Reaching behind your back is a classic trigger of rotator cuff painStrong evidence
Rotator cuff related pain, also called subacromial pain, is "the most common reason for shoulder pain", NHS inform says, and it is usually brought on by "lifting the arm above shoulder level and reaching behind the back, for example when putting a jacket on". A UK care pathway reports that rotator cuff problems account for up to 70% of shoulder pain, and a review for GPs puts them at ages 35 to 75. In UK general practice, 2.36% of adults see a GP about a shoulder each year, with new cases peaking around 50.
A torn rotator cuff can make "reaching behind your back" harder, and some tears cause weakness without much pain, US orthopaedic surgeons (AAOS) say. No study explains exactly why the movement hurts. Reaching behind combines bending the elbow, taking the arm back and out, turning it inwards and tipping the shoulder blade forward, in a study of 20 healthy young adults. Impingement is "nothing to do with your tendons being trapped by bones, as was previously thought", Arthritis UK says.
Sources 1, 2, 3, 4, 5, 6, 7
Frozen shoulder: stiff in every direction, and reaching behind is lost tooMixed evidence
Frozen shoulder typically starts slowly, with pain near the top of the upper arm and "a disproportionately severe loss of passive external rotation" (turning the arm outwards), a UK shoulder society pathway says. About 2.4 people per 1,000 get it each year, typically women between 45 and 60, and 6% to 17% get it in the other shoulder within 5 years. People with diabetes are about 5 times as likely to have it, and thyroid disease is linked too (about 1.9 times, in case-control studies).
Reaching behind the back is lost as well: an NHS trust leaflet names "twisting movements such as reaching behind the back or head". In one small study (17 people, abstract only), the affected side reached behind with much less inward turning at the shoulder joint (31 against 66 degrees), which the authors call the main cause. One person's account in a review in the British Medical Journal: "I could not lift my arm up, nor could I put it behind my back. I could wear only clothes with front fastenings".
Sources 8, 9, 10, 11, 12, 3
The front and the top of the shoulderEmerging evidence
The biceps tendon at the front causes "Pain with reaching backward, such as to put on your seatbelt in a car or to put a coat on", AAOS says. The subscapularis, the biggest rotator cuff muscle and the main inward turner, can tear after the arm is forced backwards or after a dislocation over 40. In the lift-off test, a clinician asks you to lift the back of your hand off your lower back; together with two similar tests, it picks up about 80% of these tears, a review says.
The joint at the end of the collarbone (the acromioclavicular or AC joint) hurts most reaching across the body. In 153 people in primary care, 14% had AC joint pain confirmed by an injection, the usual tests didn't pick them out, and reaching behind the back wasn't one of the features that told them apart. AC joint arthritis "typically causes few to no symptoms".
Sources 13, 14, 15, 16, 17
Other causes, and how rare they areEmerging evidence
Shoulder arthritis causes a global loss of movement, especially turning the arm out, usually from 60; an X-ray confirms it. Throwers can lose inward turning in the throwing arm, though in 2,195 athletes it wasn't clearly linked with injury. Calcium in a tendon causes sudden, severe pain that "usually necessitates an early secondary care referral".
Shoulder pain can come from the neck "even when you don't feel pain in your neck", NHS inform says, with pins and needles or burning; a pinched nerve in the neck affected about 83 people per 100,000 a year in one US population, most often at 50 to 54. A tumour at the top of the lung (a Pancoast tumour) is rare, about 5% of one kind of lung cancer as one review reports, but shoulder pain was its commonest first symptom. A tumour mimicking frozen shoulder over 40 is rare. The NHS gallstones page doesn't mention shoulder pain.
Sources 18, 17, 19, 2, 20, 21, 22, 8, 23
Tears on scans are common without painStrong evidence
In an ultrasound screening of a Japanese village, full-thickness cuff tears were found in 10.7% of people in their 50s, 26.5% in their 70s and 36.6% in their 80s, and two-thirds of the tears caused no symptoms. Across 30 studies, rotator cuff changes rose from 9.7% at 20 or under to 62% at 80 and over, "regardless of symptoms". A UK care pathway says a tear on a scan "does not imply a definite need for surgery in every case".
Sources 24, 25, 2
When a scan helpsStrong evidence
Usually not at first: an ultrasound or magnetic resonance imaging (MRI) scan of the rotator cuff "is rarely indicated in primary care", though an X-ray can help if you aren't improving. Sudden weakness after an injury needs urgent referral and a scan. Frozen shoulder is diagnosed by examination, and an X-ray tells it from arthritis. After a dislocation, "You may be offered a scan to check if your shoulder has been damaged", the NHS says. Reaching up your back or lifting your hand off it shows how far you can reach and how strong the front tendon is, but doesn't diagnose on its own.
Sources 2, 8, 18, 26, 14
How it usually goesStrong evidence
New shoulder pain "should begin to settle within 6 weeks" in many cases, NHS inform says, and the NHS suggests shoulder exercises for 6 to 8 weeks. In UK general practice, about half of people see a GP about it only once, and 13.6% are still consulting in the third year.
Frozen shoulder gets better, but slowly: NHS inform says it "can take more than 2 years to recover from", and some people keep slight stiffness that "tends to not limit their day to day activities". In 223 people followed for an average of 4.4 years, 59% were normal or near normal and 41% had some symptoms, almost all mild. Most improvement comes early, and there's no evidence of neat phases ending in full recovery. No one can promise a date.
Sources 20, 27, 4, 28, 29, 30
