Chapter 25 Shoulder and Elbow Disorders in the Athlete

The throwing athlete is at increased risk for injury to the shoulder because of the large and repetitive forces generated during the throwing motion. Injuries may occur from a single supraphysiologic load or from repeated subthreshold loads that cumulatively damage the tissue. The act of throwing has been classified, with some variation, into five stages: windup, early cocking, late cocking, acceleration, and deceleration with followthrough. This classification centers on the essential physical challenge of each phase (Figure 1). In the late cocking phase, the throwing arm is externally rotated with the arm abducted; the greater this external rotation, the greater the velocity that can be obtained from the internal rotator muscles in the acceleration phase. At the completion of the throwing motion, the arm has been internally rotated through an arc of approximately 180°. This arc is traversed at speeds approaching 7,000° per second, with great forces applied to both accelerate and decelerate the arm.1 As such, the arm is structurally challenged to move within a wide arc while maintaining the location of the humeral head within the glenoid fossa. The needed range of motion is attained by allowing just enough soft-tissue laxity, and the humeral location is maintained by the action of both static and dynamic stabilizers. Static stability is provided by the capsule and ligaments, and dynamic stability is conferred by the eccentric contraction of the rotator cuff muscles. If the capsule and ligaments are insufficiently lax, the forces of throwing applied to these tight tissues may damage them. Likewise, if the capsule and ligaments are too loose or if the rotator cuff does not hold the humeral head in contact against the glenoid, forceful abutment between the articular surfaces may lead to injury. The cuff itself is subject to repetitive microtrauma with every throw as it eccentrically contracts to stabilize the humeral head in the glenoid.