These exercizes are divided into upper and lower ab exercises. Note that there aren't two separate muscles that you can truly isolate, so all the exercises stress the whole abdominal wall. However there are "clusters" of muscle separated by connective tissue (these make up the "washboard" or the "six-pack"). You can focus on the upper clusters by moving just the torso and the lower clusters by moving the pelvis.

For the lower abs, in increasing order of difficulty:

lying leg raises
reverse crunches
vertical lying leg thrusts
hanging knee raises
hanging leg raises

For the upper abs:

ab crunches
1/4 crunches
cross-knee crunches
pulldown crunches

Lower Ab Exercises

Lying Leg Raises

Lie on your back with your hands, palms down under your buttocks. Raise your legs about 30cm (12") off the floor and hold them there. Now trying to use just your lower abs, raise your legs by another 15cm (6"). Do this by
tilting the pelvis instead of lifting the legs with the psoas. Make sure your knees are slightly bent.

If you're big or have long legs or both, you should probably avoid this exercise. For people with legs that are too heavy for their lower abs strength, this exercise pulls the lower back into an exaggerated arch which is bad (and painful). If you have this problem you can either try bending your knees slightly and making sure you keep your lower back fairly flat, or just try another exercise.

Reverse Crunch

This exercise can be done on the ground or on an incline situp board. All you need is something behind your head to hold. If you use the incline board, use it with your feet lower than your head.

Lying on your back, hold a weight or a chair leg (if lying on the floor) or the foot bar (if using the situp board). Keep the knees slightly bent.

Pull your pelvis and legs up so that your knees are above your chest and then return to beginning position.

This exercise is very similar to a hanging knee raise, but a little less intense.

Vertical Lying Leg Thrusts

Initial position:

Lie on your back.
Put your fists under your buttocks to form a cradle.
Raise your legs in the air 20-30cm (10-12") off the ground, knees slightly bent.
If you feel any strain on your lower back, bend your knees a little more.
Raise your head and shoulders off the ground slightly if you can to help keep the abs stressed.

The exercise itself has four phases:

1.Raise your legs until your feet are above your pelvis; focus on contracting the abs.
2.Thrust your heels to the ceiling, breathe out, keep contracting the abs raising the pelvis out of the cradle of your fists.
3.Lower out of the thrust back to your fists, leaving your feet above your pelvis.
4.Lower your legs back to the initial position.

Hanging Knee Raises

You need a chin-up bar or something you can hang from for this. Grab the bar with both hands with a grip a bit wider than your shoulders, cross your ankles and bring your knees up to your chest (or as close as you can get). Your pelvis should rock slightly forward. Pause at the top of the movement for a second and then slowly lower your knees by relaxing your abs. Don't lower your legs all the way. Repeat the movement using just your abs to raise your knees.

Make sure that you don't start swinging. You want your abs to do the work, not momentum. It's important that you don't move your legs too far or your psoas muscle will be doing a lot of work and possibly causing back
problems as in a situp.

Make sure your pelvis moves, your lower back stays neutral or slightly rounded, not arched, and that your abs are doing the work, not your hips.

Hanging Leg Raises

Just like knee raises except you keep your legs straight. This requires good hamstring and lower back flexibility,

The American Council on Exercise's Aerobics Instructor book warns that they have the same back problems as conventional situps. This makes sense since, like situps, the legs are kept straight and the hips move. The Aerobics and Fitness Association of America (AFAA) also regards hanging leg raises as dangerous.

For safety you should probably stick to leg thrusts and knee raises.

If you do do hanging leg raises, make sure your lower back stays neutral or rounded.

There is an isometric variant done by gymnasts called the "L-Support", which basically consists of taking the leg
raise position with the legs held straight at a level just above the hips. The position is held for 10 seconds.
When you can complete this easily, try a higher position. The same cautions about back position still hold.

Upper Ab Exercises

Ab Crunches

Lying on your back, put your knees up in the air so that your thighs are at a right angle to your torso, with your knees bent. If you like you can rest your feet on something, like a chair. Put your hands either behind your head or gently touching the sides of your head.

Now, slowly raise your shoulders off the ground and try to touch your breastbone to your pelvis, breathing out as you go. If you succeed in touching your breastbone to your pelvis, see a doctor immediately.

Although the actual movement will be very small (your upper torso should move through less than 30 degrees) you should try to go as high as possible. Only your spine should bend, your hips should not move. If the hips
move, you are exercising the psoas.

Do these fairly slowly to avoid using momentum to help.

You can increase the difficulty of the exercise by extending your hands out behind your head instead of keeping them at the side. Make sure you don't jerk your hands forward to help with the crunch, keep them still.

1/4 Crunches

Same as an ab crunch except that you raise your shoulder up, instead of pulling them toward your pelvis. You can do these quickly, in fact it's hard to do them any other way.

Cross-Knee Crunches

Like ab crunches, take the lying, bent-knee position, but this time crunch diagonally so that you try to touch each shoulder to the opposite hip alternately. At the top position, one shoulder and one hip should be off the ground.

Pulldown Crunches

Drape a towel or rope around the bar of a pulldown machine so that you pull the weight using it instead of the bar. Kneel facing the machine and grab hold of the towel and put your hands against your forehead. Kneel far
enough away from the machine so that the cable comes down at a slight angle.

The exercise is the same movement as an ab crunch, but using the weight instead of gravity. The emphasis is still on crunching the abs, pulling the sternum (breastbone) towards the pelvis and making sure you exhale all your air at each contraction.

MOST IMPORTANT OF ALL is to really concentrate on working the abs fully with each rep. DON'T just go through the motions. Focus, contract intensely and make them grow. Good luck.

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LAte