Alright, I've had a long hard think about what the hell Mark's power is going to develop into, and I've come up with flexible muscles.

Again, they will take energy to activate, but they can move as fast as he wants them to without causing damage. How fast he moves them depends on how much energy he puts into them. However, if he were to land a punch at 1000mph, his fist would turn to dust, and there is no way to counter that. However, using this energy doesn't really tire him physically, but mentally. He'll still be fit as a fiddle, but he'll still feel exhausted.

Is that better?