The goal of this randomized crossover trial is to determine whether different levels of shoulder adduction pressure during an external rotation exercise change shoulder muscle activity in adults with chronic shoulder pain and adults without shoulder pain. The main question is whether adding a small amount of shoulder adduction pressure increases activity of the infraspinatus muscle without increasing activity of the deltoid muscles. Researchers will compare three levels of adduction pressure (0, 5, and 10 mmHg) during the same external rotation exercise. Participants will perform the three exercise conditions in a randomly assigned order while researchers measure shoulder muscle activity using surface electromyography.