Meeting Abstract

91.6  Saturday, Jan. 7  Acceleration mechanics in Desert Kangaroo Rats (Dipodomys desertii) MCGOWAN, Craig; University of Idaho

Wallabies are known for their ability to decouple metabolic cost from hopping speed, largely due to elastic energy recovery from long, thin ankle extensor tendons. Smaller species of bipedal hoppers, such as kangaroo rats, have relatively thicker ankle extensor tendons and do not benefit from substantial elastic energy recovery. In a previous study of tammar wallabies, we showed that despite having thin tendons, the ankle joint was primarily responsible for modulating mechanical work during accelerations. However, the majority of the work was likely being transferred from more proximal muscles via biarticular muscles. Further, tammar wallabies changed limb posture such that ankle joint moment (and tendon stress) was independent of acceleration. The goal this study was to determine if kangaroo rats modulate joint level mechanical work for acceleration in a similar way as wallabies, or if relatively thinker ankle extensor tendons enable kangaroo rats to generate higher peak ankle moments or do more mechanical work with their ankle extensor muscles. We measured ground reaction forces and high speed video from eight desert kangaroo rats (D. desertii) over a range of hopping accelerations (-7.8 m/s to 10.1 m/s). We used an inverse dynamics analysis to calculate joint moments, power and work. In addition, we used a geometric model to estimate muscle work by the ankle extensor muscles. Consistent with tammar wallabies, our results showed that the ankle joint played the greatest role in modulating mechanical power. However, the geometrical model revealed that the ankle extensor muscles absorbed little mechanical energy in early stance, but generate a large amount of mechanical work in late stance, especially in large accelerations. These results suggest that despite exhibiting similar joint level mechanics during accelerations, the functional role of the ankle extensor muscles differs substantially between tammar wallabies and desert kangaroo rats.