The deep (ulnar) layer originates mainly from the supinator crest on the posterolateral surface of the ulna.
From their sites of origin, the two layers wrap around the posterior and lateral aspect of the head, neck, and proximal shaft of the radius to insert on the lateral surface of the radius superior to the anterior oblique line and to the insertion of the pronator teres muscle.
The supinator muscle supinates the forearm and hand.