Hey, I kind of hope this discussion continues...
I can hit 60-70% of my full potential of my serve by serving
with arm action only. Shoulder joint, elbow joint and pronation
can contribute up to 70% of racquet head speed, I've read.
So these must be the primary muscle group for serving !
I do this sometimes when I lost rhythm on my serve.
Actually action at wrist joint contribute large amount for the
final racquet head speed simply because it's the joint closest to
the racquet ! But I guess it should not be intentional.
It should snap a bit naturally because the huge kinetic energy
is going up and whipping action at the end !
Any stiff joint will put a break on transfering kinetic energy
being built from bottom up. As I grow older, I don't lose
much of muscle power(up until mid 50s,I've read from somewhere) but I get less flexible
and thus the deadly explosiveness of my big serve is disappearing !!
"I mean, you have to get emotionally involved. Otherwise, you're doing the wrong thing, you have the wrong job." - Wilander, after French Open 2008