Objective The primary aim was to investigate the effectiveness of adding more resistance exercise to usual care on pain mechanisms (including temporal summation, conditioned pain modulation (CPM) and local pain sensitivity) and pain catastrophising in people with subacromial impingement at 16 weeks follow-up. Second, to investigate the modifying effect of pain mechanisms and pain catastrophising on the interventions' effectiveness in improving shoulder strength and disability Methods 200 consecutive patients were randomly allocated to usual exercise-based care or the same plus additional elastic band exercise to increase total exercise dose. Completed add-on exercise dose was captured using an elastic band sensor. Outcome measures recorded at baseline, 5 weeks, 10 weeks and 16 (primary end point) weeks included temporal summation of pain (TSP) and CPM assessed at the lower leg, pressure pain threshold at the deltoid muscle (PPT-deltoid), pain catastrophising and the Shoulder Pain and Disability Index. Results Additional elastic band exercise was not superior to usual exercise-based care in improving pain mechanisms (TSP, CPM and PPT-deltoid) or pain catastrophising after 16 weeks. Interaction analyses showed that pain catastrophising (median split) modified the effectiveness of additional exercises (effect size 14 points, 95% CI 2 to 25), with superior results in the additional exercise group compared with the usual care group in patients with less pain catastrophising. Conclusion Additional resistance exercise added to usual care was not superior to usual care alone in improving pain mechanisms or pain catastrophising. Additional exercise was, however, superior in improving self-reported disability in patients with lower levels of pain catastrophising at baseline.