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 to 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.
You can vary almost any curl by curling one arm at the time. Do the completed curl with one arm before starting the movement with the other arm.