Originally Posted by sennoc
As example, pronation of forearm is one of the most important sources of energy at topspin forehands and (supination) at topspin backhands (30-40%!).
Sennoc, your posts have literally been a treasure trove of information - thanks! The mistake I made today was to think about all of these things while playing doubles, and I couldn't get a first serve in to save my life... luckily, my second serve was working well!
The other thing is the significance of the high elbow. I found that I could not get my elbow high unless my body was tilted. One significance of it therefore is that a high elbow means a good cartwheeling motion. Of course, as you point out, it also enables better utilization of upper arm rotation.
I have a question regarding the portion of your post that I have quoted above. Isn't there a lot of biceps action and also upper arm rotation in the topspin forehand, that contribute both to spin and power? Here also, isn't the wrist by itself too weak to do anything other than to orient the racquet head correctly? Try to isolate the pronation movement by holding your upper arm with your non-dominant hand to prevent it from rotating in a topspin forehand motion, to see what I mean. After reading tricky's posts regarding transverse adduction of the shoulder, I now believe that is also a major power source.
A similar argument applies to the one-handed backhand as well, except the shoulder rotation is external.