Dumbbell hammer curls are a classic arm mass building exercise. There is no supination or pronation (turning) of your wrist during hammer curls. Your palms are always facing eachother.
They also strengthen your grip which can improve everything from your dead lifts to bench press.
To perform properly you should have your elbows in preferably against your sides.
Some people say you are only supposed to bring your arms up to 90 degrees and hold.
This has some merit because the brachialis, which is deep to the biceps is mostly active during isometric contractions.
To build the brachialis means the lower part of your upper arms could increase in size significantly.
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