This study will determine how the brain controls movements by sending messages to the spinal cord and muscles. Researchers want to know if strengthening a hand muscle will increase the strength of the same muscle in the other hand, and if these changes happen in the brain or spine, or both. Plasticity in this study refers to the capacity for continuous changes of the neural pathways in the brain and nervous system. Researchers have seen cases in which using a muscle extensively (force production) in one muscle group has increased the strength of the same muscle group on the opposite side of the body (force transfer). This situation happens without the unused muscle becoming larger-suggesting that the practice causes changes in some parts of the brain or spine. It is vital for scientists to know how this effect works, so that they can create new rehabilitation methods for people who cannot move or who have difficulty moving one side of their body.