Which muscle abducts the shoulder and inserts at the deltoid tuberosity of the humerus?

• A. Pectoralis Major
• B. Deltoid
• C. Latissimus Dorsi
• D. Internal Intercostals

Answer: (B)

Abduction of the shoulder is carried out by the deltoid, particularly its middle fibers, which pull on the humerus to lift the arm away from the body. The deltoid inserts at the deltoid tuberosity on the lateral side of the humerus, so this combination of inserting at that point and performing abduction fits perfectly. The other muscles have different roles or insertions: pectoralis major flexes, adducts, and medially rotates the arm; latissimus dorsi extends, adducts, and medially rotates the arm; internal intercostals act on the ribs for breathing and do not attach to the humerus. (Note that the supraspinatus helps initiate abduction in the first 15 degrees, but it does not insert at the deltoid tuberosity, so the muscle described here is the deltoid.)