Your abdominal muscles get a workout as the stabilizing factor in many exercises, but the most productive workout for your abs targets them specifically. There are many exercises you can use to work your abs in isolation, but traditional crunch exercises can help you train your abdominal muscles just as effectively as a number of commercially available ab machines.
Crunches: Lie flat on your back and place your hands behind your ears without interlocking your fingers. Raise your thighs to the vertical, and bend your knees at a 90-degree angle. Contract your abs and bring your knees and head toward each other by rolling your spine until your shoulders are about 4 inches from the floor. Slowly unroll your spine and return to the starting position without relaxing your abs. Perform three sets of 12 reps.
Reverse Crunches: Lie on your back with your arms by your sides and palms flat on the floor. Bend your knees and hold your feet close to your buttocks. Use your abs to raise your pelvis off the floor, and roll your spine until your knees touch your chest. Keep your feet tight to your buttocks, and roll back down without touching your feet to the floor. Perform three sets of 15 reps.
Oblique Crunches: Lie on your back with your thighs vertical and your knees bent so that your calves are parallel to the floor. Place your hands behind your ears. Without changing the position of your hands, crunch your abs to bring your right shoulder off the floor and bring your right elbow toward your left knee. Return to the starting position and repeat with your left shoulder and right knee. This counts as one rep. Perform two sets of six reps.
Bicycle Crunches: Position yourself with your lower back flat on the floor and your shoulders raised off the floor in the crunch position. Support your head with your hands, but do not use your hands to pull your head during the movement. Extend your right leg while bringing your left knee and right elbow together. Repeat on the other side by extending your left leg and bringing your left elbow and right knee together, to complete one rep. Perform three sets of 12 reps.