Strength training of a limb muscle on one side of the body produces functionally and clinically meaningful increases in motor output of the same muscle on the opposite side of the body. Such adaptations occur without muscle hypertrophy, implying a role for the ipsilateral motor cortex and the spinal cord in force transfer. The goal of this project is to determine the interhemispheric and spinal mechanisms of force transfer in healthy young adults. We hypothesize that the effects of unilateral practice on interhemispheric inhibition (IHI) and on the excitability of the contralateral corticospinal projections are linked, and these effects are graded according to the nature of muscle activation. In a series of experiments on healthy adults age 18 to 60, we will characterize the motor cortical inhibitory and excitatory mechanisms of interhemispheric practice-induced transfer of muscle force.