Lie flat on the mat, bend your knees and push your feet against the mat. Your knees are bent at approximately a 90-degree angle with your feet in a shoulder-width position, and touch your head with your hands just behind your ears. Keep your elbows out.
While pushing the small of your back down to the floor and contracting your abdominals, begin to roll your shoulders off the floor, breathe out as you go up and breathe in as you go down. Repeat as required.
Make sure to maintain correct stance of your feet and sufficient angle at your knees.
Along with breathing (abdominal breathing), control a neutral pelvis position.
Do not lock your fingers behind your head as it may lead to upper-back injury; instead, put your hands lightly on either side of your head. Keep elbows out.
Do not relax abdominal muscles as you go back to the starting position to avoid lumbar lordosis.
Repeat in a series of at least 20 repetitions.
Crunches are a very popular staple of fitness classes: coaches like to prescribe hundreds of them as a remedy for excessive abdominal fat. Especially women tend to consider tons of crunches the silver bullet for almost anything.
This approach is not based on scientific facts. Every high-repetition exercise will lead to (among other things) depletion of body fat reserves.
There are many exercises burning fat much more effectively than crunches (which are a very short and limited movement).
So the only rationale for doing crunches in order to achieve flat belly is that they will strengthen the abdominal muscles. In such case they should be done in the 8-20 range per set – higher rep number may increase endurance but it will not build the muscle fibers.
In fact, crunches are an exercise like any other and abdominal muscles are pretty much the same as other muscles. Due to their use in daily life we may be forced to train them more often for best results but that’s it.
For advanced athletes it is necessary to increase the resistance. This can be done with a plate which you hold above you in stretched arms.
Holding the plate behind the head (or even locking your fingers behind your head if you train with no additional weight) will result in wrong posture of the head and can lead to problems with upper back.
Crunches are effective when lying on the mat with legs in 45-degree angle, with legs in 90-degree angle and on the decline bench.
You should not lift your upper back too high, some 4 inches (or 10 cm) will do, with the lower back on the floor (bench).
Notice: there is a difference between crunches and sit-ups. In sit-ups you lift your upper body completely into or very close to sitting position. This movement is mainly strengthening internal muscle called psoas, not external abdominal muscles.