The shoulder is the most mobile joint in the body, so it relies more on muscles than on bone for stability. The rotator cuff holds the ball in the socket while larger muscles move the arm.
Rotator cuff
Four muscles, the supraspinatus, infraspinatus, teres minor and subscapularis (SITS), wrap around the head of the humerus. They keep it centered in the shallow glenoid during movement.
Prime movers
The deltoid lifts the arm to the side, front and back. The pectoralis major and latissimus dorsi pull the arm across and down, and the trapezius and serratus anterior move the scapula.
Common injuries
The supraspinatus tendon runs under the acromion and is the one most often torn or impinged. Pain when raising the arm between about 60 and 120 degrees is typical of impingement.
| Muscle | Main action | Test |
|---|---|---|
| Supraspinatus | Starts abduction | Empty can test |
| Infraspinatus and teres minor | External rotation | Resisted external rotation, elbows at sides |
| Subscapularis | Internal rotation | Lift-off or belly press test |
At the bedside
Test each cuff muscle against resistance and compare sides when you examine a painful shoulder. After a shoulder dislocation, check deltoid strength and the skin over it, since the axillary nerve can be injured.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).