The purpose of this study was to evaluate the strength of the shoulder before and after the impingement test. This study included 153 cases of impingement syndrome, 20 cases of partial-thickness rotator cuff tear, and 60 cases of full-thickness rotator cuff tear. We divided each of the groups into 2 subgroups according to the mean percentage decrease in pain to evaluate the correlation between pain and muscle weakness. We also divided the impingement syndrome group into 2 groups based on stiffness. After the test, the subgroup with stiffness had a greater reduction in pain compared with the subgroup without stiffness. The results show no statistically significant difference in the strength of the shoulder, except for increased external rotation strength in patients with full-thickness rotator cuff tears and increased supraspinatus strength in patients with impingement syndrome with stiffness. In cases in which the continuity of the rotator cuff is maintained, pain is not an important factor in the cause of shoulder weakness. Preoperative muscle testing can be done despite pain, but if a full-thickness rotator cuff tear is confirmed, re-examination of external rotator strength is necessary. In patients with impingement syndrome, particularly with shoulder stiffness, supraspinatus strength can be decreased.