Meeting Abstract
Within large bipedal hoppers, adaptions of the ankle extensor muscle-tendon units that facilitate economical elastic energy storage also limit their ability to generate mechanical work when moving up slopes. Therefore, a division of labor exists such that the large proximal muscles behave as motors while the distal muscle tendon units behave like springs. However, unlike larger hoppers, kangaroo rats have relatively thick ankle extensor tendons which are unlikely to stretch during normal hopping loads. In a previous study of desert kangaroo rats, we used a joint level analysis of hopping on inclines to show that both proximal and distal joints contribute to providing the mechanical power necessary to raise the animal’s center of mass. However, a simple geometrical model based on kinematics and moment arms suggests that the underlying muscle dynamics may not reflect the work being delivered at specific joints. Therefore, in this study we used in-vivo recordings of muscle length change and activity to directly measure how two major extensor muscles are modulated in response to a range of inclines. Sonomicrometry and EMG data were collected from the lateral gastrocnemius (LG) and vastus lateralis (VL) while animals hopped on a motorized treadmill at 0, 10, and 20 degrees incline. Preliminary results show that the VL actively stretches, likely absorbing energy, under all conditions; however, the amount of stretch decreases with increasing incline. The LG undergoes increased active shortening with increasing incline, suggesting that this muscle contributes increased positive mechanical work when hopping up a hill. Therefore, while the knee extensors appear to behave in a similar manner to those of wallabies, the ankle extensors do not, suggesting there is no division of labor.
