Supraspinatus

What it is

A rotator cuff muscle occupying the hollow above the spine of the scapula. Its tendon passes beneath the acromion — the bony roof of the shoulder — and attaches to the top of the greater tubercle of the humerus.

That passage under the acromion is the whole story of this muscle. The tendon runs through a confined space, between bone above and the head of the humerus below, and everything that goes wrong here relates to it.

It is one of four cuff muscles, remembered as SITS: Supraspinatus, Infraspinatus, Teres minor, Subscapularis.

What it does

Two things, and the second is more important than the first.

It initiates abduction — the first part of lifting your arm out to the side. Deltoid, pulling almost straight up along the humerus at that point, has poor leverage to start the movement; supraspinatus is better placed and gets it going before deltoid takes over.

It centres the humeral head in the socket. The shoulder is a very shallow joint — a large ball on a small, flat socket — and it is held together by soft tissue rather than bony fit. All four cuff muscles compress the head into the glenoid continuously so the joint does not simply travel. This stabilising job is what the cuff is actually for, and it happens during every arm movement regardless of direction.

What loads it

Abduction against resistance, and holding a load out to the side. It also works continuously in any overhead activity as a stabiliser.

What is commonly got wrong about it

That deltoid starts abduction. It is the other way round, and the mistake is everywhere.

"Shoulder impingement" as a settled diagnosis. The traditional model held that the acromion pinches this tendon, that its shape determines who gets pain, and that surgically shaving the bone fixes it. That model has been substantially undermined. The CSAW trial randomised 313 patients across 32 UK hospitals to decompression, to arthroscopy alone as a placebo, or to no treatment. Decompression showed no benefit over the placebo operation, and both surgical groups beat no treatment by a margin the authors judged not clinically important.¹ Many clinicians now avoid the term "impingement" entirely.

That a tear on a scan explains the pain. Pooling 30 studies and 6,112 shoulders, rotator cuff abnormality was found in 9.7% of people aged 20 and under and 62% of those 80 and over — rising with age at a similar rate whether or not the shoulder hurt. The authors conclude that cuff degeneration is common enough to count as a normal part of ageing.² A scan finding is not automatically the cause of a problem. What that means for any individual shoulder is a conversation with a clinician, not a website.

That "cuff exercises" mean bands only. The cuff stabilises under load. Light rotation work has its place, but the muscle's job is resisting large forces at the joint.

Sources

1. Beard DJ, Rees JL, Cook JA, et al. Arthroscopic subacromial decompression for subacromial shoulder pain (CSAW): a multicentre, pragmatic, parallel group, placebo-controlled, three-group, randomised surgical trial. The Lancet 2018; 391(10118):329–338. doi:10.1016/S0140-6736(17)32457-1 PMID 29169668 313 patients across 32 UK hospitals. Decompression showed no benefit over arthroscopy-only placebo (mean difference −1.3 points on the Oxford Shoulder Score, 95% CI −3.9 to 1.3). Both surgical groups beat no treatment by a margin the authors judged not clinically important.
2. Teunis T, Lubberts B, Reilly BT, Ring D. A systematic review and pooled analysis of the prevalence of rotator cuff disease with increasing age. Journal of Shoulder and Elbow Surgery 2014; 23(12):1913–1921. doi:10.1016/j.jse.2014.08.001 PMID 25441568 30 studies, 6,112 shoulders. Rotator cuff abnormality rose from 9.7% in people aged 20 and under to 62% in those 80 and over, with a similar rise regardless of symptoms. The authors conclude cuff degeneration is common enough to be considered a normal aspect of ageing.

Every source here was checked against PubMed rather than recalled. Where this site says something is disputed or unsupported and no source appears, that is deliberate: it means the claim is being conceded rather than cited.