A jackknife sit-up is a variation of the traditional sit-up. This sit-up incorporates lifting the arms and the legs into the sit-up motion. There are variations to this exercise in which an individual can choose to add more resistance.
This exercise is classified as a strength exercise that focuses on the abdominals, or core. As with the traditional sit-up, the individual lies on his back. The individual stretches his arms above his head while lifting the legs at a 45-degree angle. When the individual reaches the upright position, the arms are parallel to the legs. A medicine ball can be added for extra resistance.