Pitching has been investigated more thoroughly than any other overhead activity, with the shoulder most often the primary focus. It is recognized that the baseball pitcher uses the entire body in the pitching motion, beginning with the lower extremities and advancing to the trunk, shoulder, elbow, wrist, and hand. An alteration in any segment of this chain can affect the outcome. Consistent accuracy through repetition of coordinated high-velocity activity is the key to successful pitching. It is also the source of injury.
Pitching is a smooth, continuous motion that occurs during a relatively brief period. Depending on the investigator, baseball pitching is divided into three (Pappas et al., 1985), four (Jobe, Moynes, Tibone, & Perry, 1984), five (Braatz & Gogia, 1987), or six (Park, Loebenberg, Rokito, & Zuckerman, 2002/2003a) phases. Of these studies, the most commonly used phases include windup, early cocking, late cocking, acceleration, and follow-through.
Windup, the setting phase of the pitching motion, occurs when the individual positions the body such that the glove (non-throwing) side is facing the target. The purpose of the windup is to set a rhythm that establishes a synchronized timing of the body parts (Braatz & Gogia, 1987). At the start, the two hands are together and near the body anywhere from the belt to the head as he takes a step back with the leg contralateral to the throwing arm. This contralateral leg is the stride leg while the ipsilateral leg is the support leg (Dillman, Fleisig, & Andrews, 1993). With the weight transfer, the body rotates 90° as the stride leg flexes at the hip and knee so the pelvis rotates towards the throwing shoulder and the lumbar spine flexes slightly. The body winds up so that all segments of the body from the legs to the arms are able to contribute to the ball’s propulsion (Pappas et al., 1985). This phase is a minimally demanding portion of the pitching motion. Speed, energy expenditure, and forces generated are all at low levels.
Cocking begins when the hands separate and ends when abduction and maximum lateral rotation of the shoulder is achieved (Pappas et al., 1985). Cocking is divided into early cocking and late cocking according to the contact of the forward foot on the ground. In early cocking, the scapula is retracted and the humerus is abducted, laterally rotated, and horizontally extended. The elbow flexes. The stride leg begins to extend the knee; abduct, medially rotate and extend the hip; and evert and plantar flex the ankle (Braatz & Gogia, 1987). The non-throwing shoulder is abducted and its elbow is extending. The body’s center of gravity is lowered because the support knee and hip are flexing and the hips and pelvis begin to rotate forward.
Late cocking begins when the stride foot hits the ground (Jobe et al., 1984). At the time of foot contact, both arms are elevated about 90° and in line with each other along the plane of the shoulders. Anterior stress on the glenohumeral joint is predominant at this time, with the body in front of the arm. The deltoid is strongly active during early cocking. When maximum shoulder lateral rotation and abduction to at least 90° occur, the static stabilizers of the shoulder, the glenohumeral capsule and ligaments, serve to limit further motion. Active stabilizers, including the forward flexors, lateral rotators, the subscapularis, pectoralis major, and latissimus dorsi, act as additional restraints to control motion. Scapular stabilizers such as the pectoralis minor and serratus anterior are also active in late cocking. Reciprocal inhibition of the other rotator cuff muscles, the teres minor, supraspinatus, and infraspinatus, is also taking place as these muscles attempt to resist the superior subluxating forces that occur when the trunk is in a forward lean and the shoulder is maximally laterally rotated (Jobe et al., 1984). At the end of late cocking, the lumbar spine hyperextends to add to the shoulder’s lateral rotation (Braatz & Gogia, 1987). The supraspinatus and infraspinatus are particularly active in late cocking. By the end of this phase, the shoulder medial rotators are on maximum stretch, the body is “wound” optimally for the elastic energy transfer, and the pelvis leads the shoulders to face the target legs and trunk begin their acceleration for energy transfer to the arm. Right before the end of this phase, the body laterally tilts to the non-throwing arm side. Shoulder rotation to the target and lateral trunk motion are facilitated by the non-throwing arm’s motion from a position of abduction at the start of late cocking to adduction and extension at the end (Braatz & Gogia, 1987).
Acceleration starts with maximum shoulder lateral rotation and abduction and ends when the ball leaves the fingers (Jobe et al., 1984). The movements in this phase include scapular protraction, humeral horizontal flexion and medial rotation, and elbow extension. Just prior to ball release, the shoulder is still at about 90° of abduction. The glenohumeral joint’s capsule is wound tight to provide an elastic force release and the accelerator muscles are also maximally stretched (Perry, 1983). During this phase the speed of the arm has increased significantly in a relatively brief period, beginning from almost 0°/s at the end of cocking to 7500°/s by the end of acceleration, a time of 50 msec (Pappas et al., 1985). The serratus anterior and pectoralis major are strongly active during this phase as the arm moves forward and the scapula protracts (Jobe et al., 1984). The subscapularis and latissimus dorsi are contracting concentrically as the arm moves into medial rotation during acceleration (Jobe et al., 1984).
Follow-through occurs from the point of ball release to the completion of the motion when the support leg moves forward and contacts the ground to stop forward body motion (Jobe, Tibone, Perry, & Moynes, 1983). It is divided into early and late follow-through according to the point of maximal shoulder medial rotation. Early follow-through is completed rapidly, in less than 0.1 s (Moynes, Perry, Antonelli, & Jobe, 1986). Trunk rotation and scapular motions occur and are diminished to a varying extent from one style to another, depending on the individual thrower. The deltoid is strongly active during early follow-through (Jobe et al., 1984). The rotator cuff, especially the lateral rotators, must decelerate the arm after ball release and work against the momentum distraction forces occurring at the shoulder. The biceps is also working at high levels eccentrically to reduce distraction forces at the elbow (Jobe et al., 1984). Some of the forces produced during acceleration are absorbed by the stride leg; it is planted during acceleration and flexed knee position absorbs some of the forces (Braatz & Gogia, 1987). After the ball is released, the throwing arm continues to move across the body toward the opposite hip with the scapula continuing to protract; this cross-body motion helps to minimize irritation to the rotator cuff since the concomitant scapular motion keeps the coracoacromial arch structures from impinging on the rotator cuff (Braatz & Gogia, 1987).
It is during the follow-through that injuries to the posterior shoulder occur. The body must now dissipate the energy that has been developed to accelerate the ball. This is one reason it is important for the body to continue to move after the ball is released. An abrupt stop in arm motion will prevent this energy dissipation and cause these tremendous forces to be absorbed primarily by the shoulder. Flexing the trunk, flexng the support knee, and allowing the arm to continue along its path of movement across the body and to the opposite leg all assist in dissipating this energy and reducing distraction forces on the shoulder (Braatz & Gogia, 1987).