A painful arc is pain felt at about half range in a movement, in the absence of pain before and after the mid-range (see p. 213). Pain may or may not recur at the end of the movement. An arc results from a momentary impingement of a lesion, lying in such a position that it is caught between the lesser or greater humeral tuberosities and the overlying anterior part of the acromion, the coracoacromial ligament or the acromioclavicular joint1–4 (Fig. 1). A painful arc is mainly found on elevation, sometimes on medial rotation of the arm. Irrespective of the precipitating movement, the diagnostic significance remains the same: temporary impingement.

An arc on elevation is usually most pronounced during the active movement, and more on the way up than on the way down. This is due to the muscular activity, which pulls the humeral head closer to the acromion during active elevation.⁵

Sometimes the patient tries to avoid the painful moment by changing the direction of the elevation when the pain is felt. Indeed, some patients abduct until they feel the pain start at mid-range and then bring the arm in front of the body to avoid further impingement.

Occasionally the patient may experience the pain at mid-range only on the way down, or even on passive medial rotation. This has the same meaning as a painful arc on the way up. If the examiner has the impression that passive elevation or medial rotation is limited, it is important to insist that the movement is gently continued, otherwise the patient may voluntarily stop the movement too early, due to pain. The apparent limitation may be overcome by persistence, and it may be possible for the patient to then get beyond the painful moment. Thus, what might initially be regarded as a limited movement can be proved to be a painful arc with full range, which has completely different diagnostic implications.

Neer introduced the term ‘impingement syndrome’, now widely recognized and further divided into a ‘subacromial’ and a ‘subcoracoid’ impingement syndrome.^6–9 Cyriax always considered painful impingement as a sign rather than a syndrome: it does not implicate just one lesion but one of several.^10,11 Since the exact diagnosis is in the majority of the cases based on other tests – very often on resisted movements – a painful arc usually has value as a localizing sign, defining exactly in which particular part of the structure the lesion lies.

Movement in the subacromial space is between a convex, cuff-covered proximal humerus and a concave surface consisting of the coracoacromial roof and the inferior part of the acromioclavicular joint.¹² The gliding surface between these two moving spheres is the bursa. Pathology in one or more of these structures may partly interfere with movement and thus cause the clinical sign, ‘painful arc’ (Fig. 2).