How to do a Jackknife Sit-Up
Reviewed by Jim Parker, CPT, B.A. Kinesiology
About this exercise
- Sets Logged
- 1,462,904
- Popularity Rank
- 248th
- Difficulty
- Beginner
- Abs Strength
- 88 mSCORE 37th
- Equipment Required
- Bodyweight-only
Workouts with Jackknife Sit-Up
Target muscles worked
Instructions for Proper Form
Jackknife Sit-Up is a combination of a Sit Up and a Leg Pull-In. Like the other two exercises, this is a bodyweight core exercise that offers the benefits of both. It’s very similar to a Cocoon Crunch, however, you’ll be raising your entire torso off the ground, rather than just your shoulder blades. This is a great exercise for anyone looking to add some extra challenge to their core workout.
- Lie your back onto floor with your knees extended and your arms extended on the floor overhead.
- Keep your lower back flush against the floor by rotating your pelvis upward.
- Flex at your hips to elevate your extended legs while simultaneously bringing the bottom of your ribcage in towards your belly button and lifting your upper back off the floor.
- Your arms should follow the movement so that they are parallel with your extended legs that are about 45 degrees from the floor.
- Control the movement as you return to the starting position.
Common Form Mistakes
Resting Between Reps
Keep your core engaged for the duration of the exercise, even between reps. This exercise is designed to add instability that your core needs to compensate for. Keeping your core engaged throughout the exercise will improve the effectiveness of the exercise.
Sets & Reps Calculator
Average Jackknife Sit-Up standards by male, female, gender, weight, age and height
Use this calculator to see Fitbod's possible first recommendations for you. This would be your starting line, based on more than 4.5 billion logged sets from 1.1 million users.
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Alternatives & Variations to Jackknife Sit-Up
1. Crunches
94 mSCORESWAPPED 1,658 TIMESCrunches are one of the most common exercises out there. It’s an easy to perform, bodyweight exercise that targets your abdominals. Crunches are very similar to Sit Ups, however, instead of raising your upper body all the way into a near sitting position, you only raise your shoulder blades off the ground. This is a great exercise for beginners looking to build core strength.
2. Leg Raise
96.5 mSCORESWAPPED 1,132 TIMESLeg Raise is a simple bodyweight core exercise that involves laying on the ground and suspending your legs in the air. While your core does play a significant role in raising your legs, the real benefit comes from controlling their descent back to the floor.
3. Plank
92.8 mSCORESWAPPED 1,062 TIMESPlanks are one of the most common core exercises out there. This is a bodyweight exercise that is performed for time rather than reps. Planks don’t require any movement, instead they force you to place yourself in a position that requires you to engage your core, and maintain that muscle activation for the duration of the exercise. This is a great way to help build strength and stability in a way that will translate to a variety of other exercises, and your daily life.
4. Bicycle Crunch
91.8 mSCORESWAPPED 981 TIMESBicycle Crunch is a more difficult version of a standard crunch. Instead of raising your torso off the ground for each rep, you keep your torso off the ground, and your core engaged. By cycling your legs, you add more instability to the movement, which leads to better core activation.
5. Russian Twist
96.2 mSCORESWAPPED 861 TIMESRussian Twist is one of the more common core exercises. This exercise targets multiple muscles in your core, but the reason for including this exercise is usually to target your obliques. By raising your feet off the ground, engaging your core, and twisting to each side, you can better target this specific muscle in your core.
6. Sit Up
93.8 mSCORESWAPPED 835 TIMESSit Up is one of the most common exercises out there. It’s a bodyweight exercise that targets your core. Sit Ups are very similar to Crunches, however, rather than raising just your shoulder blades off the ground, you raise your entire upper body. This gives you a bit more range of motion than Crunches, however you’ll start to lose some tension in your core at the top of the movement.
7. Scissor Kick
94.2 mSCORESWAPPED 795 TIMESScissor Kicks are a bodyweight, core exercise that involve raising your feet off the ground, and kicking your legs up and down. By raising your legs off the ground, you’re already working your core. By continually alternating which leg is raised, you’re constantly forced to keep your core engaged for stability as well as to keep each leg raised.
8. Oblique Crunch
86.7 mSCORESWAPPED 736 TIMESOblique Crunch is a variation on the more standard Crunch. Similar to Crunches, this is a bodyweight core exercise. By reaching across your torso, you can help isolate your obliques. This is a great exercise for anyone looking to specifically target their obliques.
9. Reverse Crunch
87.7 mSCORESWAPPED 705 TIMESReverse Crunch is a variation on the more standard crunch, although it may appear closer to something like a Leg Raise. Rather than engaging your core to bring your chest towards your pelvis, you instead elevate your legs, and bring your pelvis in towards your chest.