Photo Study of Novak Djokovic's Strokes

Here, Novak's largest muscles are all loaded up to deliver energy to his serve through a kinetic chain. When he straightens up from his big knee bend and body arch, the released energy will be transferred to his arm and ultimately into an extremely fast whipping upward of the racquet. This degree of body arch usually indicates a strong component of topspin in a serve. In many players, it would signal a twist serve, which is usually at least 80% topspin.