Guide · 7 min read
The rotator cuff muscles, explained
The rotator cuff is four muscles that run from the scapula to the top of the humerus and hold the head of the humerus against the shallow glenoid cavity while larger muscles move the arm. The shoulder trades bony stability for range of movement, and the cuff is most of what it gets in return.
Students usually meet the cuff as the initialism SITS - supraspinatus, infraspinatus, teres minor, subscapularis - and stop there. That is enough to list them and not enough to answer a question about them. This guide works through the four in the order that makes their attachments and nerve supply predictable, using the strings on each muscle's own reference card.
What the four have in common
All four originate on the scapula and insert on the proximal humerus, and every one of their cards carries the same clause in its action line: stabilises the glenohumeral joint. That is the shared job. Their individual actions - abduction, lateral rotation, medial rotation - are what they do on top of it.
Because they all cross the same joint from the same bone, the difference between them is almost entirely a matter of where on the scapula they start and where on the humerus they finish. Learn those two coordinates and the rest of the card follows.
| Muscle | Origin | Insertion | Main action | Nerve |
|---|---|---|---|---|
| Supraspinatus | Supraspinous fossa of the scapula | Superior facet of the greater tubercle | Initiates abduction (first ~15°) | Suprascapular (C5, C6) |
| Infraspinatus | Infraspinous fossa of the scapula | Middle facet of the greater tubercle | Laterally rotates the arm | Suprascapular (C5, C6) |
| Teres minor | Superior part of the lateral border of the scapula | Inferior facet of the greater tubercle | Laterally rotates the arm | Axillary (C5, C6) |
| Subscapularis | Subscapular fossa (anterior scapula) | Lesser tubercle of the humerus | Medially rotates and adducts the arm | Upper and lower subscapular (C5, C6, C7) |
The tubercle rule does most of the work
Three of the four insert on the greater tubercle of the humerus and one does not, and that split explains the actions without any further memorising.
Supraspinatus, infraspinatus and teres minor take the superior, middle and inferior facets of the greater tubercle respectively - in that order, top to bottom, matching the order in which their bellies sit on the back of the scapula. The greater tubercle is lateral, so all three pull the humerus laterally or upward: supraspinatus starts abduction, and the two below it laterally rotate.
Subscapularis is the exception on every count. It is the only cuff muscle on the anterior surface of the scapula, filling the subscapular fossa, and the only one to insert on the lesser tubercle rather than the greater. Because the lesser tubercle faces medially, it is the only one that medially rotates the arm - and, from the front, the one that resists anterior displacement of the humeral head.
Supraspinatus and the first 15 degrees
Supraspinatus is the cuff muscle most often asked about, because it is the one whose action is genuinely non-obvious. Its card records that it initiates abduction of the arm through roughly the first 15 degrees, after which the much larger deltoid takes over and carries abduction on to about 90 degrees.
The two muscles are therefore not interchangeable, and their nerves differ as well: supraspinatus is supplied by the suprascapular nerve and deltoid by the axillary nerve. This is also the muscle most commonly torn or impinged in the cuff, and its card notes the two examination findings that go with it - a painful arc and a positive empty-can test.
Two nerves, not four
The nerve supply of the cuff is more economical than the four-muscle list suggests, and learning it as three groups rather than four entries removes most of the confusion.
The suprascapular nerve supplies two of them. It arises from the upper trunk of the brachial plexus with roots C5 and C6 and reaches its targets through the suprascapular notch, beneath the transverse scapular ligament, then curves around the spinoglenoid notch into the infraspinous fossa - which is exactly the route needed to reach supraspinatus and then infraspinatus. Entrapment along that path weakens the initiation of abduction and lateral rotation together, because one nerve serves both.
The axillary nerve supplies teres minor, and nothing else in the cuff. It comes off the posterior cord with roots C5 and C6 and passes through the quadrangular space with the posterior circumflex humeral artery, winding around the surgical neck of the humerus. Its other muscle is the deltoid, which is why an injury at the surgical neck produces weak abduction plus the 'regimental badge' patch of numbness over the lateral shoulder.
Subscapularis is supplied by the upper and lower subscapular nerves, carrying C5, C6 and C7. The library does not hold a separate card for those two branches, so this is the one cuff innervation you learn from the muscle's own entry.
What is not in the rotator cuff
Two muscles are routinely misfiled into the cuff, and both are worth naming explicitly because a practical examiner will use them as distractors.
Teres major is not a cuff muscle. Its own card says so. It arises from the posterior surface of the inferior angle of the scapula - not from a fossa - and inserts on the medial lip of the intertubercular groove rather than on a tubercle, so it adducts and medially rotates the arm instead of stabilising the joint. It is supplied by the lower subscapular nerve, and it forms the lower border of the quadrangular and triangular spaces.
Deltoid is not a cuff muscle either. It originates from the lateral third of the clavicle, the acromion and the spine of the scapula, and inserts on the deltoid tuberosity well down the shaft - it caps the joint from outside rather than wrapping the head from within. Teres minor is the one to check yourself on: it sits directly beside teres major, shares part of its neighbourhood, and is in the cuff while teres major is not.
How to drill the cuff on this site
Four muscles is a small enough set that it is worth being able to produce the whole table from a blank page, in order, with attachments and nerves. That is the standard a spot test applies.
The four cards sit in the upper limb region alongside the nerves that supply them, and each one lists the other three under Related, so you can move around the group without going back to a list.
- Write out the SITS order from memory, then add the facet of the greater tubercle for the first three and the lesser tubercle for the fourth.
- Say the nerve after each muscle before you check it - two suprascapular, one axillary, one subscapular pair.
- Run the flashcards for the upper limb and treat teres major as a deliberate trap rather than a mistake.
- Use the innervation and action modes in the quiz to confirm you can go in both directions: muscle to nerve, and nerve to muscle.
- Read the nervous system page for the suprascapular and axillary nerves in the context of the rest of the plexus.
Frequently asked questions
What are the four rotator cuff muscles?
Supraspinatus, infraspinatus, teres minor and subscapularis, usually remembered as SITS. All four run from the scapula to the proximal humerus and stabilise the glenohumeral joint, which is the job they share; abduction and rotation are what each adds on top of it.
Which rotator cuff muscle inserts on the lesser tubercle?
Subscapularis, and it is the only one. The other three insert on the greater tubercle, taking its superior, middle and inferior facets in that order. Because the lesser tubercle faces medially, subscapularis is also the only cuff muscle that medially rotates the arm.
Is teres major part of the rotator cuff?
No. Teres major arises from the posterior surface of the inferior angle of the scapula and inserts on the medial lip of the intertubercular groove of the humerus, so it adducts and medially rotates the arm rather than stabilising the joint. It is supplied by the lower subscapular nerve.
Which nerve supplies the rotator cuff?
There is no single nerve. The suprascapular nerve (C5, C6) supplies supraspinatus and infraspinatus, the axillary nerve (C5, C6) supplies teres minor, and the upper and lower subscapular nerves (C5, C6, C7) supply subscapularis. Learning it as three groups is easier than four.
Why is supraspinatus described as initiating abduction?
Its card records that it starts abduction through roughly the first 15 degrees, after which the deltoid carries the movement on to about 90 degrees. The two muscles have different nerves - suprascapular for supraspinatus and axillary for deltoid - so weakness in each presents differently.