Overhead athletes most frequently subject their shoulders to the unstable position of maximal abduction and external rotation during a throw. It is important to obtain a thorough understanding of the biomechanical stresses placed on specific shoulder structures as they correlate to distinct phases of the throwing motion [8]. It is also necessary to take into account that the legs and trunk (core) play a significant role as force generators in the transfer of energy to the ball. The scapula aids in allowing for extreme ranges of motion by providing a stable platform for the humeral head to rotate. This is part of the “kinetic chain concept.” There are six phases included in the description of a throw, which are achieved in approximately 2 s, and have been described in Chap. 1 but are reviewed briefly in this chapter (Fig. 2.1).

Phase I is the wind-up or readying phase, in which there is the least amount of stress placed on the shoulder. Toward the end of this phase, the dominant shoulder is in slight internal rotation and abduction.

The phases of a throw described above are more descriptive of a baseball pitch. Although similar, there are some subtle differences when comparing the pitch with a throw in different sports. In football, the increased weight of the ball provides changes in mechanics (Fig. 2.2). It has been shown that quarterbacks rotate the shoulder sooner and assume maximal external rotation of the shoulder earlier on in the throw. This allows for more time in acceleration during internal rotation. Also, the shoulder horizontal adduction is increased and greater elbow flexion is achieved in the late cocking phase. This decreases the potential load on the shoulder. Arm velocity is decreased in the football throw because the quarterback must have a more erect position and a complete follow-through that is observed in the baseball pitch does not occur in football. Due to the decreased force generated when throwing the ball, shoulder injuries are not as common.

Due to the repetitive motion of the dominant upper extremity and the extreme stresses of the throw, adaptive changes occur that affect the stabilizing structures. In overhead athletes, the ability to externally rotate the humerus is very important to generate high velocities to the ball. This leads to an increase in humeral retroversion and a relative capsular laxity. It has been shown in many high-level throwing athletes that the dominant shoulder exhibits increased external rotation and decreased internal rotation in abduction. These changes occur due to laxity within the anterior inferior glenohumeral ligament (AIGHL), which normally functions as a restraint to anterior translation of the arm in abduction and external rotation. The coracohumeral ligament (CHL) is also a restraint to external rotation at the side and may develop laxity, especially in baseball pitchers, and as a result, an increased sulcus sign may be seen. Retroversion of the humeral head (an average of 10–20°) is also a finding in overhead athletes that may lead to increased external rotation of the dominant shoulder. The increase in external rotation can range from 9° to 16°. However, there are instances when the increase in external rotation does not make up for the loss of internal rotation. This typically occurs later in the athlete’s career and attributes to a loss of overall rotational motion of the dominant shoulder (Fig. 2.3). This is the general process and evolution of glenohumeral internal rotation deficit (GIRD).

Muscular changes are also seen in the dominant arm of throwing athletes. Hypertrophy of the shoulder girdle muscles is common, while atrophy of the infraspinatus can be seen as well. External rotation strength, exhibited by the infraspinatus and teres minor muscles, has been found to be weaker in the dominant as compared to the nondominant shoulder. Conversely, muscles used in internal rotation, such as the subscapularis, latissimus dorsi, pectoralis major, and teres major, are typically stronger in the dominant throwing arm.

A very detailed history must be obtained for accurate diagnosis and treatment of shoulder injuries. Obtaining the mechanism of injury, duration, location, and timing of symptoms will give clues to the diagnosis. The age of the patient will also aid in possible differential diagnoses. For instance, young athletes will frequently have physeal injuries, labral pathology, or instability. On the other hand, older patients will more likely have pathology of the rotator cuff. Overhead athletes in the middle of their career may acquire laxity and rotator cuff pathology.

It is also important to inquire about specific history unique to overhead athletes, such as the phase of the throwing cycle that symptoms usually occur. Pain that occurs during the cocking phase may suggest labral pathology, internal impingement, or instability. During the late cocking or acceleration phases, anterior instability may be seen. In regard to posterior labral pathology or instability, this typically occurs with pain at the follow-through phase. Pain during deceleration or ball release may point toward a diagnosis of rotator cuff pathology. Timing of symptoms during the game, complaints of loss of throwing velocity, lack of command while throwing, and recent decrease in the number of pitch counts during games can also provide clues as to the possible diagnosis. It is also important to ask at what age the athlete began throwing and amount of rest in between seasons. Symptoms pertaining to the experience of numbness, tingling, or discoloration should raise the possible concern of neurologic or vascular pathology.