52 patients with shoulder impingement syndrome were randomized to receive thoracic spinal manipulative therapy (SMT) or a sham SMT treatment. The goal of this study was to investigate the effect of SMT on shoulder pain and functional outcomes following treatment and determine the mechanism by which SMT improves these outcomes in individuals with subacromial shoulder impingement. The results of this study demonstrated that the use of thoracic SMT did not alter thoracic spine mechanics and that the treatment did not provide improvements in pain or function when compared to a sham for patients with subacromial shoulder impingement.