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Specifically, the biceps curl works the muscles in the front of the upper arm. Nicole L. Campbell: To do a biceps curl with a dumbbell, hold a dumbbell with your palm facing upward. Slowly curl the weight up by bending your elbow, keeping your elbow close to your body. Then slowly lower the weight to the starting position. You'll feel tension in the muscles in the front of your upper arm. When you're doing biceps curls, don't swing your arm or elbow.
The preacher curl places your arms in front of your body and allows you to work your biceps with a great degree of isolation. Isolating your biceps is of course useful to focus your mind in order to achieve a great contraction which will serve to build new muscle. The dumbbell curl performed on an incline bench offers similar advantages in that the angle of the arm is similar to that when performing a preacher curl.