The infraspinatus occupies most of the infraspinous fossa below the scapular spine and is the primary external rotator of the shoulder, playing a central role in the rotator cuff force couple that resists the tendency of the powerful internal rotators to translate the humeral head anteriorly during pushing activities. It shares its nerve supply with the supraspinatus via the suprascapular nerve, making both muscles vulnerable to a single nerve injury. External rotation strength testing is considered the most sensitive functional screen for posterior rotator cuff integrity.
| Origin | Infraspinous fossa of the scapula |
|---|---|
| Insertion | Middle facet of the greater tubercle of the humerus |
| Nerve Supply | Suprascapular nerve (C5, C6) |
| Blood Supply | Suprascapular artery; Circumflex scapular artery |
| Actions | External rotation of the arm at the shoulder, the primary action; Stabilises the posterior aspect of the glenohumeral joint, resists anterior translation |
|---|
Beyond external rotation, the infraspinatus is constantly active during overhead sports to decelerate the rapidly internally rotating arm during the follow-through phase, making it one of the most heavily loaded muscles in throwing athletes and a frequent source of posterior shoulder pain and fatigue.
Infraspinatus atrophy without a history of trauma raises suspicion for suprascapular nerve entrapment at the spinoglenoid notch, a condition seen in volleyball players from repetitive traction on the nerve during spiking. External rotation weakness is a consistent finding after posterior shoulder dislocations and in throwers with glenohumeral internal rotation deficit (GIRD). The posterior shoulder impingement experienced by overhead athletes often involves the posterior labrum and infraspinatus undersurface.
The infraspinatus belly fills the infraspinous fossa below the scapular spine and is easily palpable during resisted external rotation with the elbow at 90°. It can be distinguished from the teres minor by its larger volume and more superior position.
An incomplete tear through the posterior rotator cuff, usually on the articular surface at the posterior superior humeral head, the 'peel-back' lesion in overhead athletes, producing posterior shoulder pain with horizontal adduction and internal rotation.
Compression of the nerve as it wraps around the spinoglenoid notch causes isolated infraspinatus weakness and atrophy with preserved supraspinatus function, visible as a hollow below the scapular spine with normal shoulder abduction.
