2. Deltoid

Another of the main shoulder muscles is the deltoid. It lies over the top of the shoulder and has three origin points, one from the clavicle and two from the scapula. It inserts into the humerus, the bone at the top of the arm. The deltoid flexes, rotates, abducts, and extends the humerus, depending on which fibers are activated.

deltoid humerus flex nojustice / Getty Images


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