Crunches focus on upper abdomen, especially on rectus abdominis (the abs) and, to a lesser extent, the internal and external obliques. They make it possible to bend the spine without the hip joints taking part, which provides maximum stimulation of the abdominal muscles. It must be noted however that they should be done on a flat bench with feet unlocked.  If a Roman chair is used, the feet are locked in the special roller, the hip joints participate actively in the movement and its effect on the abdominal muscles is lessened.

There are variants of this movement.
The movement can be done by lifting the feet and resting them on the wall (make sure your trainers are clean or you might get into trouble with the gym manager). Another variant is to put the feet on the ground and bend the knees at about a 45° angle. With arms extended before the chest for balance, use constant contraction, decreased amplitude and press lifting. Really deep concentration is called for.
It is very important to direct strong nerve impulses at the regions of the abdomen. There is a tendency for stretching the neck so as to push the torso upwards in the beginning of the crunching up. This lessens the effect of the exercise on the abdomen and can lead to cervical vertebrae injuries. It is important as well to lower the upper part of the torso by strong muscle contraction, not allowing it to fall down too quickly and perform the next concentric phase using momentum.

1.Take the correct initial position with your feet on a small, firmly fixed bench. The lower part of the legs must be at right angles to the thighs. Place yourself well on the bench, so that your thighs are perpendicular to the floor.
2. Put your hands on either side of your head, elbows pointing forward. If you elbows point aside there is the risk of swinging them to the front while you are crunching up, which will lead to a sudden jerk of the torso.

3.Crunch up slowly, with strong nerve impulses in the abdomen.
4. Finish the concentric phase in the utmost point of the full amplitude of the movement.
5. Start the eccentric phase lowering the torso by strong contraction of the abdominal muscles. They should be worked to the maximum in order to reduce the strain on the hip girdle.
6. Finish the eccentric phase in its final point (3.5 cm above ground). You must not push up from the ground so as not to perform the next concentric phase by momentum.

Important:
1. Use a horizontal bench and don’t lock in your feet.
2. Put your hands on either side of your head and don’t crane it upwards while lifting the torso. Concentrate on the abdominals all the time.
3. Slowly lower the torso during the eccentric phase and don’t push away from the bench in its final point.