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The pronator teres is a superficial muscle of the anterior forearm, located on the palm-side of the arm below the elbow. Responsible for pronation, or rotating the forearm palm-down, its two heads act on the proximal radioulnar joint, the articulation between the radius and ulna bones just beneath the hinge of the elbow. Populations to which this muscle is significant include players of racquet sports such as tennis or ping-pong as well as swimmers, who use it to pronate the forearm during the freestyle stroke. As such these individuals may be at risk for pronator teres overuse injuries like tendonitis or muscle strains and tears.
Because the pronator teres has two separate heads, or sections that converge into a single muscle, it has more than one point of origin in the arm. Its humeral head arises from just above the medial epicondyle, one of two rounded bony prominences at the base of the humerus bone, on the medial supracondylar ridge, as well as from the common flexor tendon attached to the medial epicondyle. Both are found just above the elbow on the inside of the upper arm. Its ulnar head, the smaller and deeper of the two sections, originates beneath the elbow on the inside of the coronoid process of the ulna, an uneven triangular projection on the anterior side of the bone. The two heads converge and cross the forearm diagonally to insert via a tendon partway down the lateral side of the radius.
Along with the pronator quadratus, which is located nearer to the distal or lower end of the anterior forearm, the pronator teres is one of only two muscles responsible for pronation. In anatomical position, the position used in the study of anatomy to locate structures on the body relative to one another, the arms are at the sides with the palms facing forward. Pronation is the act of rotating the palms backward without an accompanying rotation in the shoulder joint; from anatomical position the hands would simply turn to face the other direction.
This specific forearm movement initiated by the pronator teres and pronator quadratus involves changing the position of the radius and ulna relative to one another. In anatomical position, the ulna is medial, or closer to the midline of the body, and the radius is lateral, or farther from the midline, and the two are parallel. When pronation occurs, particularly when the elbow is flexed to 90 degrees, contractions of the pronator teres muscle pull the radius anteriorly and medially to the ulna. In other words, as the forearm rotates palm-down, the radius bone crosses in front of the ulna bone toward the midline of the body so that the two form an "X." As the muscle lengthens back to its neutral position, so the bones return to parallel.