Axon regeneration in peripheral nerves is enhanced by exercise in the form of modest daily treadmill training. This project will study three inter-related aspects of exercise as a potential therapy for treatment of peripheral nerve injuries to help move it closer to actual translation. Marked sex differences in the efficacy of different training paradigms to promote axon regeneration exist and androgens are implicated in both sexes. In Aim 1, the androgen dependence of the effect of exercise will be studied in conditional androgen receptor knockout mice. The effects of different exercise paradigms on the lengths of regenerating axons, the numbers of neurons whose axons regenerate successfully, and the expression of brain derived neurotrophic factor (BDNF) and its receptor, trkB, in motoneurons, dorsal root ganglion neurons, and growth cones of regenerating axons will be studied both in systemic and neuron-specific knockouts. As a therapy, different exercise paradigms will be needed for use when different nerves are injured. Establishing guiding principles for their design will be important. The most obvious of these principles may be that the activity of neurons whose axons are regenerating is both necessary and sufficient for the enhancement of regeneration produced by the exercise, but this has not been studied in vivo. In the experiments in Aim 2, axotomized neurons will be activated directly using optogenetic technologies, to simulate their activity during treadmill training in male and female mice, and the sufficiency of this activation will be assayed by measuring the growth of regenerating axons and the expression of BDNF and trkB in neurons and growth cones. Using similar technologies in different mice, neuronal activity will be blocked during exercise and the effects assayed similarly to determine the requirement for activity for the effects of exercise. Whether exercise or optical activation of neurons that results in enhanced axon regeneration also leads to improved functional recovery will be studied in Aim 3. Nerve conduction studies will be used to monitor the time course of muscle reinnervation and H reflex restoration in exercised mice and optically activated mice. Kinematic analyses of limb movements during walking on different slopes will be used to measure these effects on functional recovery behaviorally Effects of sex and training/activation paradigm will be studied separately. This potential therapeutic use of exercise addresses an important public health issue. It is low-tech and requires only modest supervision by skilled practitioners. Patients can assume a lot of the responsibility for their own functional recovery. At the end of this study we will have acquired evidence in support of three significant aspects of the efficacy of exercise that will add to its translational promise.