The Rotator Cuff: Part One
The rotator cuff is a group of four scapulohumeral muscles that aid in movement of the glenohumeral joint (shoulder joint) throughout much of its range of motion. They are considered scapulohumeral muscles because of their origin on the “shoulder blade” and insertion on the humerus. The shoulder girdle is a ball-and-socket joint just like the hip, so it has three degrees of freedom, which makes it highly moveable. The issue with this, however, is the humeral articulation with the glenoid of the scapula. The bony anatomy of the shoulder causes it to be anatomically unstable. This instability is somewhat alleviated by the labrum, which helps deepen the socket, but the instability remains. So, what does this mean? The shoulder relies heavily on dynamic stabilization from muscles and tendons, which is offered primarily by the scapulohumeral muscles. The rotator cuff group offers vital stabilization and movement of the upper extremity. As said before, this group includes four muscles, all of which offer specific actions at the shoulder.
The four rotator cuff muscles or “SITS” muscles include: the supraspinatus, infraspinatus, teres minor and subscapularis, all of which form a “cuff” which helps to form the joint capsule at the shoulder.
The supraspinatus is the most superior of the rotator cuff, sitting atop the scapula. This is the only rotator cuff muscle which does not directly help with rotation. Its main action is to initiate arm abduction (taking arm away from the body) by assisting the deltoid.
The infraspinatus sits below the supraspinatus and spine of the scapula. Its individual action along with the teres minor is external rotation of the arm. This is commonly the muscle aggravated or injured during low bar back squats, pitching or overhead activities, just to name a few.
Working inferiorly, the teres minor sits below the infraspinatus on the shoulder blade. This individual muscle, again works with the infraspinatus to externally rotate the arm. It also is responsible for arm adduction, or bringing the arm towards the body. The majority of the muscle belly and tendon is covered by the deltoid muscle group.
The final rotator cuff muscle, the subscapularis, arises from a different area of the scapula. It sits in the subscapular fossa on the anterior (front) side. Its main responsibility is internal rotation of the arm, while also assisting the teres minor in arm adduction.
As we stated at the beginning of this article, although these muscles arise from different areas on the scapula, they all attach to the anterior side of the humerus. In addition to their individual actions, as a group, they provide the vital stabilization of the shoulder by holding the humeral head in that shallow glenoid cavity. This is a very commonly injured muscle group because of the responsibility they have in stabilizing this highly moveable joint. In this series of articles, we will talk about the causes of rotator cuff injuries, how to prevent them and how to rehabilitate them. If you have any questions, comments or concerns feel free to let us know and remember to always #HealByMoving.