DALEY, M.A.; Harvard University, Bedford: Dynamics of ankle extensor function during stabilization in running guinea fowl

How do animals maintain stability in the face of unexpected disturbances during locomotion? This study investigates how limb and muscle dynamics respond to sudden changes in substrate level during avian bipedal running. Guinea fowl ran along a runway with a camouflaged “false floor” section made of paper, imposing a sudden drop in substrate. Ground force and high-speed video recordings were taken simultaneously with in vivo measures of muscle force, length and activity from the lateral gastrocnemius (LG) and digital flexor-IV (DF-IV). The goals of the study are 1) to investigate the dynamics of the control response and 2) to test the hypothesis that the DF-IV plays an important role in mediating the intrinsic response within the disturbed step, while the LG plays a greater role in mediating recovery in subsequent steps. During steady, level running the LG and DF-IV develop force during the support phase of the stride. The LG shortens throughout force development, while the DF-IV stretches until peak force and shortens during force decline. When a bird initially encounters the false floor, limb and muscle dynamics do not differ from control runs. However, shortly after contact the limb breaks through and continues to extend until the substrate is again encountered. Concurrently, muscle force rapidly declines at the loss of resistance and increases again at ground contact resulting in a double peaked pattern of force development. The pattern of length change is largely unchanged for the LG, while DF-IV changes from a stretch-shorten to a shorten-stretch pattern. These results show that force development by both muscles responds rapidly to the disturbance, but DF-IV length dynamics are more sensitive than those of the LG to changes in the interaction between the limb and environment. (Supported by NIH R01-AR047679 and an HHMI Predoctoral Fellowship)