Adaptive mechanisms in the control of interlimb coordination during locomotion

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Interlimb coordination is essential for dynamic stability when navigating in an ever-changing environment and to change locomotor speed, involving complex dynamic interactions at different levels of the central nervous system and emergent properties of the musculoskeletal system. Despite its obvious importance, the dynamic control of interlimb coordination is poorly defined in terrestrial mammals. The long-term goal of my research program is to elucidate neurophysiological and biomechanical mechanisms that control interlimb coordination during locomotion. To accomplish this goal, cats are trained to step on a treadmill with four separate surfaces whereby the speed of the forelimbs and hindlimbs or of the left and right sides are independently controlled. The belts can operate at the same speed (tied-belt locomotion) or at different speeds (split-belt locomotion). Thus, speed or speed differences between limbs can be systematically manipulated to challenge interlimb coordination, revealing adaptive mechanisms. Chronic implantations are used to record muscle activity and evoke reflexes while three-dimensional motion capture acquires detailed locomotor kinematics. Comparisons are made in intact and spinal-transected cats to define the contribution of spinal mechanisms in coordinating the limbs. The FIRST OBJECTIVE is to characterize reflex modulation during tied-belt and split-belt locomotion. Reflexes are essential to quickly adjust to external perturbations. We hypothesize that reflex amplitude 1) will be modulated with task and 2) will be less variable on a step-by-step basis during split-belt locomotion. We also hypothesize that results will be similar in intact and spinal cats, indicating that reflex modulation is primarily determined at a spinal level. The SECOND OBJECTIVE is to characterize patterns of muscle activity during tied-belt and split-belt locomotion. We hypothesize that 1) muscle synergies make use of the bifunctional nature of some muscles, 2) synergies can extend to more than one limb and 3) they are primarily controlled at a spinal level. The THIRD OBJECTIVE is to characterize the dynamic coordination within and between limbs during tied-belt and split-belt locomotion. We hypothesize that interlimb coordination self-organizes when adjusting to speed and to constraints imposed on the interlimb pattern. The proposed research will provide a new depth of understanding on the neuromechanical mechanisms that control interlimb coordination during locomotion. This is of particular significance because knowledge of how the neural and biomechanical systems interact to dynamically coordinate the limbs during real movements is crucial for designing biologically realistic robots and efficient neuroprosthetic devices as well as for maximizing athletic performance and motor rehabilitation.