Infraspinatus (Rotator Cuff)
The infraspinatus is the largest muscle of the rotator cuff. Its origin is the large surface on the back of the scapula below the acromion process. Its insertion is on the posterior of the greater tubercle near the top of the humerus.
Primarily, the infraspinatus contracts to externally rotate the arm. As a muscle of the rotator cuff, its job is also to hold the head of the humerus firmly against the glenoid fossa of the scapula.
Follow-throughs with large lateral motion across the chest stress the infraspinatus by stretching it while it is contracting (eccentric contraction) to decelerate the arm. Repeated stress in this manner eventually leads to tearing of the rotator cuff.