Turn your palms toward each other to perform hammer curls.
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When you exercise your arms, it's easy to focus on the larger muscle groups, such as the biceps and triceps in your upper arms. The brachioradialis is more difficult to target because it snakes across more than half of each arm. The muscle attaches to your upper arm, near your elbow, and extends to your wrist. The brachioradialis helps flex your elbow, so any activity in which you bend your elbow relies in part on this muscle. As with all strength-training exercises, warm up with five to 10 minutes of cardio activity before you work the brachioradialis.
Curling exercises are typically the go-to activities to strengthen the biceps. Holding a barbell with a different grip, however, shifts the emphasis to the brachioradialis. Instead of an underhand grip, hold the barbell with your arms extended downward and your palms facing your body. Perform an otherwise-standard curl by flexing your elbows and raising the bar toward your shoulders. Keep your elbows tight to your sides and your upper arms as still as possible throughout the exercise.
The hammer curl is another variation of the standard biceps curl that shifts the focus to the brachioradialis. Stand straight with a pair of dumbbells in your hands. Let your arms hang naturally at your sides and aim your palms at your body. Curl each dumbbell toward your shoulder, keeping your wrist in the same position, so your palms face each other if you lift both weights simultaneously. You can also curl one arm at a time. Alternatively, do hammer curls with a low-cable machine by grasping a rope attachment that's secured to the cable.
Performing curls on a preacher bench offers another way to target the brachioradialis. Do standard preacher curls -- with the backs of your upper arms on the bench's pad -- using a barbell, but with an overhand grip. Do the same exercise, using the same grip, with a low-cable machine by attaching a straight bar to the cable and placing the preacher bench in front of the machine. You can also find some lever machines that replicate reverse-grip preacher curls.
As a rule of thumb, perform 12 repetitions of weight-training exercises. Use a weight that leaves your arms fatigued after 12 reps, but make sure to do each rep with correct form. Train the brachioradialis one to three times per week -- during a total-body workout or during upper-body or arms-only sessions.