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 movements of pronation and supination have already been described (page 304). They are performed by five muscles, two pronators and three supinators. The pronators are the pronator radii teres and the pronator quadratus. The supinators are the brachioradialis (supinator longus), the supinator (brevis), and the biceps.
The pronator radii teres arises by two heads, one from the medial (internal) condyle and the other, much smaller, from the inner surface of the coronoid process. The median nerve passes between these two heads. The muscle crosses the forearm obliquely and inserts by a flat tendon into the middle of the outer surface of the radius. It rotates the radius inward and tends to draw it toward the ulna and flex it on the humerus. The influence of this muscle is marked in displacing the radius when fractured.
The pronator quadratus arises from the volar (palmar) surface of the lower fourth of the ulna and inserts into the lateral and anterior surface of the radius. By its contraction it rotates the radius toward the ulna and in cases of fracture tends to draw the bones together and thus endanger the integrity of the interosseous space (Fig. 326).
The brachioradialis arises from the upper two-thirds of the lateral (external) supracondylar ridge of the humerus and inserts into the base of the styloid process of the radius. When the hand is in a state of pronation contraction of the brachioradialis will tend to supinate it. It also acts as a flexor of the elbow, as has already been pointed out. It is superficial and is an important guide both to the radial (musculospiral) nerve and to the radial artery.
The supinator arises from the lateral condyle, the external lateral and orbicular ligaments, and the triangular surface of the ulna below the lesser sigmoid cavity. It winds around the posterior and external surfaces of the radius and inserts into the upper and outer portion, covering its head, neck, and shaft as low down as the insertion of the pronator radii teres muscle. It lies deep down beneath the mass of extensor muscles and supinates the radius. It is pierced by the deep branch of the radial (posterior interosseous) nerve which bears the same relation to it as does the external popliteal nerve to the peroneus longus muscle in the leg.
The biceps muscle has already been described. Arising by its long head from the upper edge of the glenoid cavity and by its short head from the coracoid process it inserts into the posterior portion of the tubercle of the radius. While its main function is that of flexion of the elbow, still, from the manner in which it winds around the tubercle of the radius, it acts as a powerful supinator when the hand is prone and it is a disturbing factor in the displacements which occur in fractures of the bones of the forearm.
Fig. 326. - The pronator and supinator muscles of the left forearm.