You'll use the same pair of dumbbells for each exercise in this routine. Choose a weight that just barely allows you to do 12 biceps curls. If it's too light, you won't build as much muscle, says Houston-based trainer Carter Hays, C.S.C.S. Too heavy and your form will break down first, your ligaments later. Proceed in circuit fashion, moving from one exercise to the next without rest. At the end of the circuit, rest for 60 seconds. Repeat for a total of four rounds.

Hold a pair of dumbbells in front of your chest with your palms in. Bend at the hips and knees to lower yourself until your thighs are parallel to the floor. Stand and press the weights overhead, rotating your wrists so that your palms face forward. Lower the dumbbells in front of your chest so your palms face you again. Do 12 reps.

Lie with your head and upper back on a Swiss ball and raise your hips so your body forms a straight line from knees to shoulders. Hold the weights along the sides of your chest with your palms toward each other. Press the weights up, then lower them halfway down. Keeping the same bend in your elbows, slowly lower the dumbbells behind your head until they touch the ball. Reverse until the weights are back over your chest, press them up, and lower them to the starting position. Do 12 reps.

Stand holding a pair of dumbbells at arm's length at your sides. Step forward with your left foot until your right knee is an inch or two off the floor and your left thigh is parallel to the floor. Then do a biceps curl. Push back up to the starting position and repeat, this time stepping out with your right foot. That's one rep. Once you've completed 12 curls, place the weights on the floor and continue lunging for 12 more repetitions.