Delays and the control of movement in legged animals

The ability to respond quickly to a perturbation is essential to the survival of an animal. As animal size increases, several features that influence response time also change—larger animals suffer from longer sensorimotor delays, proportionally weaker muscles, and heavier body segments. Larger animals also have more time available to react due to longer characteristic movement times. I studied how these features affect neural control of the fastest perturbation responses, and I used simple neuromechanical models to estimate how response time changes with animal size. In chapters 2 and 3, I quantified the scaling of inertial delays—the time required to physically reposition body segments and regain stability after a perturbation. In chapters 4 and 5, I quantified the scaling of the fastest response times to a perturbation under two control configurations—feedforward vs. feedback control. I tested whether they are affected more by the force generation capacity of muscles or by sensorimotor delays. I developed two tasks representing common perturbation response scenarios in animal locomotion: a distributed mass pendulum approximating swing limb repositioning (swing task), and an inverted pendulum approximating whole body posture recovery (posture task). I parameterized the anatomical, muscular, and inertial properties of these models using literature scaling relationships. I found that inertial delays depended both on movement task and movement size. Inertial delays got longer with larger movements, and scaled faster in the posture task than the swing task. As movement size increased, inertial delays exceeded sensorimotor delays, and this occurred for smaller movements in larger animals. Across animal size and task, force capacity of muscles limited feedforward control response times, while sensorimotor delays limited feedback control response times and forced the use of lower controller gains to prevent instability. Feedback control response times also exceeded available movement times in animals of all sizes, while feedforward control did so only for the largest animals. Feedback control was about four times slower than feedforward control in the smallest animals, but only around two times slower in the largest animals. Thus, both small and large animals are more likely to use feedforward control to react quickly against perturbations.