Start: Sit up straight at the end of a bench. With an underhand grip, hold two dumbbells straight down, at arm's length.
Finish: Curl the weights toward your shoulders. Stop and squeeze when the dumbbells are 6 o 8 inches in front of your shoulders. Hold the contraction for a beat, then slowly return to the starting position. Don't lean back to help yourself lift more weight, and keep your upper arms against your sides throughout the exercises.
Curl one dumbbell halfway up and leave it there as you do the entire set with your other arm. Then switch arms and do the same thing. The isometric hold in one bicep makes this one a killer.
Stand with your back against a wall while you perform the curls.