Background: Shoulder external rotation exercises are commonly used to improve the stabilizing ability of the infraspinatus. Although the side-lying wiper exercise (SWE) is the most effective shoulder external rotation exercise to maximize infraspinatus activity, the effect of adduction force on the infraspinatus and posterior deltoid has not been demonstrated. Objects: This study was conducted to investigate whether horizontal adduction force increases infraspinatus activity and decreases posterior deltoid activity. Methods: Twenty-eight healthy subjects (male: 21, female: 7; age=23.5±1.8 years; height=170.1±7.4 ㎝; weight=69.4±9.6 ㎏) were recruited. Subjects were asked to perform the SWE under two conditions: (1) general SWE and (2) SWE with adduction force using pressure biofeedback. Surface electromyography (EMG) signals of the infraspinatus and posterior deltoid were recorded during SWE. Paired t-tests were used to compare the EMG activity of the infraspinatus and posterior deltoid between the two conditions. Results: Posterior deltoid muscle activity was significantly decreased following SWE with adduction force (7.53±4.52%) relative to general SWE (11.68±8.42%) (p<.05). However, there was no significant difference in the infraspinatus muscle activity between the SWE with adduction force (28.33±12.16%) and the general SWE (26.54±13.69%) (p>.05). Conclusion: Horizontal adduction force while performing SWE is effective at decreasing posterior deltoid activity.