We are presenting direct comparison of high-speed (240 fps) and a normal speed video (30 fps).
Both videos shows the part of the service motion in tennis, which is invisible to the human eye, thanks to the racket head speed at this stage of the serve.
In both videos you can see a motion of hitting arm from a position of maximum external rotation of the shoulder to the point of contact and the action of a forearm (pronation) and wrist.
240 fps
Tennis Serve - High-Speed Video Footage...
30 fps
Tennis Serve – Normal Speed Video Footage...