The influence of testing positions and resistance locations on muscle activities during shoulder internal rotation strength measurements in young healthy adults

Abstract Background Although some tests of shoulder internal rotator strength including subscapularis are commonly used in clinical practice, the differences in shoulder muscle activities other than subscapularis muscle among those tests are not well understood. The purpose of this study was to examine the activities of the superficial shoulder muscles in addition to internal rotation strength during two belly-press and three lift-off test positions. Methods Thirteen healthy young adult men (age 29.5 ± 5.4 years) were recruited for the present study. They performed isometric shoulder internal rotation against manual resistance during the belly-press test position s with two different resistance locations and the lift-off test with three different arm positions. The surface electromyographic activities of the superficial shoulder muscles, including the deltoid (anterior, middle, posterior), pectoralis major, long head of triceps and latissimus dorsi muscle, were collected and compared between the two belly-press tests, and among the three lift-off test positions (P  Results The belly-press test position with resistance to elbow showed significantly greater activities of the anterior and middle deltoid muscle than the original belly-press test; but showed significantly smaller activities of pectoralis major, triceps and latissimus dorsi muscle than the original belly-press test. Among the three lift-off tests, all muscle activities, except for the pectoralis major, were greater in the lift-off at L4/5 than in the lift-off at buttock and thigh. Lift-off at thigh showed significantly smaller activity of pectoralis major than the lift-off at L4/5 and buttock. Conclusions The findings of the present study suggest that clinician should give attention to compensatory motions by excessive shoulder extensor and adductor muscle activities for the original belly press test, by excessive deltoid muscle activities for the modified belly-press, and by excessive shoulder extensor muscle activities for the lift off test in the inferior arm positions.