Shoulder Anatomy and the Rotator Cuff: The 4 Muscles, Their Actions, and Why They Tear

The rotator cuff consists of four muscles that form a tendinous cuff around the glenohumeral joint: Supraspinatus, Infraspinatus, Teres minor, and Subscapularis — remembered by the mnemonic SITS. The glenohumeral joint is the most mobile joint in the body (ball-and-socket with a shallow glenoid fossa), which means it sacrifices stability for range of motion. The rotator cuff compensates by holding the humeral head pressed against the glenoid fossa during movement. Without the cuff, the deltoid would pull the humerus superiorly out of the socket every time you abducted your arm. Quick reference: Supraspinatus — initiates abduction (first 15 degrees), innervated by suprascapular nerve (C5-C6). Most commonly torn. Infraspinatus — external rotation (strongest external rotator), innervated by suprascapular nerve (C5-C6). Teres minor — external rotation (assists infraspinatus), innervated by axillary nerve (C5-C6). Subscapularis — internal rotation (largest and strongest rotator cuff muscle), innervated by upper and lower subscapular nerves (C5-C7). All four insert on the greater tubercle of the humerus EXCEPT subscapularis, which inserts on the lesser tubercle. This insertion pattern is a high-yield exam point: the mnemonic is that subscapularis is the odd one out — it is the only one on the anterior side (lesser tubercle) because it is the only internal rotator. Snap a photo of any shoulder anatomy diagram or rotator cuff question and AnatomyIQ identifies each muscle, traces the innervation, and explains the clinical significance — including the tests used to isolate each muscle. This content is for educational purposes only and does not constitute medical advice.

Supraspinatus: Origin — supraspinous fossa of the scapula. Insertion — superior facet of the greater tubercle of the humerus. Action — initiates abduction (first 0-15 degrees), after which the deltoid takes over. The supraspinatus passes under the acromion through the subacromial space — this narrow passage is why supraspinatus is the most commonly impinged and torn rotator cuff muscle. Every time you raise your arm, the tendon slides under a bony arch. Infraspinatus: Origin — infraspinous fossa of the scapula. Insertion — middle facet of the greater tubercle. Action — external rotation of the shoulder, especially powerful when the arm is at the side. Also assists with horizontal abduction. Infraspinatus is the second most commonly torn rotator cuff muscle and is tested clinically with the external rotation lag test. Teres minor: Origin — lateral border of the scapula (upper two-thirds). Insertion — inferior facet of the greater tubercle. Action — external rotation (assists infraspinatus) and weak adduction. Distinguished from teres major by innervation (teres minor = axillary nerve; teres major = lower subscapular nerve) and function (teres minor = external rotation; teres major = internal rotation and adduction). This distinction is a classic exam trap — teres minor and teres major sound similar but do opposite things. Subscapularis: Origin — subscapular fossa (entire anterior surface of the scapula). Insertion — lesser tubercle of the humerus. Action — internal rotation (the strongest internal rotator of the shoulder) and adduction. The subscapularis is the largest rotator cuff muscle and forms the anterior wall of the rotator cuff. It is tested clinically with the lift-off test (Gerber test) — the patient places the dorsum of their hand on their lower back and tries to push it away; inability to maintain this position indicates subscapularis weakness or tear. AnatomyIQ generates clinical correlation questions from anatomy facts — snap a photo of the SITS muscles and it tests you on origins, insertions, actions, and the clinical tests for each.

Two nerves innervate all four rotator cuff muscles: Suprascapular nerve (C5-C6): innervates supraspinatus AND infraspinatus. It passes through the suprascapular notch (under the superior transverse scapular ligament) to reach the supraspinous fossa, then winds around the spinoglenoid notch to reach the infraspinous fossa. This tortuous course makes it vulnerable to injury at two points: the suprascapular notch (compression from a cyst, traction injury, or fracture of the scapular neck) and the spinoglenoid notch (ganglion cysts from labral tears). Suprascapular nerve injury causes weakness of both abduction initiation (supraspinatus) and external rotation (infraspinatus), with atrophy visible in both fossae. Axillary nerve (C5-C6): innervates teres minor AND the deltoid. It wraps around the surgical neck of the humerus — making it vulnerable to anterior shoulder dislocations and surgical neck fractures. Axillary nerve injury causes loss of deltoid function (cannot abduct past 15 degrees — supraspinatus can still initiate but deltoid cannot take over) and loss of teres minor function (weakened external rotation). The sensory test: loss of sensation over the regimental badge area (lateral deltoid skin). This is the most important clinical test for axillary nerve integrity after shoulder dislocation. Subscapular nerves (upper and lower, C5-C7): innervate subscapularis (both nerves) and teres major (lower subscapular nerve only). These are short nerves from the posterior cord of the brachial plexus and are less commonly injured in isolation. The clinical reasoning pattern: if a patient cannot initiate abduction AND has weak external rotation with atrophy in two scapular fossae — think suprascapular nerve. If a patient cannot abduct past 15 degrees AND has loss of sensation over the lateral deltoid — think axillary nerve (check for shoulder dislocation history). AnatomyIQ traces nerve pathways and generates clinical scenario questions — describe a patient presentation and it identifies the nerve, the site of injury, and the expected examination findings.

Supraspinatus accounts for 80-90% of rotator cuff tears, and the reason is purely anatomical: it runs through the narrowest space in the shoulder. The subacromial space is a corridor between the acromion (above) and the humeral head (below). The supraspinatus tendon passes through this space every time the arm is elevated. In a healthy shoulder, the space is about 1 cm — just enough for the tendon and the subacromial bursa that cushions it. But with repetitive overhead motion (throwing, swimming, painting), the tendon becomes inflamed (tendinitis) and swollen, reducing the available space. The acromion then mechanically compresses the tendon with each elevation — this is subacromial impingement. The vascular watershed: the supraspinatus tendon has a zone of relative hypovascularity (poor blood supply) approximately 1 cm from its insertion on the greater tubercle. This critical zone receives blood supply from both the muscle side and the bone side, but the overlap is incomplete — creating an area of reduced healing capacity. Tears almost always occur in this watershed zone because the tissue cannot repair the microtrauma from repetitive impingement as effectively as well-vascularized tissue. Acromion morphology matters: the shape of the acromion varies between individuals. Type I (flat) = least impingement risk. Type II (curved) = moderate risk. Type III (hooked) = highest risk — the hook narrows the subacromial space and mechanically abrades the supraspinatus tendon. This classification (Bigliani classification) appears on anatomy exams and explains why some people are anatomically predisposed to rotator cuff tears. The clinical presentation of supraspinatus impingement: painful arc between 60-120 degrees of abduction (the range where the tendon is maximally compressed), positive Neer test (pain with passive forward flexion), and positive Hawkins test (pain with internal rotation at 90 degrees of forward flexion). A complete tear produces a positive drop arm test — the patient cannot slowly lower the arm from 90 degrees of abduction. Snap a photo of any shoulder impingement or rotator cuff question and AnatomyIQ explains the anatomy behind the clinical presentation, the relevant tests, and the structural factors that predispose to injury.