I've played both positions, and there is a lot more movement at first base. You are involved in so many more plays at 1B., and even on the groundballs that go through the IF, the first baseman is running to cover the bag at the crack of the bat. You can't wait to see if the ball goes through before you head to first or it is too late. At 3B, you can stand and watch plays to the right side of the IF. First base has it's share of diving plays as does 3B. It just doesn't have the long throw. I really don't see the argument that 3B is more taxing. It can be more frightening, because it is the hot corner. You need quick reflexes at 3rd, but other than that, first base has much more movement. As far as I know, Youk's physical issues have not been related to his throwing arm.