Meeting Abstract

57.4  Jan. 6  Distal hindlimb muscles play a key role in stability of running over rough terrain DALEY, M.A.*; BIEWENER, A.A.; University of Michigan, Ann Arbor; Harvard University, Cambridge mdaley@umich.edu

Using a novel obstacle treadmill experiment, we test the hypothesis that distal limb muscles rapidly modify work output for stabilization tasks. We examine the in-vivo force-length performance of the lateral gastrocnemius (LG) and digital flexor-IV (DF) of guinea fowl as they negotiate camouflaged obstacles that repeat every 10-12 steps. We expect the LG and DF to differ in stabilization performance due to differences in architecture. Although both have pinnate, short fibered architecture, the DF has a long, compliant tendon that crosses all of the distal joints, whereas the LG has a relatively short, stiff tendon. The results confirm that the LG and DF play distinct roles, consistent with observed differences in architecture. The LG acts as a �length-dependent actuator�, producing energy when the limb begins stance in a more crouched posture. The DF acts as a �velocity-dependent actuator� absorbing or producing energy depending on how rapidly the limb is loaded. The results suggest that intrinsic mechanical factors explain much of the change in distal muscle performance, enabling rapid posture and load dependent changes in limb work. Additionally, the DF might act as an important sensor, eliciting rapid proprioceptive feedback that excites agonist muscles through heterogenic reflex pathways. These muscles provide a direct link between limb posture and limb energy performance that likely helps maintain stability and prevent injury during running over rough terrain. The results provide further support for the idea that limb muscles exhibit a proximo-distal gradient in function and motor control. Further, they provide insight into potential performance tradeoffs of different muscle-tendon architectures for efficiency, stability and agility.