Journal of shoulder and elbow surgery
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J Shoulder Elbow Surg · May 1995
Shoulder muscle forces and tendon excursions during glenohumeral abduction in the scapular plane.
Muscle force values and tendon excursions across the glenohumeral joint during unconstrained glenohumeral abduction in the scapular plane were evaluated with a dynamic shoulder testing apparatus. This evaluation was achieved by simulated rotator cuff and middle deltoid activity applied in four plausible muscle force ratios: (1) equal force to each tendon, (2) 2:3 ratio of force applied to the middle deltoid/supraspinatus tendons, (3) 3:2 ratio of force applied to the middle deltoid/supraspinatus tendons, and (4) zero force applied to the supraspinatus tendon to simulate supraspinatus paralysis. The glenohumeral joint was then moved to 5 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, and maximum glenohumeral abduction while muscle forces, tendon excursions, and glenohumeral joint kinematics were monitored. ⋯ Tendon excursion for the middle deltoid (6.4 +/- 0.2 cm) and supraspinatus (3.8 +/- 0.2 cm) were proportionately larger than those for the subscapularis and infraspinatus. Humeral head translations on the glenoid were less than 2 mm in all four conditions evaluated; therefore the glenohumeral joint behaves kinematically as a "ball-and-socket" articulation during glenohumeral abduction. Simulated supraspinatus paralysis does not change normal joint kinematics and does not prevent full glenohumeral abduction.