1. Bicycle Exercise
Lie on the floor with your lower back pressed flat to the ground. Put your hands beside your head. Do not cradle your head unless you can do this exercise without pulling on your neck. Bring knees up to about 45- degree angle and slowly go through a bicycle pedal motion. Touch your left elbow to your right knee, then your right elbow to your left knee. Keep even, relaxed breathing throughout.
Continue alternating sides for 1-3 sets of 12-16 reps.
2. Captain's Chair Leg Raise
The captain's chair leg raise is the second most effective move for the abdominal muscles as well as the obliques. You can do a variety of exercises on the captain's chair, which is a rack with padded arms that allows your legs to hang free and can be found in most health clubs and gyms. The key to keeping this move safe and effective is, first, to avoid swinging the legs or using momentum to bring the legs up. Second, keeping the knees bent will help you focus more on the abdominals and less on the hip flexors.
To perform this move stand on the chair and grip the handholds. Put your back against the pad. Contract your abs and slowly lift your knees towards your chest. Slowly lower your knees back down and repeat for 1-3 sets of 12-16 reps. If you do this right, you should feel your abs contracting as you lift and lower your knees. Do not lift your legs too quickly, do not arch your back, and do not swing your legs up.
3. Exercise Ball Crunch
An exercise ball can be a very effective tool when working on your abdominal and oblique muscles. The exercise ball crunch is the third most effective exercise for the abs. On an exercise ball, it is harder to cheat when do abdominal exercise because your abs are typically singled out.
To do this exercise, lie on the ball with the ball under your lower back. Cross your arms over the chest and slowly pull the bottom of your ribcage toward your hips while keeping your lower back on the exercise ball. Slowly return to your starting position all the while making sure the ball does not roll. Perform 1-3 sets of 12-16 reps.
4. Vertical Leg Crunch
The vertical leg crunch is 4th best abdominal exercise. This is only the 2nd exercise without some type of equipment on this list. The vertical leg crunch is a type of modified crunch.
To perform this exercise lie on the floor with your legs extended straight up in the air with knees crossed. You can place your hands behind your head or across your chest. Contract your abs and slowly lift your torso toward your feet. Your legs should stay fixed while you do this and there should not be any jerky motions. Slowly return to starting position and do 1-3 sets of 12-16 reps.
5. Long Arm Crunch
The long arm crunch is ranked the 5th most effective ab exercise, changing the traditional floor crunch by straightening the arms behind you. When trying to figure out how to place your hands just think of Superman. Straightening your arms out adds a bit more challenge and difficulty to the standard crunch. This move also emphasizes the upper part of the abs, but will also workout the rest of the abdominal muscles to a lesser extent.
To do this straighten your arms out behind your head. Contract the abs and slowly lift your torso off of the mat or floor. You should feel your abs contract while you are lifting. Slowly return to starting position. Perform 1-3 sets of 12-16 reps.
None of these exercises require a large monetary investment. I provided this list because ab machines as seen on tv such as the Abdoer Twist are only effective if you first have an understanding of how to engage your muscles before you attempt to utilize a machine. If people don't learn how to engage their abdominal muscles first, they will tend to use their other muscles as a crutch. This will not get any real use out of the abdominal exerciser they are attempting to use.