Shoulder externally rotating exercises with pulley apparatus. Joint load and EMG

The mechanical load on the gleno-humeral joint and the muscular activity during shoulder external rotation resisted by a pulley apparatus were analysed using normalized, low-pass-filtered EMG recorded from the infraspinatus, deltoid, pectoralis major and trapezius muscles. The load moment about the longitudinal axis of the joint was compared with the distribution of maximum muscle moment over different angles throughout the range of motion. The effect of subject positioning on the joint load and the muscular activity was studied. The best adaptation between the curves for load moment and maximum muscle moment was obtained when the subject was positioned sitting with the pulley located 20 degrees posterior to a frontal plane through the shoulders at a distance of 1.3 m from the joint. Of the four muscles investigated, the infraspinatus was the most active. The method described might be used to find optimal designs of shoulder external rotation exercises with special regard to avoiding unintentional overloading of joint structures weakened by disease or trauma.