This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The elbow-joint is interposed between the long bones of the forearm below and the long humerus above. The arm muscles come down and pass over the joint to insert close to it in the bones of the forearm. The muscles of the forearm in a similar manner cross the joint and are attached comparatively near it to the humerus above. Thus we see the joint strengthened by the crossing of the various muscular insertions. The elbow having only an anteroposterior motion, the muscles must of necessity be in two main groups, one in front and the other behind the joint.
It is true that there are lateral muscles but they have little or no influence on the movements of the elbow-joint. The medial (internal) condyle gives origin to the flexor muscles of the forearm and the pronator radii teres, and the lateral (external) condyle gives origin to the extensor muscles; but the bony attachment of both these sets of muscles coincides too closely with the axis of motion to allow of their aiding to any marked extent either flexion or extension of the elbow. Their function as far as the elbow is concerned is to aid and strengthen the lateral ligaments of their special sides.
These comprise the biceps, brachialis anticus, brachioradialis, and extensor carpi radialis longior. It will be observed that the first two muscles come from above and cross the joint, while the last two arise just above the joint to pass down the forearm (Fig. 297).
The brachialis anticus arises from the humerus by two heads, one on each side of the insertion of the deltoid, and from the anterior surface to just above the elbow-joint. It passes over the joint and inserts into the base or lower and inner part of the coronoid process. It does not insert into the tip, but some distance below. Its function is purely flexion.
Fig. 297. - View of the antecubital fossa and muscles at the bend of the elbow.
The biceps arises from the upper rim of the glenoid cavity by its long head and from the coracoid process by its short head. It inserts into the posterior edge of the bicipital tubercle of the radius. Between it and the tubercle is a bursa. About 4 cm. (1 1/2 in.) above its insertion its tendon gives off a fibrous expansion which passes inward to blend with the deep fascia covering the flexor group of muscles. This is called the bicipital or semilunar fascia. The biceps tendon passes almost in the middle between the two condyles. Along its inner side is the brachial artery, which is covered by the bicipital fascia; over this fascia passes the median basilic vein, sometimes used for transfusion. The insertion of the biceps is into the radius, which is the movable bone, and not into the ulna, which is less so. As a consequence, in addition to its function of flexion it acts also as a powerful supinator of the radius.
The extensor carpi radialis longior arises from the lateral condyle and lower third of the supracondyloid ridge and inserts into the base of the second metacarpal bone.
The brachioradialis or supinator longus arises from the upper two-thirds of the lateral (external) supracondyloid ridge above the preceding muscle and as high as the insertion of the deltoid. It inserts into the base of the styloid process of the radius.
These two muscles, owing to their high attachment, so much above the axis of motion of the joint, both act as flexors. The brachioradialis also supinates the hand.