Jack knife (sport)
As a jackknife a sporting exercise is to train the abdominal muscles called. The name comes from the design - it is reminiscent of folding a pocket knife .
execution
You lie down with your back on the floor. The arms are stretched out towards the head. Now you lift your legs, still straight, and at the same time move your arms over your head in the direction of your legs. The body is, so to speak, 'folded up'. After reaching this point, your arms and legs are returned to their original position and you are ready for another repetition.
criticism
In some cases, abdominal muscle training with the aid of the 'jack knife' is no longer recommended, as weak abdominal muscles put too much strain on the lumbar spine. Furthermore, most of the work in this exercise is done by the hip flexors. In summary, this exercise can be described as outdated and non-functional (i.e. not suitable for the purpose of primarily exercising the abdominal muscles). A better alternative is belly crunching .