Background: Individuals with Subacromial Pain Syndrome (SPS) often present with a variety of contributing factors. It is possible that a subgroup exists within SPS that has primary impairments of scapular mobility and/or muscle strength. In an attempt to better identify scapular contributions in SPS, the Scapular Assistance Test (SAT) and Scapula Reposition Test (SRT) have been described. Additionally, thoracic spine thrust manipulation has been shown to be effective for shoulder pain. Problem Statement: It is currently unknown whether or not there are impairments in scapulothoracic muscle force generation or scapular mobility in individuals with SPS who have positive results on the SAT and SRT. It also remains unknown whether individuals with SPS respond differently in the immediate effects on scapular motion, scapulothoracic muscle force generation, pain, or function following different manipulation techniques. Methodology: Sixty subjects with shoulder pain were enrolled in the study. Baseline measures were obtained for scapular upward rotation and posterior tilt, scapulothoracic muscle force generation, pectoralis minor muscle length, pain, and function. Participants were randomized to receive a seated thrust manipulation, supine thrust manipulation, or sham manipulation. Measures were reassessed immediately after treatment and the Penn Shoulder Score (PSS) was reassessed at 48 hours. Results: The results indicated no significant differences in scapular upward rotation or posterior tilt, or muscle force generation based on the results of the SAT or SRT. There was a small but significant difference in pectoralis minor muscle length based on the result of the SAT. There were no significant between-group differences in scapular motion, muscle force generation, or pectoralis minor muscle length based on the treatment received. There were no significant differences in 48-hour improvement in pain, function, satisfaction, and total PSS scores. Small but significant within group changes existed on several measures. Discussion: The SAT and SRT may be ineffective in differentiating scapular movement associated impairments. Thoracic spine thrust manipulation resulted in no greater immediate improvements in scapular motion, strength, pectoralis minor muscle length, pain, or function compared to a sham treatment. The improvements in pain and function are likely not biomechanical in nature and are likely not derived from the manipulative thrust.