If you've always wondered why your abdominal muscles refuse to shape up, no matter how many hours of crunches you punish your body with, California researchers have some very welcome news for you.

According to a recent study published by the American Council on Exercise, pushing yourself to accomplish a thousand crunches every second day isn't the most effective way to tone, tighten and strengthen your gut. In fact, the classic crunch - or abdominal sit-up as it is better known - is amongst the most ineffective ways to work your abs… second only to those gimmicky abdominal exercise equipment advertised on late-night television shows. What's more, rather than carving the flab away from your abdominal muscles, crunches are actually harmful to your back, and are the most common reason for back strains, sprains and other injuries.

Over the years, numerous fitness experts have rallied to and fro about the most effective ab exercises. After years of scientific research and experiments, scientists conclude that in order to bring about the best results, abdominal exercises must stimulate the two major muscle groups in your mid-section - the rectus abdominus which is the long flat muscle extending along the length of the front of your abdomen and the obliques or the long flat muscles along the sides of your abs. The more intense the stimulation - the better the results

On the basis of these criteria, the top three ab exercises are:

Exercise One: Hanging Leg Raises

To do this exercise:

- Hang from a pull-up bar, your legs extended straight down.
- Raise your legs upwards, bringing your knees up and towards your chest.
- Slowly lower your knees to the starting position and repeat.

When performing this exercise, try not to swing your legs and use the momentum to push your knees upwards. Instead, let your abdominal muscles do the work. Use a slow and controlled motion, concentrating on your ab muscles.

Start with bringing your knees up to your hip level, and then raise your knees higher as your abdominal muscles grow stronger. A tougher and more intense variant of this exercise is the straight-leg version, where you lift both your legs towards your chest, without bending your knees.

For those of you who find bent-knee leg raises too easy, and straight leg-raises too Herculean, try the tuck-straight variant, where you bend your knees on the lift up, and straighten your legs as you bring them back down to the starting position.

Hanging leg raises really push your rectus abdominal muscles and are a time-tested favorite with athletes and body-builders alike. This exercise targets your entire core section, building you a stronger, firmer and more toned midsection, and is one of the few exercises which actually give your lower abs a thorough workout.

Exercise Two: The Plank

To do this exercise:

- Lie face down on a mat.
- Lift your torso up on your forearms with your palms flat on the floor.
- Push off the mat, raising your body on your toes. Lift your knees off the mat. Your body should be balanced on your forearms and your toes.
- Keep your back flat such that your head, shoulders, hips and heels are in a straight line.
- Tuck in your abdominal muscles to keep your hips from jutting upwards.
- Hold for 20 - 60 seconds and then slowly lower your body back to the starting position.

The Plank is an excellent strength and stability exercise which builds core strength, stability and taut abs. This exercise targets your rectus abdominus and your obliques, while also strengthening your back and glutes.

When performing this exercise, concentrate on bringing your belly-button towards your spine, by squeezing your abdominal muscles as tightly can you can. While most adrenaline-addicted bodybuilding enthusiasts scoff at any exercise that doesn't call for at least a several hundred repetitions, the plank is undoubtedly a lot more challenging than it seems at first blush. This exercise makes you push against the dual forces of gravity and your bodyweight to keep your back from arching and sagging, by tightening all your abdominal muscles.

Exercise Three: The Side Plank:

A slightly more advanced variant of the basic plank, the side plank is very effective in building core strength and stability. When performed correctly with perfect form, this exercise can tremendously strengthen and tone your abdominal and lower back muscles.

To do this exercise:

- Lie on your left side with your feet stacked, one on top of the other. Your left forearm should be placed flat on the floor, extended at a right angle from your body. Rest your left arm on your left side.
- Lift your body upwards on your forearm, with your weight balanced on your left forearm and the side of your left foot.
- Be sure that you aren't arching your shoulders and / or holding your breath. Remember to keep your right shoulder, your hip and your foot in a straight line.
- Hold this position for as long as you can, and then slowly lower your body back to the starting position.
- Switch sides and repeat

For a more advanced version of this exercise, lift your arm and extend it straight towards the ceiling, such that it forms a straight line with your supporting elbow.

If you're having trouble with balancing your bodyweight, start with lifting your body on your left forearm and the side of your left knee, maintaining a straight line with your head, right shoulder and right knee.

Although surprisingly uncomplicated, this exercise will make you break a sweat with the core stability and strength it calls for. If your oblique muscles aren't strong enough, this exercise can be very challenging. Start simple, increasing the duration and complexity of this exercise as you gain more strength and balance.

Exercising your way towards a perfect set of six-pack abs is a relatively simple and uncomplicated process… given that you learn to distinguish the fact from fiction. Rather wasting your time, and hard-earned money, on fad products, expensive gym memberships and bogus miracle weight-loss pills, focus on reliable, scientifically proven ab-toning techniques... your fitness levels (and abdominal muscles) will know the difference!