The hammer curl develops the bicep and forearm. It has particular effect on the brachialis, the muscle running along the exterior of your upper arm. It specifically targets the brachioradialis in the forearm.
This exercise is similar to a standard dumbbell curl but has a few distinctions. The hammer curl works the forearm a little more than the standard curl. It also works the entire set of arm muscles in a different way than the typical dumbbell curl. You should alternate hammer sets of hammer curls with sets of standard dumbbell curls. Include 8-15 reps in each set of curls.
How to perform the hammer curl:
Stand straight holding a pair of dumbbells, arms at your sides, turn your hands so your palms face towards each other, without moving your upper arms slowly curl the weights up until the dumbbells are close to your shoulders. Flex your biceps, and return to the starting position.