(1) Supraspinatus: This is one of the three rotator cuff muscles infamously known for giving rise to shoulder pain. It arises from the fossa above the spine of the shoulder blade bone known as supraspinous fossa. It inserts into the upper facet of the greater tuberosity of the arm bone and from the capsule of the shoulder joint.
It assists the deltoid in moving the arm away from the side of the body (abduction). It also helps to roll the arm outward (external rotation). It is supplied by the suprascapular nerve from the upper trunk of the brachial plexus. The suprascapular nerve carries the C5 and C6 nerve nerve root fibers (especially the C5 nerve root fibers) to this muscle.
The tendon of this muscle is commonly involved in degenerative processes and may rupture. It can also be entrapped under the acromion.
(2) Infraspinatus: Shoulder pain symptoms are commonly associated with a rotator cuff problem. The infraspinatus is the largest of the three muscles comprising the rotator cuff, the other two being supraspinatus and teres minor muscles.
The infraspinatus muscle arises from the infraspinous depression (fossa) of the shoulder blade bone and attaches to the middle portion of the greater tuberosity on the arm bone (humerus). It also arises from the capsule of the shoulder joint.
It functions primarily to roll the arm outward as in turning the palm forward so that the thumb is away from the midline of the body (external rotation). It is supplied by the suprascapular nerve from the upper trunk of the brachial plexus which carries the fibers of the C5 and C6 nerve roots, primarily the C6 nerve root.
Points to focus on:
a. Rotator cuff injuries occur because there is power mismatch between the muscles that roll the arm inward as in turning palm backward so that the thumb is toward the midline of the body (internal rotation).
b. There are very few muscles that can perform external rotation, namely the three rotator cuff muscles (supraspinatus, infraspinatus and teres minor) and the posterior deltoid muscle.
c. In contrast, there are many internal rotator muscles and they are large and powerful. These include the latissimus dorsi, teres major, pectoralis major and subscapularis muscles. Most of the activities that we do daily are performed with the shoulder in internal rotation. Therefore the internal rotators become extremely strong and tight.
d. The external rotator muscles are used in lengthening contraction to stabilize the shoulder joint when it is internally rotated. Also, it is prone to attrition when caught under the acromion especially with the shoulder in internal rotation. As a result, the external rotators consisting of the rotator cuff gradually fail to the point that it can even rupture.