Muscle activity pattern of the shoulder external rotators differs in adduction and abduction: An analysis using positron emission tomography

Background: The muscle activity pattern during shoulder external rotation has not been fully clarified. This study aimed to determine the activities involved in external rotation in the adducted and abducted positions using positron emission tomography (PET). Methods: Seven healthy volunteers underwent PET examinations after performing external rotation using an elastic band at both 0° and 90° of shoulder abduction in the frontal plane. External rotation exercise was performed before and after injection of fluorine 18 fluorodeoxyglucose, which was followed by PET examination. The protocols for external rotation exercise were identical between the 2 shoulder positions. To obtain control data, PET examination was also performed under resting conditions. The order of these 3 PET examinations was randomized, and they were performed at intervals of 1 week or greater. Each PET image was fused to the corresponding magnetic resonance image to identify each shoulder muscle. After this, the standardized uptake value was calculated in each muscle and was compared between the 2 shoulder positions. Results: The infraspinatus showed the greatest muscle activity during external rotation at 0° of abduction, whereas the teres minor showed the greatest activity at 90° of abduction. The teres minor-infraspinatus ratio at 90° of abduction (mean ± SD, 1.21 ± 0.23) was significantly higher than that at 0° of abduction (0.84 ± 0.15) (P < .01). Conclusion: The infraspinatus and teres minor are the main shoulder external rotators. The teres minor is more important as an external rotator in abduction than in adduction.