Originally Posted by bhupaes
Sennoc, when you say the upper arm contributes, you mean upper arm rotation as in internal shoulder rotation, right? This would move the racquet in the same way (almost) as forearm pronation, and because of this, a lot of people probably consider upper arm rotation to also be pronation.
The problem is that both kind of rotations work at different parts of kinetic chain. Internal shoulder rotation is a source of energy at serves when there is L between upper arm and forearm. Pronation (of forearm) as a source of energy works when we have L between forearm and racquet. As a result, there are totally different forces acting at elbow/wrist. If you think that pronation of forearm is a source of energy, you will force your wrist to do impossible things. This is not a source of energy, this is source of injuries.
LeeD, find another thread.