Meeting Abstract

17.5  Saturday, Jan. 4 11:30  Net joint work provides limited insight into muscle-tendon unit function during incline hopping by kangaroo rats MCGOWAN, C.P.; University of Idaho cpmcgowan@uidaho.edu

In a recent inverse dynamics analysis of incline hopping, we showed that the proportion of net joint work supplied by each joint is similar across a wide range of slopes (10-25 degrees). This suggests that unlike tammar wallabies, there is not a division of labor between proximal and distal muscles of kangaroo rats. However, inverse dynamics analyses alone may provide limited insight into the functional roles of specific muscle groups. In a previous analysis of acceleration mechanics, we showed that because the major ankle extensor muscle-tendon units (MTUs) are biarticular, coupling of knee and ankle flexion (but not extension) resulted in the MTUs doing relatively little negative work, despite the ankle joint itself absorbing a substantial amount of energy in the first half of stance. In this study, we sought to determine if a similar coupling of knee and ankle kinematics leads to a decoupling of MTU work and net joint work during incline hopping over a range of slopes. We used a combination of inverse dynamics and geometric modeling to calculate ankle extensor MTU work during hopping on slopes of 10, 15, 20, and 25 degrees. Our results show that despite an increase in both positive and net joint work at the ankle across slopes, the net work done by the ankle extensor MTUs actually decreased with increasing slope from a maximum of 0.237 J/kg at 15 degrees to 0.188J/kg at 25 degrees. The energy absorbed by the MTUs was relatively low (mean: -0.048 J/kg) and did not vary with slope. Therefore the decrease in net work was due to a reduction in positive work done by the MTUs. These results reinforce the need to use a combination of approaches to understand the functional roles of individual muscles or MTUs during dynamic movement tasks.