Biceps 21s take a standard biceps curl to the next level by working the muscle through three separate ranges of motion that ultimately require more overall muscle engagement, control, and coordination. Biceps 21s require you to complete a total of 21 reps of biceps curls, but every seven reps, you change the range of motion, maximizing burn.
Stand tall, holding a dumbbell in each hand, your feet hip-distance apart, your knees slightly bent. Position the dumbbells in front of your thighs, your palms facing away from your body, your elbows extended. Start the exercise by curling the dumbbells halfway up, stopping when your elbows form 90-degree angles. Reverse the movement and lower the dumbbells. Perform six more reps.
After the sixth rep, immediately curl the dumbbells through a full range of motion, bringing them all the way to your shoulder. From this position, lower the dumbbells halfway, stopping when your elbows are bent to 90 degrees. Reverse the movement and curl the dumbbells back to your shoulders. Perform six more reps -- you can probably guess that you're going to do seven reps for each range of motion.
For the final seven reps in this set, you'll take the dumbbells through a full range of motion. Start by lowering the dumbbells all the way to your thighs, then perform seven total reps, curling the dumbbells all the way to your shoulders, then back again.
Remember, you're not pausing or resting between each of the seven reps. You're performing the total 21 reps in a series without rest.