Which is the best exercise to isolate the Abdominals? PDF Print E-mail
 
First let's make clear that the idea of “isolation” is a myth.
There is not real isolation in any exercise.
A classic example is the dumbbell bicep curl, which is supposed to isolate the bicep.
Just do a bicep curl holding a dumbbell with your right arm without moving; you will notice that your whole musculature on your left side will be activated to hold the weight in place.
Your body operates as one unit.
Those exercises that we call isolation exercises just put more emphasis to certain muscles, nothing more.
There is a general belief that crunches are better for isolating the Rectus Abdominous (the six pack muscle) than sit ups.
It is true that the classic sit-ups, where you lock your feet, also heavily involve your Psoas.
Is that bad?
Not necessarily. The Psoas are responsible for lifting your legs above 90 degrees and flexing your hips, these movements are very important for many sports including martial arts.
The notion that the crunches do not involve the Psoas is also wrong.
If you want to train your abdominals with minimum involvement of the Psoas try the following excellent exercise:

The Janda sit up




Janda was a Czech neurologist and he invented this very difficult exercise variation.
Sit as if you want to do a classic sit up but instead of locking the legs down put a pair of kettle bells to the back of your legs, behind the heels. Another way is to have your partner hold your legs from the heels trying to lift your legs from the floor.
Flex your gluteus and hamstrings to keep your legs on the floor.
This will force the antagonistic muscles, the Psoas in this case, to relax, thus shifting the emphasis to the rectus Abdominals.
Try to make a sit up now, while keeping your gluteus and hamstrings under tension.
You will notice that it is very difficult. Even if you do hundreds of normal sit-ups do not be surprised if you manage to do 4 or 5 of Janda sit-ups.
It is a tremendous exercise that will really strengthen your abdominals.
The reason it works is because it takes advantage of the Sherrington's Law of Reciprocal Inhibition.
Muscles work in pairs - as one muscle contracts to flex a joint the antagonistic partner have to relax and elongate to permit the contraction.
So while we flex the gluteus and hams the antagonistic muscles, the Psoas, relax. Your brain signals them to relax in order to allow movement.