Biomechanics and control of landing in toads.

Abstract

Anything that jumps must land, but unlike during jumping when muscles produce energy to accelerate the body into the air, controlled landing requires muscles to dissipate energy and decelerate the body. Among anurans, toads (genus Bufo) exhibit highly coordinated landing behaviors, using their forelimbs to stabilize the body after touch-down as they lower their hindlimbs to the ground. Moreover, toads land frequently, as they cover distances by stringing together long series of relatively short hops. We have been using toads as a model to understand the biomechanics and motor control strategies of coordinated landing. Our results show that toads prepare for landing differently depending on how far they hop. For example, the forelimbs are extended farther prior to impact after long hops than after short ones. Such kinematic alterations are mirrored by predictable modulation of the recruitment intensity of forelimb muscles before impact, such that longer hops lead to higher levels of pre-landing recruitment of muscles. These differences in kinematics and muscular activity help to control the most flexed configuration of the elbow that is achieved after impact, which in turn constrains the extent to which muscles involved in dissipating energy are stretched. Indeed, a combination of in vivo and in vitro experiments has shown that the elbow-extending anconeus muscle, which is stretched during landing as the elbow flexes, rarely reaches lengths longer than those on the plateau of the muscle's length-tension curve (where damage becomes more likely). We have also been studying how movements of the hindlimbs after take-off help to stabilize animals during landing. In particular, the immediate and rapid flexion of a toad's knees after take-off leads to a repositioning of the animal's center of mass (COM) that better aligns it with ground-reaction forces (GRFs) at impact and reduces torques that would destabilize the animal. Finally, recent work on sensory feedback involved in preparation for landing demonstrates that vision is not required for coordinated landing. Toads can effectively utilize proprioceptive and/or vestibular information during take-off to help inform themselves about landing conditions, but may also use other sensory modalities after take-off to modulate landing behavior.