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How to Do Jack Knife Sit Ups: A Simple and Effective Ab Exercise

If you are looking for a simple and effective ab exercise that can strengthen your core and help you get six pack abs, you might want to try jack knife sit ups. Jack knife sit ups are a low-impact exercise that works the entire core, especially the lower abs. They are also easy to perform and can be modified to suit your fitness level.

In this article I'm going to provide an explanation of jack knife sit ups, including their purpose and how to perform them correctly. Additionally I'll share some tips to maximize the effectiveness of this exercise and avoid common errors. By following this guide you'll be able to master jack knife sit ups and experience the advantages they offer for strengthening your muscles.

What are jack knife sit ups?

Jack knife sit ups are an abdominal exercise that involves raising your legs and torso simultaneously until they meet in the middle. The name comes from the shape that your body makes when you perform this exercise, which resembles a jackknife or a folding pocket knife.

Jackknife sit ups focus on the rectus abdominis, the primary muscle responsible for sculpting six pack abs. Additionally they engage the oblique muscles located along the sides of your waist and activate the abdominis, a deep muscle that encircles your spine and aids in maintaining proper posture.

Jack knife sit ups can help you improve your core strength and stability, which can benefit your overall health and performance. A strong core can help you prevent or reduce back pain, improve your balance and coordination, enhance your posture and breathing, and protect your organs and spine from injury.

How to do jack knife sit ups properly?

To do jack knife sit ups properly, you need to follow these steps:

  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. If you’ll be exercising on a hard floor, lay down an exercise mat before you begin.
  2. Before you start make sure to take a moment to inhale deeply. Proper breathing is an aspect of exercising. By remembering to breathe in as you release and breathe out as you contract you'll ensure that your muscles receive a supply of oxygen while lifting weights. This is significant in preventing muscle cramps and hernias.
  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. 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.
  4. Hold yourself in this jack knife position for about 3 seconds. 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.
  5. Breathe in while gradually bringing down your arms and legs to the position. Take a breath and then gently bring back your arms, legs and torso to the mat. Aim to keep your arms and legs straight as possible while lowering them even though it might be challenging to avoid slightly bending your knees.
  6. Perform 10 to 12 repetitions, for each set. Strive to finish 2 to 3 sets. Following your set of 10 to 12 reps take a break of approximately 30 to 45 seconds. This allows your muscles a chance to recover without relaxing. Aim to increase your routine doing 2 to 3 sets four times a week for a minimum of six weeks in order to reap the full benefits of this exercise.

Tips and tricks to make the most of jack knife sit ups

Here are some tips and tricks to make the most of jack knife sit ups:

  • Start with your torso and legs slightly raised for an extra challenge. 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.
  • Use a medicine ball or a dumbbell for added resistance. Hold a medicine ball or a dumbbell in both hands above your head as you lie on the floor. Perform the jack knife sit up as usual, but bring the medicine ball or the dumbbell to your feet as you raise your arms and legs. Lower the medicine ball or the dumbbell back to the floor as you lower your arms and legs.
  • Do jack knife sit ups on a stability ball for more instability. Place a stability ball under your lower back as you lie on the floor. Perform the jack knife sit up as usual, but try to balance yourself on the ball as you raise and lower your arms and legs. This will engage your core muscles more and challenge your balance and coordination.
  • Avoid these common mistakes when doing jack knife sit ups. Some of the common mistakes that people make when doing jack knife sit ups are:
    • Arching or rounding your lower back. This can cause back pain and injury. Keep your spine in a neutral position and engage your core muscles throughout the exercise.
    • When performing the exercise it's important to refrain from jerking or swinging your arms and legs. Doing so can diminish the effectiveness of the exercise. Raise the likelihood of getting injured. Instead focus on executing controlled movements avoiding any reliance on momentum to lift yourself up.
    • Take care not to hold your breath for long. Doing so may lead to feelings of dizziness, nausea or even fainting. Instead make sure to breathe in a steady manner throughout the exercise.
    • Straining your neck. This can cause neck pain and injury. Keep your head in line with your torso and avoid tucking your chin or letting your head fall back.


Jackknife sit ups are an efficient abdominal exercise that can enhance your core strength and assist you in achieving well defined abs. They're user friendly. Can be adjusted to accommodate your fitness level. By using this guide you can master the technique for performing jackknife sit ups and reap the rewards of this abdominal exercise.

Kayla Thomas


Kayla Thomas, a 34-year-old sports and fitness coach living in Connecticut, USA. With seven years of hands-on experience, she's all about helping folks get fit and strong. Kayla's fitness journey began when she was a sports-loving kid, and she followed her passion to college, where she earned a degree in Exercise Science and picked up personal training and sports coaching certifications. But what really lights her up is empowering women through fitness, breaking barriers and making fitness feel welcoming for everyone. Her clients see her as not just a coach but a motivating friend. In her downtime, you'll find her swimming and cycling, always on the move. And you can follow her fitness journey on Instagram – she's all about sharing that empowerment vibe.