Hammer curls are elbow flexion with a neutral (aka palms facing eachother)
grip. Since the elbows are not supinated (palms facing up) the hammer curl
primarily targets the brachioradialis muscle which also has help from the biceps
and brachialis. Since the brachioradialis makes up a significant portion of the
forearms, hammer curls are often though of more as a forearm size building
exercise than an upper arms exercise where supinated biceps curls reign supreme.
Regardless of the target muscles, hammer curls are a great exercise and will
usually be great for forearms