The one-arm dumbbell preacher hammer curl is a good exercise for isolating the biceps and working the forearms.
- Adjust the height of the seat on the preacher bench so that when you sit on it, the back of your upper arms sit comfortably flat on the padding.
- Grab a dumbbell with your left hand and sit on the preacher bench.
- Rest the back of your upper left arm on the bench, twist the dumbbell so you're using a neutral grip (thumb facing the body), and allow your arm to fully extend.
- Place your right forearm on the top of the padding for support.
- Tense the left arm and slowly curl the dumbbell up as far as possible.
- Squeeze the bicep and slowly lower the dumbbell back to the starting position.
- Repeat for desired reps, and then repeat on the right arm.