The mechanism of the overhead action in throwing sports and tennis has been studied extensively. This motion is unnatural and highly dynamic, often exceeding the physiological limits of the joint. Owing to overload of various anatomical structures, the shoulder is susceptible to injury. Optimal shoulder function requires good kinetic chain function, optimal stability, and coordination of the scapula in the overhead action. A well balanced action of the rotator cuff muscles and capsular structures is necessary to obtain a stable centre of rotation during the overhead action. (van der Hoeven H. Kibler WB. Shoulderinjuries intennisplayers.British Journal of Sports Medicine. 40(5):435-40; 2006 May).
Primary risk factor for glenohumeral joint injuries in overhead activity athletes is the imbalance of the external rotator cuff muscles which must contract eccentrically (lengthening contractions) while the internal rotator cuff muscles contract concentrically (shortening contractions). To reduce such injuries to tennis players, upper extremity strength training program must include increasing the eccentric external rotator cuff total exercise capacity without a subsequent increase in the concentric internal rotator total exercise capacity. (Niederbracht Y. Shim AL. Sloniger MA. Paternostro-Bayles M. Short TH. Effects of ashoulderinjury prevention strength training program on eccentric external rotator muscle strength and glenohumeral joint imbalance in female overhead activity athletes.Journal of Strength & Conditioning Research. 22(1):140-5, 2008 Jan).
A study was performed to broaden the understanding of muscle function during the tennis volley under different ball placement and speed conditions by examining the activity of selected superficial muscles of the stroking arm and shoulder (flexor carpi radialis, extensor carpi radialis, triceps brachii, deltoids, and pectoralis major) and muscles related to postural support (left and right external oblique, lumbar erector spinae, and gastrocnemius) during the volley. In general, muscle activity increased with increasing ball speed. The extensor carpi radialis was more active than the flexor carpi radialis during both forehand and backhand volleys, suggesting the importance of wrist extension/abduction and grip strength. The increase in EMG levels in the forearm muscles shortly before the ball impact indicated that the subjects did not tighten their grip and wrist until moments before ball impact. Both antero-middle and postero-middle deltoids were active in most stroke phases. However, the roles of the deltoid muscles during a volley cannot be determined without knowing the actions of the other shoulder joint muscles. (Chow JW. Carlton LG. Lim YT. Shim JH. Chae WS. Kuenster AF. Muscle activation during thetennisvolley.Medicine & Science in Sports & Exercise. 31(6):846-54, 1999).