Extensor muscles of the leg are actively involved in weight support and walking activities. These muscles are extensively linked by inhibitory, bidirectional, force dependent pathways that contribute to the successful execution of a range of functional behaviors, including locomotion. These reflex pathways are thought to arise from Golgi tendon organs and are believed to regulate limb stiffness and promote inter-joint coordination during movements. The relative magnitudes of these linkages in the two directions between any two muscles vary across control decerebrate animals when quiescent, but obey a predominantly proximal to distal gradient during stepping on a treadmill. This finding indicates that the strength and distribution of these reflex pathways are subject to modulation in a task-dependent manner. Our preliminary data suggest that thoracic spinal cord lateral hemisection immediately and persistently alters the normal distribution and a dominant distal-to- proximal inhibitory gradient emerges. Animals with this lesion do not exhibit clasp-knife inhibition, a phenomenon known to be mediated by receptors other than Golgi tendon organs and that results from bilateral injury to the dorsal half of the spinal cord. The change in the strength and distribution of force feedback that we have observed is correlated with diminished limb stiffness and poor weight acceptance during locomotor tasks – both of which are challenging problems seen in humans with spinal cord injuries. These findings provide new insight into potential mechanisms contributing to disruption of motor function following injury and identify a new, potential rehabilitation strategy. Our guiding hypothesis is that SCI-induced force-feedback dysregulation results in strong inhibition directed toward proximal muscles, contributes to inadequate limb stiffness during weight support phases of movement, and can be reversed using eccentric- focused training. The current application has evolved from collaborative work, using a large animal model, across two established laboratories, bringing together expertise in spinal cord injury, plasticity, force feedback and motor control. The proposed studies are divided into two sets of experiments. The first set will carefully characterize the impact of a low thoracic hemisection on gait kinematics during subphases where inhibitory force feedback is thought to be most active (Aim 1a). These phases typically are associated with coordinated eccentric activity and/or weight acceptance/support. Pre- and post-SCI data, across time, will be compared. In the same animals, a comprehensive picture of force-feedback organization following SCI will be developed during terminal decerebrate studies at two chronic time points (Aim 1b). Both standing and walking preparations, in combination with mechanographic approaches, will be used to test task-specificity of heterogenic force feedback responses in specific muscle combinations. In the second set of experiment...