  By means of adding a collision process between the ball and racket in double
pendulum model, we analyzed the tennis stroke. It is possible that the speed of
the rebound ball does not simply depend on the angular velocity of the racket,
and higher angular velocity sometimes gives lower ball speed. We numerically
showed that the proper time lagged racket rotation increases the speed of the
rebound ball by 20%. We also showed that the elbow should move in order to add
the angular velocity of the racket.
