Jack Knife Sit Up: An Advanced Body Weight Core Exercise

Are you tired of your regular sit up routine and ready for a new challenge? Have you heard of the jack knife sit up but don’t know where to start? Don’t worry, you’re not alone. As we strive towards our fitness goals, it’s common to hit a plateau or get bored with the same exercises. The good news is, incorporating new moves like the jack knife sit up can help you break through that plateau and add some spice back into your routine. In this post, we’ll guide you through step-by-step instructions on how to correctly perform jack knife sit ups and how to modify the exercise to fit your fitness level.

Jack Knife Sit Up Summary

  • Primary Muscles: Rectus Abdominis
  • Secondary Muscles: Adductor Brevis, Adductor Longus, Iliopsoas (Psoas Major & Iliacus), Obliques, Pectineus, Sartorius, and Tensor Fasciae Latae
  • Equipment: Body Weight
  • Mechanics Type: Isolated
  • Force: Pull
  • Utility: Auxiliary
Graphic image of a fit man performing alternate cable triceps extensions.

Jack Knife Sit Up Instructions

  • Begin with sitting on the ground.
  • Then lean back and brace yourself with your arms on the ground.
  • Lift both your feet off the ground a few inches.
  • Now bring one knee up to your chest.
  • Then lower that foot and bring your other knee up to your chest.
  • Continue by alternating your legs until you are fatigued.

Video Tutorial

How to Do a Jackknife | Ab Workout

Jack Knife Sit Up Muscles

Target (Agonist)


  • Adductor Brevis
  • Adductor Longus
  • Iliopsoas (Psoas Major & Iliacus)
  • Obliques
  • Pectineus
  • Sartorius
  • Tensor Fasciae Latae

Dynamic Stabilizers

  • Rectus Femoris


  • None

Antagonist Stabilizers

  • None
Image of the skeletal muscular system with the muscles used in the jack knife sit up exercise highlighted in red and the rest in blue.

Benefits of Jack Knife Sit Up

The Jack Knife Sit Up is an effective exercise for strengthening the Rectus Abdominis, or abdominal muscles. This exercise utilizes a combination of both upper and lower body strength to activate and engage the core muscles in order to perform the movement. The Jack Knife Sit Up is particularly beneficial for targeting the Rectus Abdominis because it requires the user to raise their torso off the ground while keeping their legs straight. This exercise increases muscular strength, endurance, and stability while also helping to tone and define the abdominal muscles. Additionally, performing this exercise regularly can help to reduce and prevent back pain, improve posture, and build overall core strength.

Tips for Performing Jack Knife Sit Up

Your at the right place if you’re trying to enhance your jack knife sit up execution. Using these suggestions can allow you to take full advantage of this amazing exercise. Concurrently with building your abs muscles, improving mobility, and also a reduced possibility of injury can all be achieved with this exercise. Let’s get started and look at what these tips can do for you.

  • Engage your core: Make sure you are using your abdominal muscles to lift your body up, and that your back is in a straight line. This will ensure the most effective workout and help build your core strength.
  • Pace yourself: Don’t rush your Jack Knife Sit Ups! Focus on quality reps and make sure your form is correct before increasing your speed or reps. Doing this will help you get the most out of each rep and prevent injury.
  • Have fun: Working out doesn’t have to be a chore! Make sure to take breaks when needed, and keep your Jack Knife Sit Ups entertaining by changing up the speed and intensity. Doing this will help keep you motivated and make it more enjoyable!

Benefits and Tips Video

How to do a Jackknife | The Right Way | Well+Good

Frequent Mistakes To Avoid

When conducting jack knife sit up, avoiding typical mistakes is usually the difference between a productive workout and a debilitating injury. Also, to maximize your benefits of the exercise, proper form is required. Through preventing these common mistakes, you will raise your ability to achieve the results you want. But relax, it’s not quite as difficult as it might appear. By knowing the errors to avoid and taking the right actions, you can execute the exercise securely and effectively. So it is time for you to maximize your results from this exercise and experience the benefits of a successful workout.

  • Not using proper form: Using improper form when doing Jack Knife Sit Ups can lead to injuries and ineffective workouts.
  • Not using a stable surface: Doing Jack Knife Sit Ups on an unstable surface can lead to balance issues and potential injury.
  • Not performing the exercise with enough intensity: Without enough intensity, Jack Knife Sit Ups may not provide the desired fitness results.

Find More Bodyweight Exercises Here

Variations and Complementary Exercises

The Jack Knife Sit Up is an effective exercise for strengthening your core and obliques, but there are several variations, complementary exercises, and alternatives that work similar muscles. Below is a list of exercises that can be used to supplement the Jack Knife Sit Up and increase the intensity of your workout.

Incline Leg Hip Raise

Graphic image of Incline Leg Hip Raise.

The Incline Leg Hip Raise is a great complementary or alternative exercise to the Jack Knife Sit Up. It works the same muscles as the Jack Knife Sit Up, but in a slightly different way. The Incline Leg Hip Raise requires you to lie on an incline bench and raise your legs and hips up to a 90 degree angle. This exercise targets your abdominal muscles as well as your hip flexors, which helps strengthen your core and improve balance. This exercise is also great for building stability and muscle control in your core, which can help reduce the risk of injury when performing other exercises.

Hanging Leg Raise

Graphic image of Hanging Leg Raise.

The Hanging Leg Raise is an excellent alternative to the Jack Knife Sit Up, as it works the same core muscles while placing less strain on the lower back. This exercise involves hanging from a pull-up bar or a dip station, while keeping the legs straight and raising them up until they are parallel to the floor. The abdominal muscles are engaged throughout the entire motion, and the exercise can be made more challenging by adding weight to the feet. This exercise is beneficial for developing strength in the core and abdominal muscles, as well as improving flexibility in the hips and spine.

Hands Up Crunch

Graphic image of Hands Up Crunch.

Hands Up Crunch is a great alternative or complementary exercise to the Jack Knife Sit Up. It is an abdominal exercise that focuses on the rectus abdominis and also helps build core strength. The exercise involves lying flat on your back with your legs bent and your feet flat on the ground. You then raise your arms up and simultaneously crunch up towards your legs, engaging the abdominal muscles. It is a great way to target the same muscles as a Jack Knife Sit Up, but with less strain on the lower back.

Check Out These Top Bodyweight Exercises


Graphic image of Crunch.

Crunch exercises are a great alternative or complementary exercise to the Jack Knife Sit Up. This exercise is designed to target the abdominal muscles, specifically the upper and lower rectus abdominis muscles. It is done by lying on the floor with the knees bent, feet flat on the floor, and hands either behind or beside the head. The arms are then used to lift the upper body off the floor in an arc-like motion until the shoulder blades are raised off the floor. This exercise can be made easier or harder by adjusting the angle of the legs and arms. Crunch exercises can also be used as an alternative exercise that focuses on targeting the same abdominal muscles as the Jack Knife Sit Up while reducing strain on the lower back.

Tuck Crunch

Graphic image of Tuck Crunch.

Tuck Crunch is a great complementary exercise for the Jack Knife Sit Up as it targets the same abdominal muscles. Tuck Crunch requires you to lie on your back and bring your knees towards your chest and then curl your upper body towards them, crunching your abdominal muscles in the process. It is a more intense exercise than the Jack Knife Sit Up and can really help tone your core. Additionally, it is a much safer exercise than the Jack Knife Sit Up and can be done with minimal risk of injury.

Superman Plank

Graphic image of Superman Plank.

The Superman Plank is a great alternative or complementary exercise to the Jack Knife Sit Up. It requires no equipment and is an excellent way to build core strength, balance, and stability. By simply holding your body in a push-up position while raising one arm and the opposite leg off the floor, you can target the same muscle groups as with the Jack Knife Sit Up. The Superman Plank also helps to build posture and coordination. It can be performed from both the front and side positions, making it an effective exercise for targeting different areas of the core.

Find More Abs Exercises Here

Opposing Complementary Exercises

If you want to make sure your workout is complete, it is important to include exercises that complement the exercise Jack Knife Sit Up. To do this, you should focus on exercises that use opposing muscle groups. This will help ensure that your body is getting an even workout and that you are avoiding any potential overuse injuries. Below are some great exercises to use:

Barbell Good Morning

Graphic image of Barbell Good Morning.

The Barbell Good Morning is an excellent complement to the Jack Knife Sit Up as it utilizes the opposing muscle groups. By performing this exercise, you are working the back muscles while simultaneously stretching and strengthening the abdominal muscles. It involves holding a barbell behind your neck while you hinge forward at the hips, keeping your back straight. The range of motion is restricted to ensure that you do not overstretch the lower back. This exercise is great for developing strength in the posterior chain and for developing a strong core, which is essential for any physical activity.

Good Mornings With Bands

Graphic image of Good Mornings With Bands.

Good Mornings with Bands is an excellent exercise to complement the Jack Knife Sit Up. This exercise uses the opposing muscle group to help strengthen and engage the core. It utilizes a band, which provides resistance as you pull your body up and back, engaging the lower back and posterior chain. This exercise helps to improve your posture, increase stability, and improve core strength and power. Good Mornings with Bands is a great way to improve the effectiveness of your Jack Knife Sit Up routine and ensure that you are getting the most out of your workout.

Smith Machine Good Morning Off Pins

Graphic image of Smith Machine Good Morning Off Pins.

The Smith Machine Good Morning Off Pins is a great complement to the Jack Knife Sit Up exercise. It works the opposing muscle group of the abdominals used in the sit up, targeting the hamstrings, glutes, and lower back. The Smith Machine offers a safe and secure way to perform this exercise, as the weight can be adjusted to suit the user’s strength level and the pins provide a stable base to work from. Additionally, the machine offers the user a range of motion that allows them to really target their hamstrings and glutes.

Unleash Your Abs: Master The Jack Knife Sit Up!

Unleash Your Abs: Master The Jack Knife Sit Up! This exercise is designed to target your abs and core muscles, helping to sculpt and define your midsection. But mastering the jack knife sit up is more than just doing a few reps – it’s about getting your form and technique just right. Make sure to engage your core throughout the entire movement, keeping your back straight and your shoulders relaxed. With practice and dedication, you’ll soon become a jack knife sit up pro!

References: Wikipedia | ExRx.net | PubMed.gov | Comprehensive List of Abs Bodyweight Exercises

Pin image for jack knife sit up post. With an image of a man performing the exercise on Top and a graphic of the exercise on the Bottom.

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