral skin, and subsequent isometric activity kept the skin stiff and shortened during most of static contact while overlapping extensively with CCI activity. The concentric activity of the CCI and CCS supported Lissmann's predictions. Contrary to Lissmann, the IS had prolonged isometric activity, and the time when it shortened was not consistent with providing propulsive force. Decoupling propulsion from axial bending in rectilinear locomotion may have facilitated economical locomotion of early snakes in subterranean tunnels.