Bulging biceps and better grip are just two of benefits you will experience from using hammer-style dumbbell curls. While it is common knowledge that your biceps are trained with curls, the hammer curl variation trains other areas of your arm as well. Performing dumbbell hammer curls is easy and only requires basic knowledge of how your arms move. To set up the exercise, stand up and grab a pair of dumbbells while rotating your hands inward so that the palms of your hands are parallel to each other. The curls are performed by gently pressing your elbows into the sides of your body and then flexing at your elbow.
Your biceps, which runs across the front of your upper arm, is the biggest muscle trained when performing dumbbell hammer curls; however, your brachioradialis performs a lot of work. The brachioradialis runs from your wrist, across the inside portion of your elbow and into your upper arm bone. The rotation of your hands allows the brachioradialis to contribute more to the upward motion of the curl than traditional curl exercises even though it is smaller than your biceps. Your brachialis is also worked during these curls and provides important stabilization for your arm as it moves. It also runs across the elbow joint, but is much smaller than the brachioradialis.
Because the three biggest muscles located on the front of your upper arm are being worked, your biceps must acclimate to the resistance by getting stronger and growing in size. Size improvements in the brachialis and brachioradialis will push up on the biceps, making it "bulge" or appear fuller in size. This is further augmented by the size improvements of your forearms that are attributed to the development of the brachioradialis. Improvements in forearm size create the illusion of a smaller elbow joint. This further accentuates the size and shape of your biceps, leading to a perception of greater biceps muscularity.
Your wrist and finger flexors also work when performing dumbbell hammer curls. While these muscles are not the ones being actively trained during the lift, they do benefit from the simple task of holding the dumbbells. The more repetitions you perform of the dumbbell hammer curl, the longer these muscles must work to keep your hand in a closed position and the better trained they become.
Adding variation to your biceps routine is crucial for optimal development of biceps size and strength. However, your biceps routine should not be limited to hammer curls. Your biceps work in tandem with other muscles, like those located in your back, and failure to train them in exercises that require the utilization of both will limit overall biceps development. Perform multi-joint exercises like bent-over rows or pullups to complement dumbbell hammer curls and optimize development of your biceps.