It is important to observe that this muscle consists of two parts with different origins and passing in different directions. The posterior interosseous nerve passes between these two parts as it leaves the cubital fossa to enter the back of the forearm.
The deep part of the supinator consists of fibers that arise from the supinator crest and fossa of the ulna and wrap horizontally around the radius to be inserted into the area between the anterior and posterior oblique lines. The superficial part of the muscle arises from the distal border of the lateral epicondyle (just in front of anconeus), from the radial collateral (lateral) ligament of the elbow joint and from behind the supinator crest above abductor pollicis longus. The fibers slope downwards to overlie the horizontal deep fibers and reach the radius just above the anterior oblique line.
Nerve supply of supinator:
A branch to both parts of the supinator (C5 and 6) leaves the posterior interosseous nerve in the cubital fossa before the nerve enters the muscle.
Action of supinator:
Note that the biceps is the powerful supinator of the forearm; the supinator is to be regarded rather as a muscle that fixes the forearm in supination. Only when the elbow is completely extended is the supinator the prime mover for the action of supination, which is much weaker in this position. The muscle is too deep to be seen or palpated.