MCGOWAN, C.P.; CARD, G.; BIEWENER, A.A.: Muscle Work in Hopping Wallabies
Unlike other bipeds and quadrupeds, wallabies do not increase O2 consumption with increased hopping speed. This unique terrestrial locomotor economy has led to several studies on the mechanics of hopping and elastic energy recovery. To date, studies have focused primarily on the large Achilles tendon and the extensor muscles of the ankle in animals moving at constant speeds on a level treadmill. In the wild, these animals are faced with a wide range of terrain through which they must navigate. The primary goal of this study is to characterize whole limb mechanics for hopping at steady state-speeds, accelerating, decelerating and hopping up inclines. We wished to determine the relative distribution of work done by the muscles at each joint under these varying conditions. High-speed video and force plate recordings were used to determine external moments acting at the joints of six tammar wallabies hopping on a level and incline runway. A weighted mean muscle moment arm at each joint was determined from anatomical dissection in order to calculate the amount of muscle force required to balance these external moments. In level hopping trials hip extensors consistently contribute positive work (0.8J/Kg) increasing with increased acceleration (1.2J/Kg). Ankle extensors act to absorb energy in decelerations (-2.0J/Kg) and produce power (1.0J/Kg) for accelerations, while muscles acting at the MP joint absorb energy (-0.7 to -1.0J/Kg) under all conditions. External moments at the knee required that flexor muscles be active in the first half of the stance phase. Preliminary results suggest that this external moment is actually countered by two-joint extensor muscles of the hip and ankle, which act to flex the knee as well as extend the hip and ankle. It is likely that the extensor muscles of the knee are active at this time to aid in decelerating the animal.
