Curls can build size, but you have to use heavy weights.
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Dumbbell curls are one of the quintessential exercise for training several arm muscles, including the biceps and the forearms. In beginners, curls build both strength and size. As you become more experienced at weight training, alter the weight, sets and rep number to focus on developing additional size or strength in the arms.
Type of Curl
All curls work several major muscles of the arm, but how you hold the dumbbell determines which ones get the most action. For example, a classic curl in which you stand, hold the dumbbells alongside your thighs and curl up, rotating the palm to face the shoulder, emphasizes the biceps brachii -- the muscles at the front of the upper arm. If you do the curl in a hammer style, in which you keep the palms in a neutral grip facing each other as you bend and extend the elbows to execute the curl, you'll get more emphasis on the forearm muscles. Sit at a preacher bench and prop your upper arm against the inclined pad as you curl, and you'll put more emphasis on the brachialis, a muscle of the upper arm that works to extend the elbow and lies deeper than the biceps.
Curling for Size
If you're after big guns, put most of your focus on the classic bicep curl. Plan for three to six sets consisting of eight to 12 repetitions. Allow for a 30- to 60-second rest between sets. To build size, you use 80 to 85 percent of your one-repetition maximum -- the most weight you can curl in one effort. Other types of curls will help contribute to size too, but the classic curl will make the upper arm muscles pop.
Curling for Strength
If you're after greater strength, incorporate several types of curls in your workout so you develop the muscles of the upper and lower arm. A strength-based workout involves three to six sets of just five or six repetitions. Rest for three to five minutes between sets so your muscles have a chance to recover completely and can successfully hoist a consistent 80 to 88 percent of your one-repetition maximum for each exercise.
Balanced Strength or Size
Curls are an essential arm exercise, but they neglect the back of the upper arm -- the triceps muscles. If you only curl and don't do kickbacks, triceps extensions and dips, you'll develop an imbalance in muscle size and strength. Include at least one triceps exercise in your arm-training workouts to help you reach your fitness goals and avoid injury.