What Is a Modified Jackknife Exercise?

A modified Jackknife exercise is any movement that is a variation of the standard Jackknife Sit-Up. The Jackknife Sit-up is an intense core exercise that you do by first lying on your back with arms at your sides. Then you tighten your abdominals to lift your torso -- as in a Sit-Up -- while simultaneously bending your legs and bringing your knees to your chest to create a V-shape between your thighs and torso. Return to the start position and repeat. This movement can be modified to work the abdominal muscles more or less intensely or to target different sections of the core muscles.

  1. Assisted Jackknife

    • To make this exercise easier, you can use your arms to assist the movement. Begin with your torso already lifted off the floor by your arms, with your hands on the floor to each side and elbows slightly bent. Your legs should still be flat on the floor. Lift your legs off the floor, bending them to bring your knees to your chest, while simultaneously using your abs and arms to pull your upper-body forward slightly. Hold for a moment, then extend your legs to the starting position and lean your torso back.

    Toe-Touch Jackknife

    • In a standard Jackknife Sit-Up, the knees bend as the legs rise, and the arms are held along the body parallel to the floor. To increase the difficulty of the movement, begin by lying on your back as in the standard position, but with your arms back and extended above your head. As you contract your core and lift, keep your legs straight, with the knees only slightly bent, and bring your arms and torso up to meet them. As you reach the V-shape of the final position, extend your arms toward your feet and try to touch your toes.

    Side Jackknife

    • The variations listed above all focus on the upper and lower abdominals along the center of your stomach, but the Jackknife can be modified to work the obliques as well. Begin by lying on your left side, with your right arm bent behind your head -- as though for a crunch -- and your legs straight with the right resting on the left. Contract your right-side obliques to raise your right leg to a 45-degree angle while simultaneously lifting your torso to bring your right elbow toward your leg. Lower your leg and torso to the starting position. Repeat on both sides.

    Safety Tips

    • Always make sure to warm up before exercising. The Jackknife uses muscle groups in the torso, core and lower body, so a full-body warm-up, like jumping-jacks, is recommended. Make sure you have adequate padding beneath the tailbone and spinal-column before performing this exercise, as your body weight will press on those areas during the motion. People with tailbone injuries should not perform a full Jackknife, although they may benefit from the easier variations described. Consult a licensed physician or personal trainer if you are not sure whether you should do these exercises.