I have searched online and found that the power of a punch P=Fv, where F is the force of the punch and v is the speed. But in my mind it works like this: let's just say that only the arm speed is taken into account, then before impact, the energy of the arm travelling is E=(1/2)mv^2. That is how much work the target has to do to stop the punch. Therefore, the impact force Fi = E/L where L is the displacement before the arm reaches zero speed. In conclusion, the impact of a punch is proportional to m & v^2?