How to Do a Jack Knife Sit Up

Co-authored by Michele Dolan and Amy Bobinger, BA

Last Updated: July 30, 2024 References

Jack knife sit ups are a great low-impact exercise for strengthening abdominal muscles. They are a great way to tighten the lower abs and help you get six pack abs. Jack knife sit ups will give your abs challenging workout, but the basics are easy to master!

Method 1

Method 1 of 2:

Performing a Basic Jack Knife Sit Up

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1
Lie flat on the floor with your arms and legs extended. Your arms should be lifted so that they’re above your head. Make sure your spine is in a neutral position so your lower back isn't arched or rounded. ^{[1]
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- If you’ll be exercising on a hard floor, lay down an exercise mat before you begin.
Breathing properly is an important part of exercise. By remembering to inhale as you release and exhale as you contract, you’ll ensure that your muscles are getting plenty of oxygen as you lift weights. This is important to help prevent muscle cramping and hernias. ^{[2]
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3
Exhale and contract your abs, raising both arms and legs. Try to keep your legs straight and bring them up so they’re at about a 35-45 degree angle to the floor. At the same time, bring your arms up and over your head so that they’re parallel to your legs. Your upper body should be raised slightly off the floor. ^{[3]
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- Bring your head off the floor at the same angle as your torso. Don't tuck your chin into your chest or let your head fall back, as this can cause a neck injury.
- Use slow, controlled motions so you don't injure your back.
Frederic Delavier, Fitness Guru

To perform the jackknife sit-up, lie on your back with your legs extended, arms overhead. Contract your abs to lift your torso and legs up toward your hands. Keep your legs straight and reach your hands toward your feet. Slowly lower back down with control.
By counting to 3 while you hold this pose, your abdominal muscles will get the full benefit of the workout. Static workouts like this can increase your strength and stability.
Take a deep breath, then slowly return your arms, legs, and torso to the mat. Try to keep your arms and legs as straight as you can as you lower them, although it might be hard to not bend your knees slightly. ^{[4]
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6
Repeat 10-12 reps per set and aim to complete 2-3 sets. After your first set of 10-12 reps, allow yourself to rest for about 30-45 seconds. This will give your muscles some recovery time, but it won’t be so long that they fully relax. ^{[5]
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- Try to work up to doing 2-3 sets 4 days a week for at least 6 weeks to get the maximum benefit of this exercise.

Method 2

Method 2 of 2:

Trying Jack Knife Variations

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Start by laying in the basic starting position, then raise your torso and your legs so they’re slightly off of the floor. Perform the jack knife sit up as usual, but try to keep your torso and legs from touching the floor when you release from the position. ^{[6]
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2
Work your obliques with a side jack knife. For a side jack knife, start by lying on your left side. Bend your right arm so that your hand is behind your head and rest your left arm on the floor for support. Contract your obliques, bringing your right shoulder and hip toward each other while you lift your right leg. ^{[7]
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- Take a deep breath before you begin, then exhale as you lift your leg and arm. Inhale as you return to your starting position.
3
Try a jack knife crunch with an exercise ball. To do a jack knife on an exercise ball, you’ll start in a push-up position with your hands on the ground, about shoulder-width apart. Your shins should be resting securely on the ball. Roll the ball inwards, bending your knees until your thighs touch your stomach. Roll the ball backwards to return to your starting position. ^{[8]
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- Once you're balanced on the ball, take in a deep breath, then exhale as you roll your knees in towards your chest. Inhale when you extend your legs again.

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Tips

Work yourself up to doing a jack knife sit up by raising just your arms or legs at first.

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To make this exercise less challenging, bend your knees instead of keeping them straight.

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Don't overwork yourself. Just do what is comfortable when you begin and then begin to add more reps.

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Warnings

Avoid focusing only on sit ups to work out your abs. Too many sit ups can cause lower back problems over time. Instead, balance sit ups with other ab workouts, like planks.

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