Lie supine (on your back) on the floor with your legs straight and your arms by your sides.
Exhale as you simultaneously and slowly flex your waist and raise your knees to your chest.
Inhale as you slowly return to the starting position.
Repeat for the desired number of repetitions.
Comments and tips
Use the jackknife sit-up to develop your hip flexors and abdominal muscles, as well as to improve the strength of your core.
For the safety of your back, perform the jackknife sit-up in a slow and controlled manner.
When returning to the starting position, you may choose to keep your feet off the ground instead of setting them down on the floor. This will make the jackknife sit-up a little more challenging.
Some people think that the jackknife sit-up and the v-up are the same exercise, but they are different. The difference is that, in the v-up, the legs are kept straight and the arms are held up, by the head, whereas in the jackknife sit-up, the knees are bent and the arms are kept by the side. Keeping the legs straight and the arms up, by the head, makes the v-up more challenging.
Master the sit-up before attempting the jackknife sit-up.