There are a few exercises that we can call classics – like the Moby Dick or Casablanca of the workout -, because they fit very well in every workout. Classics are for sure push-ups, pull-ups or also the exercise of today: the crunches.
Lie down flat on your back with your knees bent and your feet on the floor. Hold your hands on your chest. Often they show to put them behind your head but I don’t like that very much, because you risk to pull to much on the head. Use only your abdominals to lift your upper torso while pushing your chest and head up towards the ceiling. Keep the lower back always flat on the floor. And then come back only a bit without touching the floor. So you will keep the tense during the whole exercise.