Supraspinatus

Supraspinatus arises from the medial two-thirds of the supraspinous fossa and its fascia. Its tendon passes beneath the coraco-acromial arch (acromion, coraco-acromial ligament and coracoid), separated from it by the subacromial bursa. It inserts on the superior facet of the greater tubercle and blends with the joint capsule.

It initiates abduction over the first ≈15° and then works with deltoid throughout the rest of the movement. It also holds the humeral head in the glenoid, which is especially important when a weight hangs from the arm. Its narrow passage under the arch and the relatively poor blood supply near its insertion (≈1 cm proximal, the "critical zone") explain why it is the tendon most prone to impingement, tendinopathy, calcific deposits and tears. It is supplied by the suprascapular nerve (C4–C6), which passes below the superior transverse scapular ligament while the suprascapular artery passes above it ("the Army goes over the bridge, the Navy goes under").