The biceps are the most powerful supinator of the forearm therefore anytime
you supinate your wrists (turn outwards) under resistance your biceps are going
to be doing the deed. This dumbbell supination exercise is meant to be a classic
'finisher' exercise for the biceps and/or forearms. Just you think your muscles
can take no more punishment, add this dumbbell supination to the mix to fully
fatigue the forearms and biceps.