"Science Daily — The risk of developing Parkinson's disease may be reduced with moderate to vigorous exercise or other recreational activities, according to recent research presented at the American Academy of Neurology's 59th Annual Meeting in Boston, April 28 -- May 5, 2007."
Big study, 143,000 people over 63 years old, over a ten year period. Moderate to vigorous exercise is defined as at least 30 minutes of exercise daily. They found these people were 40% less likely to develop Parkinson's disease than the low to no activity group. No explanations for this yet.