Brachialis

The strongest elbow flexor, the brachialis muscle stop elbow extension during deceleration. When forearm flyout occurs, the brachialis eccentrically contracts to prevent injury.

Its origin is on the lateral side of the humerus just below the insertion of the deltoid, and its insertion is shared by the coronoid process and the tuberosity of the ulna.

The brachialis does not aid in supination or pronation of the forearm.

Thanks to highfive.me.uk, I was able to find a decent diagram:

Source: highfive.me.uk