Identify the origins of the latissimus dorsi and its insertion; describe its main actions.

• A. Origins: thoracolumbar fascia, iliac crest, lumbar and lower thoracic vertebrae, sometimes lower ribs. Insertion: intertubercular groove of the humerus. Actions: extends, adducts, and medially rotates the humerus; assists in downward rotation of scapula.
• B. Origins: trapezius; Insertion: deltoid tuberosity; Actions: abduction.
• C. Origins: sternum; Insertion: greater tubercle; Actions: flexion.
• D. Origins: acromion; Insertion: olecranon; Actions: extension.

Answer: (A)

The latissimus dorsi is a large back muscle that pulls on the shoulder from a broad set of origins. It arises from the thoracolumbar fascia along the lower spine, the iliac crest, the lumbar and lower thoracic vertebrae, and often the lower ribs. Its tendon inserts into the intertubercular groove (bicipital groove) of the humerus, placing the muscle to move the arm rather than the scapula directly.

Because it attaches to the humerus, contracting the latissimus dorsi brings the arm toward the body and toward the back. This results in three main actions: the arm is extended (pulled backward), adducted (pulled toward the midline), and medially rotated (turned inward). When the arm is fixed, its pull can help move the trunk upward, as in climbing or pulling actions. It can also contribute to downward rotation of the scapula during certain movements, but its primary role is in extending, adducting, and medially rotating the humerus.

The other options mix up muscle origins, insertions, or primary actions—for example, the trapezius inserting on the deltoid tuberosity and acting to abduct; a sternum-origin muscle inserting on the greater tubercle pointing to a different muscle; or a muscle originating from the acromion and inserting on the olecranon, which describes the triceps.