HNOT, Melanie*; GILLIS, Gary B.; Mount Holyoke College; Mount Holyoke College: EMG Activity in Hindlimb Flexor Muscles During Swimming in the Toad, Bufo marinus

Work on hindlimb muscle function during anuran locomotion has focused largely on major extensors of the hip, knee and ankle. A common result from this work is that these muscles are activated simultaneously at the onset of a propulsive stroke, and exhibit one discrete burst per swim cycle. In this study, we used electromyography to explore the roles of limb flexor muscles during swimming in the toad, Bufo marinus. We measured electromyographic (EMG) activity from a hip flexor (iliacus externus), knee flexor (sartorius) and ankle flexor (tibialis anterior), as well as from a knee extensor, the cruralis. Timing of EMG activity was scaled to fractions of a swim cycle, using onset times of the cruralis to define the start and end of a cycle. Results from 3-4 individuals per muscle revealed that the sartorius exhibits a double-bursting pattern with discrete bouts of activity during limb extension and limb flexion. Flexion-related activity begins approximately 48% of the way through the swim cycle and ends as the animal prepares for its next stroke at 85% of the swim cycle. Activity in the tibialis anterior involves a single burst that begins at 63% of the swim cycle, and extends into the early portion of the next stroke, overlapping with activity in the cruralis. Activity in the iliacus externus is not in discrete bursts, but shows some level of activity through most of the swim cycle. Our results show that, unlike extensors, limb flexor muscles are not activated simultaneously during swimming. Moreover, co-activation of flexors and extensors is present during limb extension at all joints. Finally, the double-bursting pattern present in the sartorius is comparable to the biphasic activity found in various knee flexors during terrestrial locomotion in other tetrapods.