Figure 5. The resultant force vectors of the deltoid (A), supraspinatus (B), and combined deltoid and supraspinatus (C) during arm elevation from 30° to 150°. Note the superiorly oriented force vector of the deltoid (A) from 30°–120° of elevation when simultaneous supraspinatus activity is not present. The supraspinatus (B) provides a compressive force that centralizes the humeral head within the glenoid fossa throughout the range of motion.¹ This figure was published in Rockwood CA, Matsen FA, eds, The Shoulder, Morrey BF, Itoi, E, An KA, Biomechanics of the shoulder, 233–276, Copyright WB Saunders 1998.