Recent research suggests that locomotor training can improve human walking ability after neurological injury. When stroke and spinal cord injury patients practice stepping with manual assistance, they recover mobility more quickly due to task-specific motor learning. Although multiple studies support the efficacy of this rehabilitation method, there is considerable debate about the extent of motor adaptation possible in the human locomotor pattern. Some animal and clinical studies indicate that muscle activation patterns during locomotion are hardwired into the nervous system and incapable of substantial modification. This would suggest that there are limits to locomotor training as a therapeutic tool. The proposed research project will use powered ankle-foot orthoses to study human locomotor adaptation. The powered orthoses will exert a torque about the ankle joint, altering normal lower limb kinematics if muscle activity patterns are not modified. As a result, these studies will test the relative invariance of muscle activity patterns and lower limb kinematics during human locomotion. This will not only provide the opportunity to study human locomotor adaptation under controlled experimental conditions, it will also provide a means to test the hypothesis that the nervous system controls lower limb movements during locomotion based on kinematics. The overall objectives of the proposed research are 1) to determine the extent of motor adaptation possible in the human locomotor pattern and 2) to test and hypothesized neural control strategy for human walking. Healthy human subjects will walk while wearing carbon fiber ankle-foot orthoses that are powered by artificial pneumatic muscles and controlled via proportional myoelectrical control. The studies will test the hypothesis that subjects will modify their muscle activity patterns when walking with powered orthoses to maintain joint kinematics similar to normal walking. In addition to providing important insight into the neural control of human locomotion, the project will advance robotic technologies for assisting gait rehabilitation and controlling powered lower limb prostheses.