Concentration curls are resistance training exercise movements that target the biceps muscles and the brachialis muscle of the upper arm. They can be done with dumbbells as part of a weight lifting program or with a cable machine or resistance bands. Due to the muscle isolation this exercise provides, it is most often used by bodybuilders and other exercisers looking to build upper arm size and strength, although concentration curls are also sometimes part of a casual exerciser's program as well.
It is important to keep good form while doing concentration curls. Seated on a bench or other sturdy platform, lean over slightly forward and to the side, and place the back of the arm that will be worked against the inside of the leg on that side. Grasp the dumbbell and keep the back of the arm at the elbow tightly braced against the leg throughout the entire lifting motion. This should result in the arm only moving at the forearm below the elbow, while the upper arm and shoulder remain stationary. Perform the concentration curls without losing form throughout all of the reps completed, or else there will be less muscle isolation during the exercise.
Since concentration curls specifically isolate the muscles of the upper arm, they should be used in combination with plenty of other strength training exercises as part of a complete program. If building upper arm strength and size is an important part of the program, different exercises that use compound movements to target multiple muscles and even other curls besides concentration curls can be complementary. It might also provide a slightly different experience to switch between dumbbells, cables, and resistance bands when doing concentration curls and other arm exercises.
Some very advanced exercisers make modifications to this exercise to target other muscle groups in addition to the upper arm. The lower back can be worked by doing the curl while standing, bent forward at the waist, without the support of a leg beneath the arm to keep the upper portion of the arm from moving. Beginners and those with back problems might be better off leaving this form for more advanced weight lifters. To work the legs, concentration curls can be done with similar form to the seated version, only without a bench. With this form, the legs are bent into a squat and again used as a brace for the arm during the curls.