There is some controversy on whether a pitcher develops any lactic acid build up in the small muscles of the throwing arm/shoulder. There is a general consensus that running promotes greater blood flow, which helps the healing process of the broken down muscle tissue from pitching. I will have to go back through some of my stuff to find a specific reference by a medical professional. If you are really interested I would pose the same question on the ASMI board and see what they say. I glanced at it and did not see this question on their board.