This is an excerpt from Tennis Anatomy by Paul Roetert & Mark S. Kovacs.
Tennis Strokes and Shoulder Movement
For a tennis player, the shoulder is one of the most used (and sometimes overused) areas of the body. Typically, this makes it one of the most injured areas, especially in competitive tennis players. In addition to the repetitive demands on the shoulder, tennis also requires explosive movement patterns and highly intensive maximal-effort concentric and eccentric muscular work.
Groundstrokes require predominantly horizontal actions at the shoulder, using a combination of abduction and external rotation for the forehand backswing and backhand follow-through and a combination of abduction and internal rotation for the forehand forward swing and backhand backswing.
The tennis serve is a more complex sequence that uses a combination of horizontal and vertical movements. Horizontal abduction and external rotation occur during the backswing, with scapular retraction and depression into the loading phase. From the loading phase, scapular elevation, horizontal abduction, and shoulder extension move the arm toward contact. Internal rotation, shoulder extension, and adduction complete the follow-through. The muscles of the rotator cuff play a vital role in stabilizing the humerus in the shoulder during all tennis movements, but they are critical during the acceleration and follow-through phases of the serve (figure 2.3). The muscles of the rotator cuff aid in power production during acceleration and provide eccentric strength to help slow down the arm after contact during the follow-through. It has been reported that during the explosive internal rotation of the serve, shoulder rotation can reach speeds from 1,074 to 2,300 degrees per second. After contact, deceleration has to occur through eccentric strength of the rotator cuff and related musculature. At the professional level, male players reach speeds on the serve close to 140 miles per hour (225 km/h). Proper preparation of the shoulder musculature is critical.
Tennis volleys require smaller muscle and joint movements than either groundstrokes or serves. For a forehand volley, slight external rotation and slight adduction followed by abduction of the shoulder allow the player to complete the stroke. The backhand volley involves slight internal rotation and abduction followed by slight external rotation and adduction of the shoulder.