Groundstrokes: your arm should feel like a dead piece of meat?
This is what my coach told me.
If you feel like you are using ANY muscles in your arm or shoulder during a forehand or backhand (especially forehand) then you are 'arming' the ball into play, and getting far less power and spin than adequate and you WILL injure yourself eventually.
Instead, he said the ENTIRE movement should be generated by your legs and core.
I've tried playing like this and not using any muscles in my arm or shoulder whatsoever (and gripping the racket just tightly enough so that it doesn't fly out of my hands) so that my arm flogs around my body like a dead piece of meat, but I can't get any accuracy on it, and I need to literally jump off the ground and spin 360 degrees around in the air on every forehand to get any sort of power on it either.