Glenohumeral joint
The humeral head articulates with the shallow glenoid fossa of the scapula, allowing an exceptionally wide range of motion at the cost of inherent bony stability.
The shoulder complex actually involves several joints working together, though the glenohumeral joint — where the humeral head meets the shallow glenoid socket of the scapula — is the primary ball-and-socket articulation responsible for most arm motion.
Because the bony socket covers only a small portion of the humeral head, dynamic stability depends heavily on the rotator cuff muscles, the surrounding labrum, and the scapula's ability to rotate and tilt in coordination with arm movement, a relationship often described as scapulohumeral rhythm.
Key structures
Supraspinatus
This rotator cuff muscle initiates lifting the arm away from the body and is the most commonly injured tendon in rotator cuff conditions.
Infraspinatus and teres minor
These posterior rotator cuff muscles rotate the arm outward and help center the humeral head during overhead movement.
Subscapularis
The largest rotator cuff muscle rotates the arm inward and contributes significantly to anterior joint stability.
Deltoid
This large, superficial muscle covering the shoulder provides the bulk of the force for lifting the arm, working together with the smaller rotator cuff muscles that fine-tune joint positioning.
Labrum and joint capsule
A ring of cartilage deepens the socket slightly, while the surrounding capsule and ligaments provide passive restraint at the extremes of motion.
Planes & range
Shoulder abduction, lifting the arm out to the side and overhead, represents the joint's largest range of motion and requires coordinated movement between the humerus and scapula to achieve its full arc.
This trade-off between mobility and stability explains why shoulder rehabilitation places heavy emphasis on rotator cuff and scapular muscle control rather than joint mobility alone.
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