The Rotator Cuff Is a Dynamic Joint-Centring System

An Inherently Unstable Joint

The shoulder trades stability for an extraordinary range of motion — the glenoid socket is shallow, covering only a small fraction of the humeral head's surface, roughly analogous to a golf ball balanced on a tee. This architecture gives the shoulder its remarkable mobility, but leaves it dependent on soft tissue, rather than bony congruity, for stability.

Centring the Ball on the Tee

The four rotator cuff muscles — supraspinatus, infraspinatus, teres minor and subscapularis — work together not primarily to produce large rotational movements (the larger deltoid and other muscles do most of that work), but to generate a compressive force that keeps the humeral head precisely centred within the glenoid throughout the entire arc of shoulder movement (Lugo, Kung & Ma, 2008).

What Happens When This Centring Fails

When the rotator cuff is weak, fatigued or poorly coordinated, the larger, more powerful deltoid can pull the humeral head upward and forward relative to the glenoid during arm elevation, narrowing the subacromial space and increasing the likelihood of impingement-type symptoms — which is why rotator cuff rehabilitation typically emphasises coordinated, lower-load control exercises rather than simply chasing maximal strength.