Optimal Muscle Contraction
The active muscle must be optimally pre-stretched and extended beyond its resting length. This is to ensure that an optimal desired muscle contraction in any tennis groundstroke. Therefore the active muscle elongation must occur in temporal order, namely from top to bottom. The maximum amount of force in the active muscle is reached at an elongation of 20 percent of the muscle resting length
The backswing movement begins with a backward movement of the shoulder girdle. This is followed by the backward rotation of the hips (oblique muscles of the abdomen and buttocks muscles). Then it is finally followed by thigh muscles. About the last third of the backswing backward rotation, the arm is driven also. At the end of the backswing movement the tennis player must target his back towards the net. Via the elongation process, in the form of rotation around the axis of the body, an elastic tension is obtained.