Hammer curls are great at isolating upper arm bicep muscles while putting minimal stress on the lower arms.
Dumbbells are free weights that allow for a more concentrated movement and hence are part of most strength training programs when it comes to building arm strength.
This exercise isolates forearms and engages them to create more stimulation and gain in that region.
The barbell curl has stood the test of time as a great isolation workout that works both the upper arms and forearms to build strength and stimulate muscle growth in that region.