Originally Posted by tennis_balla
"In this preliminary study, we looked at high-speed video clips of the topspin forehands of the three (3) active players who have won two or more Grand Slam singles titles: Roger Federer, Rafael Nadal, and Novak Djokovic during tournament play and practice on hard courts.
Specifically, we measured the amount of topspin (in RPMs) generated and recorded the type of racket motion through the impact zone – whether or not the racket tilted forwards (to the ground) or backwards (to the sky), or remained entirely stable after impact for a minimum of 60 forehands
(struck in a wide variety of situations/court positions) per player."
If the ball makes contact above the sweet spot, it tilts toward the sky after contact; at center, it remains roughly stabble and, bellow, it further closes the face of the racket.