You'll need a 8- to 10-pound dumbbell. This works your shoulders, abs, and obliques. Do 12-15 reps per side, then crunch straight up toward toes for another 12-15 reps.
-The Trunk Rotation: You'll need an 8- or 10-pound dumbbell. Lie on your back holding the dumbbell horizontally with both hands about 6 inches over your chest. Bring your legs perpendicular to the floor, knees bent 90 degrees. Tilt your knees 45 degrees to the left as you press the dumbbell 45 degrees to the right. Alternate sides. Continue these slow rotational twists for 30 seconds. Using the weight to counterbalance your leg, zero in on your abs and your waistline muscles. Don't swing the weight or use momentum to drive the movement. Next, bring your legs and arms back to the center. Now, crunch straight up, reaching the dumbbell horizontally toward your toes. The trunk rotation targets your abs, obliques, which are the muscles that run down your side; and your shoulder muscles. Continue these crunches for 30 seconds. If you need to make this exercise easier, do it without the dumbbell.