Dumbbell one arm reverse preacher curl

Sit on a preacher bench with your chest against the pad and your arm extended over the edge of the bench, holding a dumbbell with an underhand grip. Lower the dumbbell slowly until your arm is fully extended. Curl the dumbbell back up towards your shoulder, keeping your upper arm stationary. Squeeze your biceps at the top of the movement, then slowly lower the dumbbell back down to the starting position. Repeat for the desired number of repetitions, then switch arms.