Now let’s look at the muscles. There are three sets of muscles to look at: the ones that flex and extend the elbow, the ones that pronate and supinate the forearm, and the ones that flex and extend the wrist. We’ll look at each set of muscles separately. Later on in this section we’ll see them all together.
First the muscles that flex and extend the elbow. There are three flexors, and one extensor. The three flexors are brachialis, biceps, and brachioradialis.
Here’s the brachialis muscle. It arises from this broad area on the anterior humerus. It’s inserted here, on the ulnar tuberosity. The action of brachialis is to flex the elbow, which it does equally well whether the forearm is pronated or supinated.
The biceps muscle, its full name is biceps brachii, lies in front of the brachialis. It’s a more complicated muscle. For a start, it has two heads: a long and a short. To get a good look at them, let’s take away the anterior third of the deltoid muscle, and also pectoralis major.
Here’s the long head of biceps, here’s the short head. The tendon of origin of the short head merges with that of another muscle, coracobrachialis. Their common tendon of origin arises from the coracoid process.
The tendon of the long head makes a strange journey. It runs up the bicipital groove, and passes inside the shoulder joint, to reach its origin from the supraglenoid tubercle of the scapula.
The two heads unite to form a single belly, which narrows to form this unusual two-part tendon. The main part dives down between he radius and the ulna, and inserts on the radial tuberosity. On its lateral edge the tendon fans out, here it is in the intact forearm, into a thin sheet of fascia, the bicipital aponeurosis, which becomes continuous with the deep fascia surrounding the forearm. The aponeurosis gives the biceps an indirect attachment to the ulna.
The biceps flexes the elbow. It does this more efficiently when the forearm is pronated , because then it’s fully stretched when it starts its action. The biceps can also be a powerful supinator of the forearm, as we’ll see later.
The last of the three elbow flexors is brachioradialis. It arises halfway up the humerus, just below the deltoid tuberosity. It’s inserted all the way down here, on the distal radius. Brachioradialis is an efficient flexor of the elbow, whether the forearm is pronated or supinated.
The action of the flexors is opposed by just one extensor muscle, the triceps. The triceps muscle has three heads, a long head, a lateral head, and a medial, or deep head.
The long head arises, as we saw in the last section, from the infraglenoid tubercle of the scapula. The lateral head arises high up on the lateral side of the posterior humerus. The medial head arises from a broad area lower down and more medially. As we’ll see, the radial nerve runs next to the bone, between the lateral and medial heads.
The three heads of triceps converge, to form this massive tendon, which inserts here, on the olecranon. Contraction of the triceps extends the elbow.
Just for completeness, we need to mention this tiny muscle, the anconeus. It runs from the lateral epicondyle to the lateral aspect of the proximal ulna. Anconeus is a very minor elbow extensor.