1 · A shallow socket held by muscle
The shoulder socket is shallow and the bone barely restrains the humeral head, so the four rotator cuff muscles actively hold it in the glenoid.The shoulder has the largest range of motion of any joint, and the price is stability. The humeral head is a big ball; the glenoid on the scapula is a shallow, small dish — like a golf ball on a tee, free to point anywhere, but the bone gives almost no restraint.So what holds it? The rotator cuff — four small muscles (supraspinatus, infraspinatus, teres minor, subscapularis) whose tendons wrap the head and actively cup the ball into the dish like a hand. Remember: the shoulder is not a passively locked hinge but a machine kept stable by muscle, moment to moment.