Now let’s look at the muscles that produce pronation and supination. There are two of each.
Of the two pronator muscles, the larger and more proximal one is pronator teres. Along with several other muscles, it arises from the medial epicondyle. In addition it has a small deep head of origin which arises from this part of the ulna.
Here’s the deep head of pronator teres. The median nerve passes between the two heads of pronator teres as it enters the forearm. Pronator teres inserts here, halfway down the lateral surface of the radius. Here’s its action: pronation.
The second pronator muscle is pronator quadratus, which arises from the anteromedial aspect of the ulna, and inserts here, on the anterior surface of the radius. Here’s the action of pronator quadratus.
Now let’s look at the two muscles which produce supination. The one that we haven’t seen yet is simply called supinator. Here it is. It arises from the lateral epicondyle, from the anular ligament, and from this ridge on the ulna, the supinator crest. It’s inserted on the radius, along a line ending just above the insertion of pronator teres. The deep branch of the radial nerve runs through the supinator. It enters here, and emerges under here. Here’s the action of supinator. It’s a nice match for pronator teres.
The other supinator muscle we know about already. It’s the biceps. The insertion of the biceps on the radial tuberosity gives it plenty of power to rotate the radius, especially when the elbow is flexed. When the biceps is working as a supinator, its flexing action is held in check by the simultaneous action of the triceps.
Because of the great strength which biceps contributes, supination is a more powerful action than pronation.