Rotator Cuff Function: SITS Muscles, Tears and Impingement

Last updated: September 2026

The rotator cuff is the group of four muscles — supraspinatus, infraspinatus, teres minor and subscapularis — whose tendons wrap the head of the upper arm bone and hold it centred in the shoulder socket while bigger muscles swing the arm.

That centring job is why a tear changes so much at once: pain, weakness, and a shoulder that no longer tracks as it did. Below: what each muscle does, how surgeons grade the damage, and what the grade changes about recovery.

Key takeaways

What does the rotator cuff do?

Two jobs at once: it rotates the arm, and it holds the ball in the socket while that happens. The head of the humerus sits in a shallow socket on the shoulder blade, and the AAOS describes the four tendons as a cover over that head which “helps lift and rotate the arm, while also stabilizing the head of the humerus within the joint.”

The stabilising half is the part patients rarely hear about until it fails. With a tear, the balanced pull is interrupted and the humeral head can ride upward — a 2025 review of massive tears calls this “interruption of the axial force pair, causing the upward subluxation of the head of the humerus.” Our shoulder anatomy and labrum anatomy pages cover the bones and the cartilage rim this sits inside.

What are the 4 SITS muscles and what does each one do?

SITS is the standard mnemonic — Supraspinatus, Infraspinatus, Teres minor, Subscapularis — and each starts on the shoulder blade and inserts on the humerus.

Muscle	Origin on the scapula	Insertion on the humerus	Primary action
Supraspinatus	Supraspinous fossa	Superior facet, greater tuberosity	Abduction — lifting the arm away from the body
Infraspinatus	Infraspinous fossa	Middle facet, greater tuberosity	External rotation
Teres minor	Lateral border of the scapula	Inferior facet, greater tuberosity	External rotation
Subscapularis	Subscapular fossa	Lesser tuberosity or humeral neck	Internal rotation

Source: Physiopedia, Rotator Cuff. The three greater-tuberosity facets sit in a row down the outside of the bone, which is why tendon order matters to a surgeon reading an MRI.

The four do not act in isolation: reaching behind your back uses internal rotation, turning the palm outward uses external rotation, and lifting a bag off a high shelf uses all four to steady the joint while the deltoid does the heavy work — Cleveland Clinic describes the tendons as levers pulling bone.

How are rotator cuff tears classified?

By depth first, then by size — and increasingly by how many tendons are involved. The AAOS defines a partial tear as one that goes only part of the way through the tendon thickness, leaving it attached but thinner than normal, and a full-thickness tear as detachment from the bone: “full-thickness incomplete” when part of the tendon is off, “full-thickness complete” when there is effectively a hole in it.

Size is measured in centimetres across the defect. The Cofield bands, as applied in one published series:

Grade	Size	Share of that 41-shoulder series
Small	Less than 1 cm	34% (14 shoulders)
Medium	1 to 3 cm	51% (21)
Large	3 to 5 cm	10% (4)
Massive	More than 5 cm	5% (2)

Source: Medicine, 2018, 41 shoulders after arthroscopic repair.

Two caveats belong with that table. The 2025 review of massive tears points out that “the tear’s size varies based on the patient’s size and arm position at the measurement time,” which is why Gerber’s alternative definition — a complete tear of two or more tendons — is often treated as the more useful one. And massive tears are a small slice of the caseload carrying a heavy share of the trouble: the same paper puts them at “roughly 20% of all cuff tears and 80% of recurring tears.”

How common are rotator cuff tears, and at what age?

Common, and often silent. Almost 2 million people in the United States see a doctor about a cuff tear each year, per the AAOS. Population screening puts the underlying rate higher: the 2022 review cites prevalence estimates of 5 to 30 percent overall, a screening of 1,366 shoulders in which 20.7 percent had tears, and a second of 664 people in which 22.1 percent did — asymptomatic tears significantly outnumbering symptomatic ones.

Age is the dominant variable: zero below 20, almost 37 percent above 80, around 20 to 30 percent past 60. Tears also tend to grow — in a 224-person follow-up averaging 5.1 years, 49 percent showed progression, at a median of 2.8 years. The AAOS adds that most tears are wear-related rather than traumatic, are more common in the dominant arm, and that tendon blood supply declines with age.

Degenerative tears begin in a predictable spot: where the back of the supraspinatus meets the front of the infraspinatus, 13 to 17 mm behind the biceps tendon. None of this means a tear on a scan is what is causing your pain — the practical reason our do I need shoulder surgery page exists.

What is shoulder impingement, and how is it different from a tear?

A tear is torn tissue; impingement is a space problem. The AAOS separates three causes of cuff-related shoulder pain: tendinitis, inflammation of the tendons; bursitis, inflammation of the bursa; and impingement, where the gap between the acromion and the cuff tendons narrows as you raise the arm, so the bone rubs on the tendon and the bursa beneath it.

They overlap in practice — an irritated tendon swells, a swollen tendon has less room, and less room irritates it further. Initial treatment is non-surgical: rest, anti-inflammatory medication, physical therapy, and a steroid injection into the bursa if those do not settle it. Recovery “may take several weeks to months,” in the AAOS’s own words; our non-surgical alternatives page goes through that ladder.

Calcific tendinopathy is a separate entity again — calcium deposits inside the cuff tendons, visible on X-ray. How does the tear grade change the surgery?

It changes whether the tendon is trimmed, reattached, patched or bypassed entirely. The AAOS puts the simple version like this: a partial tear may need only a trimming or smoothing procedure — a debridement — while a full-thickness tear is usually stitched directly back to the bone. Repairs are most often arthroscopic, but an open or mini-open repair may suit a large or complex tear, or one needing a tendon transfer.

At the far end, the tendon may not be reachable. The 2022 review describes an irreparable tear as one with grade 4 fatty atrophy and retraction back to the socket, and lists the fallbacks: a latissimus dorsi transfer or a superior capsular reconstruction where there is no arthritis, and a reverse shoulder replacement where the head has migrated upward and secondary arthritis has set in.

The signs that push a surgeon toward operating, per the AAOS: symptoms lasting 6 to 12 months, a tear larger than 3 cm with good surrounding tissue, significant weakness and loss of function, or a tear from a recent acute injury. Our surgery types overview and success rates pages cover each procedure and its published outcomes.

Why does the cuff hurt most at night, and what does that mean for sleep?

Because the cuff is a stabiliser, and lying down removes the arrangement it had all day. Night pain is not a sign that something has gone wrong — “pain at rest and at night, especially if you are lying on the affected shoulder” is the first item on the AAOS symptom list for a tear, and it is also the dominant complaint after a repair. The Philadelphia Hand to Shoulder Center attributes the post-op version largely to inflammation, noting that swelling can increase when you lie down with the arm poorly supported.

The re-tear figures are what the restrictions are built around — and that is where published protocols go quiet. Reading 11 rotator cuff protocols for our sleeping in a sling dataset, nine told patients to keep the sling on at night, two never mentioned sleep, and none named a week at which lying on the operated arm becomes allowed. Surgeons fill the gap individually: OrthoNebraska’s Matthew Dilisio, MD suggests a small pillow between the operated shoulder and the ribs, and the Philadelphia guidance is a reclined or back position with the arm supported and pillows blocking a roll onto the repair. The Flexicomfort Shoulder Pillow is a 56 × 40 × 15 cm foam wedge with two adjustable straps built for that gap between ribs and arm.

Methodology: based on published clinical guidance, peer-reviewed research and the makers’ production specs for the pillows named. General information, not medical advice. Our night pain timeline and rotator cuff sleep phases pages carry the week-by-week detail.

Frequently asked questions

What the anatomy means for your own sleep setup

The cuff’s day job is to hold the humeral head centred while something else moves the arm. At night nothing is moving the arm, so the setup has only two jobs: keep the torso inclined, and keep the operated arm from drifting into rotation or dropping behind the body. That is why wedge angle and arm support come up more often than mattress choice on our sleeping after shoulder surgery hub.

Two pieces cover it in most beds: a foam wedge that holds an incline where a pillow stack flattens, and a strapped wedge between ribs and arm so the elbow does not sink.