The Janda sit-up was first popularised by fitness instructor Pavel Tsatsouline as a superior alternative to conventional sit-ups as well as crunches. However it was actually invented by Czechoslovakian exercise physiologist Vladimir Janda. The idea behind the Janda sit-up is to achieve a higher than normal level of abdominal isolation using the principle of reciprocal inhibition which simply means that whenever a muscle is contracted, the muscle opposite to it relaxes, an effect caused by the nervous system in order to achieve maximum efficiency. To put it simply, during a normal sit-up because of a neurological phenomenon know as irradiation, tension in any muscle being worked spreads to neighboring muscles as well and in the case of normal sit-ups or crunches irradiation results in an unwanted activation of the hip flexors thus taking much of the stress away from the abdominal muscles. This is where reciprocal inhibition helps to find a way around the problem. By contracting the hamstrings and glutes, the hip flexors are made to relax which is achieved practically by a partner placing each hand under the calves of the person performing the sit-up and asking them to pull on the hands with their calves while performing the sit-up, all of which results in greater abdominal isolation.