The biceps is composed of two heads — the short and long. Both converge onto the same tendon to cause elbow flexion, such as during barbell curls. Many lifters perform their dumbbell curls the same way as the barbell using a palms-up grip, which is highly effective at targeting the elbow flexors. Of note, because your arms are at your sides, the contribution of the short and long heads is relatively even.
Standard Dumbbell Curl
Dumbbell curls should be in virtually everyone’s arsenal of biceps exercises. Because there aren’t any compound moves for the bi’s, having a variety of moves to shock the relatively small biceps musculature is critical. As you curl the dumbbells up, in order to ensure the front delts aren’t taking over, be sure to lock your elbows at your sides — don’t allow them to travel forward. You may not be able to bring the dumbbells as high, but the emphasis on the biceps will be even greater.
Make This Change: Start With a Neutral Grip
This subtle change at the start of the move can make some not-so-subtle size and strength changes to your upper arms. When you begin each rep with a palms-in (neutral) grip rather than palms-up, it forces you to supinate (turn your wrist up) during the curl to arrive in the palms-up position. That twist allows you to take your biceps through its full range of motion, helping to develop the muscle more fully. Then at the top of the curl, you can continue with even more twisting to turn your palms out even more. (Even as you read this sentence, curl one arm up then twist your wrist slightly and you can feel the greater contraction.)
Get The Benefits From Both
Since there isn’t an arsenal of multijoint moves to target the biceps, your next best bet is to keep them responding and growing by attacking them with a nice variety of exercises. That includes barbell curls with various grip widths to dumbbell curls with different angles. While the standard palms-up dumbbell curl is phenomenal at mimicking the barbell curl while forcing stabilizers to contribute, supinating your wrists on some sets can heighten the peak contraction even further. Be sure to regularly change up your curls to ensure optimal development.