Muscular mechanisms of snake locomotion: an electromyographic study of the sidewinding and concertina modes of Crotalus cerastes, Nerodia fasciata and Elaphe obsoleta.

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Synchronized electromyography and cinematography were used to determine the muscle activity of colubroid snakes during sidewinding and concertina locomotion. The primary muscles studied were the three largest, most superficial epaxial muscles: the Mm. semispinalis-spinalis, longissimus dorsi and iliocostalis. Sidewinding locomotion of Nerodia fasciata and Crotalus cerastes was the result of continuous posterior propagation of contractile blocks consisting of several adjacent muscle segments. During sidewinding, the activity of the M. longissimus dorsi and M. iliocostalis was primarily unilateral, beginning when a body region was convex and ending when it was maximally concave on the side of the active muscle. Unilateral activity of the M. semispinalis-spinalis correlated with lateral flexion in addition to bilateral activity that correlated with dorsiflexion of the vertebral column. During concertina locomotion of N. fasciata and Elaphe obsoleta, muscle activity also involved blocks of several simultaneously active adjacent muscle segments, but all major activity was unilateral and was not propagated posteriorly in a simple continuous fashion. Muscle activity during concertina locomotion correlated either with lateral flexion towards the side of the active muscle or with the maintenance of static contact with the sides of a tunnel. The number of simultaneously active adjacent muscle segments and the maximum duration of continuous muscle activity varied significantly between Nerodia and Elaphe and among the different widths of tunnels. Theoretical considerations combined with observed differences suggest that the more elongate body of Elaphe is advantageous for performing concertina locomotion. There was no consistent evidence that nonhomologous muscles with tendinous interconnections functioned as single units during either of these two locomotor modes. Although individual segments of the studied epaxial muscles span several vertebrae, via long, tendinous connections, consistent kinematic correlations with muscle activity were observed only between the contractile portion of a muscle segment and the vertebrae adjacent to that contractile portion.