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.
Sit on a ball as you perform a seated curl or any variation of it.