Brachialis is a flexor muscle in the upper arm. It lies deep to biceps brachii, and is a more powerful flexor of the elbow.
Brachialis arises from the anterior surface of the humerus, particularly the distal half of this bone. It attaches distally at the coronoid process and the tuberosity of the ulna. The ulna does not rotate, so brachialis's only action is flexion. Pronation/supination of the forearm does not affect its action.
These are the coracobrachialis muscle, which like the biceps attaches to the coracoid process of the scapula, and the brachialismuscle which connects to the ulna and the humerus.
It has also been proven through several tests into muscle group stimulation, that supination of the forearm with an isometric grip allows for close and normal-grip bench press exercises to have a much more profound effect on the biceps brachii and the clavicular portion of the pectorialis major.
Originally, supination of the forearm was attributed as a function of the brachioradialis muscle.