THROWING INJURIES OF SHOULDER

OVERHAND THROWING

The biomechanical and electromyographic activity of overhand throwing has been investigated to give a relative model of function in a controlled environment. Electromyographic sequence activity appears fairly consistent regardless of generated velocities and is discussed here. Whiteley  presented an excellent review of biomechanical investigations on this topic. The overhand throw as it relates to pitching has been divided, depending on the investigator, into three, four, five, or six phases. This discussion explores a five-phase model consisting of (1) windup, (2) early cocking, (3) late cocking, (4) acceleration, and (5) follow-through.

Windup

The windup is an activity that is highly individualized. Its purpose is to organize the body beneath the arm to form a stable platform. As with all overarm activities, it is vital that the body perform in sequential links to enable the hand to be in the correct position in space to complete the assigned task. The hand can be placed in an infinite number of localities, and it is essential that the scapula humeral rhythm place it in an optimum setting for the task of propulsion. The drawing of the humerus into the moment center of the glenoid fossa is accomplished during the first 30 of elevation as the arm is brought upward by the deltoid and supraspinatus. Because of these many individual styles, no consistent pattern of muscle activity occurs during the windup phase.

Early Cocking

Early cocking is the period when the dominant hand is separated from the gloved hand, and it ends when the forward foot makes contact with the mound. The scapula is retracted and maintained against the chest wall by the serratus anterior. The humerus is brought into 90 of abduction and horizontal extension, with a minimal external rotation of approximately 50. This position is accomplished by activation of the

anterior, middle, and posterior deltoid. The external rotators of the cuff are activated toward the end of early cocking, with the supraspinatus more active than the infraspinatus and the teres minor as it steers the humeral head in the glenoid. The biceps brachii and brachialis act on the forearm to develop the necessary angle of the elbow. As the body moves forward, the humerus is supported by the anterior and middle deltoid as the posterior deltoid pulls the arm into approximately 30 of horizontal extension. At

this time, the static stability of the humeral head becomes dependent on the anterior margin of the glenoid, notably the inferior glenohumeral ligament and the inferior portionnof the glenoid labrum.

Late Cocking

Late cocking is the interval in the throwing motion when the lead foot makes contact with the mound, and it ends when the humerus begins internal rotation. The lead foot applies an anterior shear force to slow the lower extremity and to transfer energy. The foot serves as an anchor; the forward and vertical

momentum is transformed into rotational components. During this time, the humerus is moved into a position more forward in relation to the trunk and begins to come into alignment with the upper body. The extreme of external rotation, an additional 125, is achieved to provide positioning for the power phase or acceleration. This is the first of two critical instants. The supraspinatus, infraspinatus, and teres minor are active in this phase but become quiet once external rotation is achieved. Deceleration of the externally rotating humerus is accomplished by the contraction of the subscapularis. It remains active until the completion of late cocking. The serratus anterior and the clavicular head of the pectoralis major have their greatest activity during deceleration. The biceps brachii aids in maintaining the humerus in the glenoid by

producing compressive axial load. At the end of this phase, the triceps begins activity by providing compressive axial loading to replace the force of the biceps. The capsule becomes wound tight in preparation for acceleration.