During the wind-up phase, muscle activity is low,  Small forces are placed on the shoulder and few injuries occur during wind-up.

 

During the early cocking phase, muscle activity increases. The serratus anterior

and trapezius muscles demonstrate moderate to high activity and, in concert with the middle deltoid and supraspinatus, help keep the humeral head congruent with

the glenoid.

 

The late cocking and acceleration phases( ABER motion) are when the shoulder begins to experience an increase in forces. It is also when the shoulder is maximally externally rotated and horizontally abducted. The subscapularis, pectoralis major, and latissimus dorsi all demonstrate high to very high activity and provide stability to the joint. The action of the deltoid begins to diminish ,and the rotator cuff muscles increase their activity. The infraspinatus and teres minor show very high activity, whereas the supraspinatus is the least active. The rotator cuff muscles provide a compressive force, contributing to the stability of the joint.

 

During deceleration and follow-through( ABER to ABIR position), the excess kinetic energy not transferred to the ball is dissipated through the shoulder. This is characterized by a powerful deceleration force provided by the posterior musculature. Opposing muscles tend to fire simultaneously to accomplish this control. The teres minor demonstrates the highest activity of the posterior musculature, and the middle and posterior heads of the deltoid are the most active antagonists. The subscapularis also demonstrates high activity as well in maintaining humeral head position and preventing subluxation.

 

 Injury to the rotator cuff can occur during the deceleration phase as it resists horizontal adduction, internal rotation, anterior translation, and distraction forces. This repetitive action may result in microtrauma over time and may lead to tensile failure of the rotator cuff.