Testing the propulsive role of m. peroneus longus during quadrupedal walking in Varanus exanthematicus.

Abstract

Some varanid lizards show a prominent and highly distinctive lateral calcaneal process. It has been posited that this structure serves as a lateral "heel" to increase the moment arm for m. peroneus longus, allowing it to function as a powerful propulsive muscle. However, to confirm that m. peroneus longus serves this function requires electromyographic data showing activity during tarsal plantarflexion in the late part of the stance phase. Muscle activity patterns of m. peroneus longus, m. tibialis anterior, and mm. gastrocnemii were collected from two savannah monitors (Varanus exanthematicus) during quadrupedal walking. Across strides, m. peroneus longus shows an early onset just before hindlimb touchdown and an offset that is highly correlated with that of mm. gastrocnemii. These patterns are consistent across individuals. However, the fact that the first onset of m. peroneus longus appears to be around the end of swing phase, with activity continuing throughout the remainder of stance, suggests that this muscle likely serves other functional purposes during locomotion beside propulsion. This, paired with the fact that qualitative comparisons of m. peroneus longus activity across other lizard species reveal remarkably similar patterns, suggests the propulsive role of m. peroneus longus in V. exanthematicus was probably built upon existing muscle activity patterns present in ancestral squamates and then exaggerated through modifications to lateral calcaneal heel and the associated proximal expansion of the fifth metatarsal.