Meeting Abstract
Frogs exhibit a varied suite of locomotor behaviours. Intrinsic to this repertoire is the property of selected joints to rotate with three degrees of freedom. In the hip joint, this is reflected by the complex musculature surrounding the acetabulum. Hip muscles acting in the flexion-extension and adduction-abduction planes are well-known contributors to locomotor behaviours, such as jumping. Long axis rotation (LAR) of the femur is less frequently studied, in part due to the challenges associated with measuring LAR in-vivo. We have performed a kinematic simulation to assess the effect of LAR on muscle function during jumping, using empirical data gathered from Kassina maculata (the red-legged running frog) and a musculoskeletal model implemented in the physics engine MuJoCo. We imposed a ± 20° LAR onto the femur of K. maculata during a static point in the early, mid and late stance phase of a jump. The three-dimensional moment arms of hip muscles were measured across this range of LAR, and are presented in the context of anatomical reference planes. We found that the imposition of LAR switched the function of the obturator externus, gluteus maximus, iliacus externus, and iliolumbaris muscles. These muscles are generally of low power, with a mean PCSA of 2.7 mm2 (compared with the more powerful knee extensor muscles), so the functional significance of moment arm sensitivity to LAR requires further investigation. We speculate that these muscles may shift their functions if the degree of LAR changes during locomotor behaviours. Because of their low force capacity, we expect them to play an important role in modulating power produced by larger muscles. We will test our model’s predictions by measuring LAR in jumping frogs using xray reconstruction of moving morphology (XROMM).
