" Scientists induced Parkinson’s in rats by killing the dopamine cells in their basal ganglia, and then forced half of them to run on a treadmill twice a day in the ten days following the “onset” of the disease. Incredibly, the runners’ dopamine levels stayed within normal ranges and their motor skills didn’t deteriorate. In one study on people with Parkinson’s, intensive activity improved motor ability as well as mood, and the positive effects lasted for at least six weeks after they stopped exercising. "

― John J. Ratey , Spark: The Revolutionary New Science of Exercise and the Brain