Background: Based on the understanding of the muscle activation relationship between the infraspinatus and posterior deltoid muscles to according to the angle of motion during external rotation on glenohumeral joint, effective shoulder joint strengthening exercise for the prevention and rehabilitation of shoulder injury due to muscle strength imbalance can be performed by achieving the ideal muscle activity ratio during exercise.
Objectives: To compare and analyze the muscle activation changes and activity ratio of the infraspinatus and posterior deltoid muscles according to the glenohumeral external rotation angle.
Design: Quasi-randomized trial.
Methods: The study included 48 healthy male and female adults who provided informed consent for participation in the study. All the subjects performed isometric glenohumeral external rotation by setting the angle of motion to 30°, 45°, and 60° using a 5 kg resistance weight pulley. On surface electromyography, the differences in muscle activation and activity ratio between the infraspinatus and posterior deltoid muscles were investigated.
Results: A significant difference in muscle activation was found in the comparison between the infraspinatus and posterior deltoid muscles according to the glenohumeral external rotation angle (P<.05). The muscle activation levels of the infraspinatus and posterior deltoid muscles were highest at the external rotation angles of 30° and 60°, respectively. The muscle activity ratio between the infraspinatus and posterior deltoid muscles also showed a significant difference (P<.05) and was highest at the shoulder external rotation angle of 30°.
Conclusion: The findings of this study suggest that muscle activity is the highest at the shoulder external rotation angle of 30° in healthy individuals.

INTRODUCTION
SUBJECTS AND METHODS
    Subjects
    Intervention methods
    Measurement methods
    Statistical Analyses
RESULTS
    General characteristics of the subjects
    Changes in muscle activation between the IS andPD muscles according to the glenohumeral externalrotation angle
    Changes in muscle activity ratio between the IS andPD muscles according to the glenohumeral externalrotation angle
DISCUSSION
CONCLUSION
REFERENCES

• Dongseok Yang(Department of Physical Therapy, Daegu Hospital of Korea)
• Wonho Choi(Department of Physical Therapy, Gachon University of Korea) correspondence