Where do the deltoid fibers insert?

• A. Deltoid tuberosity of the humerus.
• B. Greater tubercle.
• C. Lesser tubercle.
• D. Olecranon.

Answer: (A)

The key idea is where the deltoid attaches at the distal end of the upper arm. The deltoid inserts on the deltoid tuberosity of the humerus, a roughened area on the lateral midshaft of the humerus. This insertion lets the deltoid pull the humerus upward to raise the arm and, with its different fiber parts, contribute to flexion and medial rotation (anterior fibers), abduction (middle fibers), and extension or lateral rotation (posterior fibers).

The other sites belong to different muscles: the greater tubercle is where several rotator cuff muscles insert, the lesser tubercle is the insertion for subscapularis, and the olecranon is where the triceps brachii attaches.