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 sitcomfortably 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 then slowly lower the dumbbell back to the starting position.

Repeat for desired reps, and then repeat on the right arm.

Exercise Tips:

Keep the rep timing slow, and control the dumbbell on the way down.

Only your forearm should be moving.