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.

Background: The serratus anterior (SA) muscle is one of the important muscles in the upward rotation of the scapula when the arm is raised. Insufficient muscle activity of the SA can cause deformation of the shoulder rhythm resulting in shoulder pathology. Objects: This study intends to compare SA and upper trapezius (UT) activity during the conventional wall-slide and push-up plus exercises for SA muscle strengthening and the scapular upward rotation (SUR) exercise. Methods: A total of 30 subjects participated in this study, and we measured the muscle activity of the SA and UT muscles during the wall-slide, push-up plus and SUR exercises. The one-way repeated ANOVA was used to compare SA and UT muscle activities during the 3 exercises. Results: During the SUR exercise, SA muscle activity was 79.88% maximum voluntary isometric contraction (MVIC), which was significantly higher than its activity during the other 2 exercises. The UT muscle activity was 47.53 %MVIC during the SUR exercise, indicating a significantly higher UT muscle activity than during the other 2 exercises. Conclusion: These findings suggest that the SUR exercise can maximize SA muscle activity to strengthen the SA while keeping UT muscle activity at an appropriate level.

Background: Flexion-relaxation phenomenon (FRP) was a term which refers to a sudden onset of myoelectric silence in the erector spinae muscles of the back during standing full forward flexion. Hamstring muscle length may be related to specific pelvic and trunk movements. Many studies have been done on the FRP of the erector spinae muscles. However, no studies have yet investigated the influence of hamstring muscle flexibility on the FRP of the hamstring muscle and lumbopelvic kinematics during forward bending. Objects: The purpose of this study was to examine the flexion-relaxation ratio (FRR) of the hamstring muscles and lumbopelvic kinematics and compare them during forward bending in subjects with different hamstring muscle flexibility. Methods: The subjects of two different groups were recruited using the active knee extension test. Group 1-consisted of 13 subjects who had a popliteal angle under 30°; Group 2-consisted of 13 subjects who had a popliteal angel above 50°. The kinematic parameters during the trunk bending task were recorded using a motion analysis system and the FRRs of the hamstring muscles were calculated. Differences between the groups were identified with an independent t-test. Results: The subjects with greater hamstring length had significantly less lumbar spine flexion movement and more pelvic flexion movement. The subjects with greater pelvic flexion movement had a higher rate of flexion relaxation during full trunk bending (p<.05). Conclusion: The results of this study suggest that differences in hamstring muscle flexibility might cause changes in people’s hamstring muscle activity and lumbopelvic kinematics.