What, the endless hours of standing around doing nothing on the field compared to the endless hours of standing around doing nothing in the dugout?
I'm guessing here, but I'm pretty sure the condition coaches have each and every player on a plan to keep them in shape. Heck, Holt and Pedroia are just two players who were playing everyday who this year had a hammy thingamajig. And there are others I probably can't think of right now.