Jack Knife: Classic Body Weight Ab Exercise You Can Do Anytime

Do you struggle with performing the jack knife exercise correctly? You’re not alone. Many people find it challenging to engage the right muscle groups and maintain proper form throughout the movement. But don’t fret, the complexity of the jack knife is understandable, especially if you’ve just started your fitness journey. The good news is, with a little bit of guidance and practice, you can master the jack knife and reap its numerous benefits. In this post, we’ll walk you through step-by-step on how to perform a jack knife correctly and help you avoid common mistakes.

Jack Knife 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 woman performing alternate cable triceps extensions.

Jack Knife Instructions

  • Lay on the ground flat on your back, your arms above your head on the floor.
  • Then simultaneously lift your legs straight up and your upper body, bringing your hands and your feet together.
  • Then slowly lower your body back until it is flat on the ground.
  • Repeat your Jack Knives for a full set.

Video Tutorial

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

Jack Knife Muscles

Target (Agonist)


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

Dynamic Stabilizers

  • Rectus Femoris


Antagonist Stabilizers

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

Benefits of Jack Knife

The Jack Knife exercise is a great way to target the Rectus Abdominis muscle in a strength training or fitness routine. The Jack Knife exercise is a compound movement, which means it works multiple muscles at once. It specifically works the Rectus Abdominis, which is the main muscle used to crunch and flex the torso. This exercise helps to strengthen the core muscles and increase overall abdominal strength, providing greater stability and balance when performing other exercises. Additionally, it helps to improve posture and can even reduce lower back pain. The Jack Knife exercise is an effective way to engage the Rectus Abdominis muscle and make it stronger.

Tips for Performing Jack Knife

Your at the right place if you’re trying to enhance your jack knife execution. These tips will help you get more out of this amazing workout, while helping you reap all the benefits it has to offer. Concurrently with shaping your abs muscles, improving movability, and even a reduced possibility of injury can all be accomplished with this exercise. So let’s get started and see what these tips can accomplish for you.

  • Start with proper form: To ensure you get the most out of the exercise, start with proper form. Make sure your back is flat, your stomach is tight and your head is in line with your spine throughout the motion. This will help you to maximize your effort and reduce the risk of injury.
  • Keep your core tight: As you move through the Jack Knife, focus on keeping your core tight and engaged. This will help you to maximize the abdominal activation of the exercise and get the most out of each rep.
  • Pace yourself: Don’t rush through the exercise. Take your time and focus on each rep. This will help you to maximize the benefit of each rep and reduce the risk of overexertion or injury.

Benefits and Tips Video

Ab Jackknife - Medicine Ball Workout Video - Women's Health

Frequent Mistakes To Avoid

Avoiding common mistakes can be the distinction between a successful training session and an injury when executing jack knife. Moreover, achieving optimal results from this exercise requires appropriate form, and making sure you don’t perform common mistakes can allow you to perform the exercise correctly and obtain optimal results. Not only can avoiding these errors increase your strength, but it can also help you feel more assured and inspired when at in the gym. So let’s get started on avoiding those common mistakes and making this exercise a normal part of your fitness routine.

  • Not using proper form: Using improper form when performing the Jack Knife exercise can lead to injuries, as well as reduce the effectiveness of the exercise.
  • Not controlling the movements: Not controlling the movements during the Jack Knife exercise can result in poor form and instability, reducing the effectiveness of the exercise.
  • Not focusing on the core: It is important to focus on engaging and activating the core muscles when performing the Jack Knife exercise, otherwise you won’t get the maximum benefit from it.

Find More Bodyweight Exercises Here

Variations and Complementary Exercises

The exercise Jack Knife is a great core workout, but if you’re looking for something different, there are a few variations, complementary, or alternative exercises you can do to target similar muscles.

Hip Thrust Crunch

Graphic image of Hip Thrust Crunch.

The Hip Thrust Crunch is a great complementary exercise for the Jack Knife. It targets the same core muscles as the Jack Knife, but with a slightly different focus. In the Hip Thrust Crunch, you begin by lying flat on your back with your knees bent. Then, you thrust your hips up and crunch at the top of the movement. This exercise puts more emphasis on the lower abdominal muscles than the Jack Knife, helping to target and strengthen the entire core. It is also a great alternative exercise for those who find the Jack Knife too difficult or uncomfortable.

Hip Lift

Graphic image of Hip Lift.

The hip lift is an effective exercise that can be used as a complementary or alternative to the Jack Knife. This exercise strengthens the abdominal muscles while also engaging the glutes, lower back, and hips. It begins in a supine position with the feet flat on the ground and the knees bent. Then, the hips are lifted upward and forward in a controlled motion until they reach their highest point. The position should be held for a few seconds before returning to the starting position. This exercise can help improve core strength, posture, and balance while also providing a great alternative to Jack Knife if one is looking for a different type of exercise.

Bridge On Knees

Graphic image of Bridge On Knees.

Bridge On Knees is a great alternative to the Jack Knife exercise, especially for those looking to improve their core strength. It works the same muscles, but with a different focus. Bridge On Knees is performed by lying on your back and lifting your hips off the ground while simultaneously bending your knees. This exercise puts more emphasis on the glutes, hamstrings, and lower back muscles than the Jack Knife exercise does. It is also a great way to stretch and strengthen the muscles of the hips and lower back. Bridge On Knees can be done as part of a warm-up, or as an alternative to the Jack Knife exercise for anyone looking to improve their core strength.

Check Out These Top Bodyweight Exercises


Graphic image of Bridge.

Bridge is a great complementary or alternative exercise to the Jack Knife. It works to strengthen the same core muscles as the Jack Knife and can be performed by anyone who is unable to do the Jack Knife due to lack of flexibility or strength. Bridge requires minimal equipment, making it ideal for those with limited access to fitness gear. It also allows for a more complete range of motion than the Jack Knife, which can help improve mobility and flexibility in the lower back and hips. Bridge is a great way to mix up your routine and challenge your core muscles in a different way.

Bench Crunch

Graphic image of Bench Crunch.

Bench crunch is a great complementary or alternative exercise for the Jack Knife. It focuses on the same abdominal muscles and helps to develop core strength. To perform a bench crunch, you lie flat on your back on a bench with your feet planted firmly on the floor. You then lift your chest up off the bench and curl your torso towards your knees. This exercise can be performed with added weight for more of a challenge and can also be done in a slow and controlled motion to increase the intensity. Bench crunches can be done as part of a regular abdominal routine or used as an alternative to the Jack Knife when you need to take a break from the more difficult move.

V Up

Graphic image of V Up.

V Up is an exercise that can be used as a complementary or alternative exercise to the Jack Knife. The V Up involves lying on the back with the arms extended above the head and the legs extended out in a V shape. This exercise is a great way to increase core strength and stability by focusing on the rectus abdominis muscles, hip flexors, and lower back muscles. The V Up also strengthens the abdominal wall and helps improve posture, balance, and stability. This exercise can be done in a variety of ways and can be modified to accommodate different levels of fitness and needs.

Find More Abs Exercises Here

Opposing Complementary Exercises

For those looking to build on the Jack Knife exercise and target opposing muscle groups, there are some great exercises that can help with this. To maximize the benefits of the Jack Knife, it is important to include exercises that work opposing muscle groups. The following exercises are great for targeting the opposite muscles as the Jack Knife.

Resistance Band Hip Extensions

Graphic image of Resistance Band Hip Extensions.

Resistance Band Hip Extensions are a great exercise to complement Jack Knife. This exercise targets the opposite muscle group of the Jack Knife, which is the glutes, hips and hamstrings. With Resistance Band Hip Extensions, you stand on the resistance band with your feet close together and then move your hips back and forth, making sure to keep your back straight. This helps to strengthen the glutes and hamstrings, which is important for overall balance and stability in the lower body. Additionally, it helps to stretch the hip flexors that were worked during the Jack Knife exercise. Resistance Band Hip Extensions are a great way to finish an intense lower body workout as they help to target opposing muscle groups, improve stability and flexibility, and help to prevent injury.

45 Degree Hyperextension

Graphic image of 45 Degree Hyperextension.

The 45 Degree Hyperextension is a great complement to the Jack Knife exercise, as it works the opposing muscle group. By working the lower back and glutes, the 45 Degree Hyperextension targets the opposing muscle group to the abdominals which are used in the Jack Knife. It is a great exercise for strengthening and toning the lower back and glutes, and helps to improve overall core strength. With the combination of both exercises, it provides an effective workout for the entire core area.

Straight Leg Cable Pull Through

Graphic image of Straight Leg Cable Pull Through.

The Straight Leg Cable Pull Through is a great exercise to compliment the Jack Knife, as it utilizes the opposing muscle group. This exercise targets the hamstrings and glutes, which are the primary muscles used in the Jack Knife exercise. To perform the Straight Leg Cable Pull Through, stand with your feet hip width apart and hold the cable attachment in front of your hips. Keeping your back flat and your chest up, bend forward slightly at the hips and extend your legs straight behind you while keeping your arms straight. As you return to the starting position, squeeze your glutes and hamstrings before repeating. This exercise is great for strengthening the posterior chain and can be an effective way to improve your Jack Knife performance.

Mastering the Jack Knife: Cut Through Your Core with Confidence!

Mastering the jack knife exercise is a great way to sculpt a strong and toned core, and it’s not as intimidating as it may seem. By understanding how to engage your abs correctly and maintaining proper form, you can cut through your core with confidence! It’s important to start with a modification or variation that suits your fitness level and gradually progress as you become more comfortable with the movement. Consistency and patience are key, as it takes time and practice to perfect your form and reap the benefits of this challenging exercise.

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

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

About The Author

Join Us On Social Media

Copyright © 2008 - | Privacy | MuscleMagFitness Powered By | MAcademyORON.org