Antagonistic Muscles Associated with the Radius and Ulna
The lower arm is flexed when the biceps brachii and the brachialis contract. The biceps brachii has two heads and originates from the medial surface of the scapula near the point where it articulates with the humerus and it inserts on the radius. The brachialis originates from the lateral, proximal end of the humerus and inserts on the ulna.
The lower arm is extended when the triceps contracts. The medial and lateral heads of this muscle originate from the humerus and the long head originates from the lateral surface of the scapula. All three heads insert as a common tendon on the ulna.
Click here to view the muscles that flex and extend the lower arm. The brachialis (1) passes under the biceps brachii (2) to insert on the ulna. While the biceps brachii (2) inserts on the radius.
In this view the medial head of the triceps (3) can be seen inserting on the elbow.
Click here to return to the forearm bones.
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