| The goal of this thesis is to investigate the neuromuscular control strategies animals use to maintain dynamic stability in rough terrain. Little is known about the behavioral strategies animals use to achieve dynamic stability and the associated neuromuscular performance requirements. The complexity of animal musculoskeletal systems appears 'redundant' for steady level locomotion because many muscle combinations could accomplish the task. However, muscles that play similar roles in steady movement might play distinct roles in more mechanically demanding tasks such as stabilization after a disturbance. I used an integrative experimental approach to reveal the interplay between muscle-tendon architecture, neuromuscular performance, and body mechanics during unsteady locomotion. I first measured in vivo performance of two guinea fowl ankle extensors during incline locomotion and revealed that they perform distinct mechanical roles due to differing tendon architecture. I then measured body mechanics, limb dynamics and ankle extensor performance as guinea fowl ran over (1) randomized, unexpected drops in substrate height (2) visible drops in substrate height, and (3) camouflaged, rapidly repeating obstacles on a treadmill. These studies reveal the critical role that limb posture and distal limb muscle performance play in the control of stable running. I provide evidence that the limb exhibits a proximo-distal gradient in motor control, in which muscles at the proximal joints operate in a feedforward manner, insensitive to perturbations, whereas the muscles at the distal joints are highly sensitive to altered interaction between the limb and ground. This control strategy improves stability by keeping limb cycling relatively constant, but allowing the distal limb to adjust rapidly to altered limb loading. In this manner, the distal muscles produce or absorb energy to control velocity and restore equilibrium limb posture in response to perturbations. The results are consistent with the hypothesis that a tradeoff exists in limb design for running efficiency versus dynamic stability, making predictions about differences in stability and distal muscle performance among species, which could be tested by expanding the approach in a comparative context. Consequently, these studies provide a foundation for revealing the relationship between limb morphology, neuromuscular function and the control of stable running. |
