Originally Posted by boramiNYC
^^^in that vid I can clearly see pronation. at trophy the hand is neutral but the upward thrust of the shoulder leads supination of the hand and rapid pronation through contact. Of course ESR/ISR is present as well.
typically 'pronation' in serve indicate not the pronated position of the hand but rotation of the hand toward the pronated hand position from neutral or supinated position.
I'm sure pronation and supination occur early in the service motion, at trophy position, etc. but have not looked much at that.
Where does the final racket head speed come from?
After the arm is straight and the racket has very little racket head speed toward the ball, in other words, when the racket head acceleration is about to start, I can clearly see forceful ISR (humerus rotation) but I cannot observe pronation (forearm only rotation) because it is too difficult or small to see.