Centrifugal force - Wikipedia, the free encyclopedia
Physics for the win!
You don't need to tense any of the muscles in your hand in order to run, but to avoid fingers flapping from the changes throughout the arm swing, people tend to point their fingers or clench their fists. My old cross-country coach always said to "Hold a potato chip, and don't break it" to get us to relax our hands when running at a moderate pace. I don't think that's really all that relevant when sprinting, though.