Your brachialis is hidden beneath the biceps. Your brachialis is the only true elbow flexor because it attaches to your ulna, the forearm bone that doesn't rotate, as opposed to the radius, which does. Because other elbow flexors like the brachioradialis and biceps attach to your radius, they also act to pronate or supinate your forearm instead of only generating elbow flexion. Despite these differences, the brachialis responds to the same general stretching technique as your biceps.
Stand in a doorway with your shoulders parallel to the doorway's opening.
Reach straight to the side and slightly back to grasp the doorjamb at shoulder level with your right hand. Back further away from the doorjamb, if necessary, so you can fully straighten your right arm.
Turn your body slowly toward the left until you feel a stretch in your upper right arm. Hold the stretch for 15 to 30 seconds, breathing normally, then switch to the other side. Do the stretch a total of three to five times on each side.
Sit down on the floor. Straighten both arms, and plant your hands on the floor behind you, slightly wider than your hips. Turn your hands so your fingers point straight back.
Scoot your hips slowly forward, keeping your weight evenly distributed, until you feel mild tension in your biceps and brachialis. You might feel a stretch across your chest, too.
Hold the stretch for 15 to 30 seconds, breathing normally, then scoot your hips back toward your hands to release the stretch. Repeat a total of three to five times.
Doing either one of these stretches is adequate; you don't have to do them both.