Meeting Abstract
Kangaroo rats (D. deserti) are bipedal hopping desert rodents that use vertical jumps to escape their predators (mainly owls and snakes). Previous research on the mechanics of these jumps indicates that the ankle is the main contributor to net joint work when comparing across jump heights. In addition, we found 30% of the net joint work measured at the ankle is transferred from proximal muscles via the bi-articular ankle extensors. Give these results, the lateral gastrocnemius (LG) muscle, a large ankle extensor, and the vastus lateralis (VL) muscle, a large knee extensor, likely both play important roles in propelling the animals up during vertical jumps. In the present study, we examined the in vivo performance of the LG and VL muscles during vertical jumping over a range of jump heights with the use of sonomicrometry and electromyography (EMG). We hypothesized that muscle shortening strain would be independent of jump height, but that relative EMG intensity would increase with increasing jump height. Preliminary data suggest that our hypotheses are supported. There appears to be a linear relationship between jump height and relative EMG intensity for both muscles, whereas net muscle shortening does not change. This suggests that the increase in mechanical work required to achieve higher jumps is likely due to great muscle force and not greater muscle length changes.
