The elbow joint consists of the humerus (upper arm bone), radius, and ulna in the forearm. The ulna is the bone on the little finger side of the forearm (remember l in ulna for little finger) and the radius radiates around it. The elbow joint muscles are the biceps brachii, brachialis, brachioradialis, triceps brachii (triceps muscle), anconeus, pronator teres, pronator quadratus, and supinator.
The Biceps brachii crosses both the elbow and shoulder joints. It is a weak flexor of the shoulder joint but is a strong flexor of the elbow joint, especially when the forearm is supinated (palm facing up).
The Triceps Brachii also assists Latissimus Dorsi in extending the shoulder joint. It contracts strongly during the up phase of a push-up, to straighten the arm.
Pronator Quadratus works in conjunction with Triceps Brachii during pronation with elbow extension.
Pronator Teres works the hardest when the elbow is flexing the hand simultaneously pronating.
The supinator muscle assists the Biceps brachii in supinating the hand, which is turning it over so that the palm faces up. To isolate the supinator muscle, supinate the hand whilst extending the elbow as this takes out the Biceps muscle.
Anconeus muscles work alongside Triceps Brachii in extending the elbow. It also acts to pull the synovial membrane out of the way of the olecranon process when the elbow is extending.
The brachialis acts to flex the elbow whether in pronation or supination, along with the Biceps brachi. As Brachialis is attached to the Ulna, which cannot rotate, it is the only true flexor of the elbow.
The Brachioradialis muscle acts to supinate the forearm from a pronated position when it flexes the elbow. When starting in a supinated position, it acts to pronate the hand as it flexes the elbow.