Brachioradialis

The second of the two main muscles at the front of the upper arm, the brachioradialis (note 1) joins the middle of the upper arm bone (humerus) to the radius (Note 2). It is the second of the main muscles responsible for bending the arm at the elbow. As with the bicep, in archery, the drawing arm brachioradialis may come into use in the early part of the draw, keeping the string close to the bow arm through the draw, and ideally, should largely relax as the drawing elbow comes around behind the string.

Note 1: Sometimes called the brachial. This is a bit general; a 'brachial' muscle is one used in 'brachiation', or swinging around in trees. The biceps, corachobrachialis, triceps, and brachioradialis are all 'brachials' in some sense.

Note 2: The radius is the forearm bone on the thumb side.