Meeting Abstract
Bouncing gaits such as running, trotting and hopping are believed to be efficient because they enable animals to utilize elastic energy storage and recovery, much like a bouncing ball. However not all bipedal hoppers have compliant tendons that can serve as springs. Although considerably smaller, kangaroo rats use a bipedal hopping gait that is very similar to that of kangaroos and wallabies. Yet their ankle extensor tendons are relatively thick and stretch very little during hopping. Inverse dynamics analyses suggest that while at the joint level kangaroo rats maintain spring-like behavior, this does not reflect the mechanical output of the underlying muscle-tendon units. Rather, dynamic coupling of the joints via biarticular muscles enables muscles to behave as pure motors or dampers. In this study, we use in vivo measurements of muscle dynamics from the lateral gastrocnemius (LG) to test the hypothesis that the ankle extensor muscles of kangaroo rats behave as motors, actively shortening during stance, across multiple grades. Data were collected while animals hopped on a treadmill inclined to 0, 10 and 20 degrees. Preliminary results of this study support our hypotheses. The LG undergoes active net shortening at all grades and the magnitude of net shortening increases with increasing grade. While there were no direct measurements of muscle force, these data suggest that the LG generates net positive mechanical work during hopping and is a primary contributor to generating the positive work necessary to raise the center of mass when hopping up an incline.
