The structure indicated is the brachialis muscle.

The brachialis muscle is one of three muscles in the anterior compartment of the arm:

1. Biceps brachii
2. Brachialis
3. Coracobrachialis

The muscles of the anterior compartment are innervated by the musculocutaneous nerve and generally serve to flex the forearm at the elbow joint.

The brachialis muscle lies deep to the biceps brachii muscle and is synergistic in its action, assisting with flexion of the forearm at the elbow joint.

Origin: Distal half of anterior humerus

Insertion: Ulna tuberosity

Action: Flexion of elbow

Innervation: Musculocutaneous nerve

Learn more about the anatomy of the arm muscles in this tutorial.