Jackknife Sit-Ups: Tone Abs, Obliques & Hip Flexors Fast!

The jackknife sit-up is an intermediate to advanced ab exercise that only requires your body weight and a little more movement than a basic ab crunch or standard sit-up.

It’s a great option if you really want to challenge yourself and will activate the entire group of core muscles.

Learn how to do it with tips, variations, and more in this exercise guide.

Jackknife Sit-Up Muscles Worked

There are several muscle groups involved in the jackknife sit-up. Here are descriptions of each one. 

Iliopsoas

Iliopsoas has a big role in performing the major flexion of the hip joint. It’s not just one muscle but rather a psoas major and minor and an iliacus which can also function separately. Lifting the legs up will utilize a lot of hip flexor strength. 

Rectus Abdominis

Jackknifes are utilized for their ab building prowess and they, like other similar variations can improve your core development if you do them properly. The rectus abdominis performs a crunching motion at the upper and lower torso which is needed to effectively execute jackknifes. 

Adductor Brevis

The adductor brevis works as the name implies, to adduct or bring the thigh toward the opposite leg or inward. You’ll need to include some activation of these muscles to help keep your legs together as you lift them up and down. 

Adductor Longus

The adductor longus is a strong adductor like its family members, the adductor brevis and magnus. 

Tensor Fasciae Latae

We use muscles that majority of the population would never know of and tensor fascia latae is one of those, however, an important muscle group. With the gluteus medius and gluteus minimus, the TFL internally rotates and abducts the hip but also assists the rectus femoris in the flexion of the hip, and assists in pelvis stability when standing and walking.

How To Do The Jackknife Sit-up

While this isn’t necessarily a complex movement, you might benefit from a few tips to ensure you do it right and get the most from it. 

Here are step-by-step instructions for the jackknife sit-up.

1. Lie on your back with your arms extended overhead and legs straight out. 
2. Keeping your legs straight or knee slightly bent, engage your core and simultaneously bring your feet and arms together over the middle of your body. You should try to touch your ankles at the top.
3. Bring your legs and arms back into the starting position and repeat for the desired number of reps. 

Here’s a video example…

Jackknife sit-up tips

• You want to make sure to keep your core muscles nice and tight. Simply going through the motions will not engage your midsection as effectively and that defeats the purpose of the movement. 
• You can bend your knees if you cannot keep your legs straight.
• The arms and legs should move up and toward each other at the same time. 

Jackknife Sit-up Benefits

There are a few benefits of doing this sit-up variation. 

Improve core development

This is an ab-focused movement which means most exercisers do it because they believe it will help to improve the appearance of their core muscles. 

Well, done right and intensely enough and it can certainly contribute to the development of your abdominal muscles. 

Strengthen your midsection

This exercise also helps to increase your core strength. 

A strong midsection is crucial for all lifting and physical activities as it keeps your body rigid and supportive, stabilizes your spine, and protects your lower back. 

Don’t need equipment

The jackknife sit-up requires no workout equipment to do which means it can be done almost anywhere. 

It’s fun and you can make it more challenging

While the jackknife sit-up is an effective core exercise, it also provides a challenge and is a great option for adding some versatility to your training. 

You can include it in supersets, circuit training, workout challenges, and more.

Not to mention, you can add ankle or wrist weights or hold some dumbbells and make it much more challenging. 

Jackknife Sit-Up Variations

The jackknife sit-up is a great ab exercise but there are similar ones that you may like as much or better, and some are easier which makes them ideal for beginners. 

Cocoons

Cocoons are a similar exercise to the jackknife sit-up but it’s arguably easier to do. The movement resembles insect cocoons when performed properly. 

You could also use it as a progression to jackknife sit-up.

Lying straight leg raise

The lying straight leg raise involves only lifting the legs without moving the arms. Many believe leg lift variations emphasize the lower portion of the abdominals and it may. But you cannot necessarily target the lower abs.

This exercise can help to increase hip flexibility to be able to perform exercises like the jackknife sit-up.

Using The Jackknife Sit-up Into Your Training Routine

It’s not rocket science really. The jackknife sit-up can fit into your routine in many different ways and there are no rules. But we do think a little strategy can make your training more enjoyable and productive. 

For example, you might use this exercise as part of a superset. Or, you could choose several exercises and so a circuit with the jackknife sit-up being thrown in somewhere. 

But you could also do it with other core movements or on its own as a finisher since it’s challenging and intense. 

Sets/reps

The best way to perform bodyweight ab exercises is to train to failure. That’s because, for many, doing more than a few reps is necessary to challenge the muscles. 

We would recommend a minimum of 2 sets for this exercise and reps to failure.

Wrapping Up

The jackknife sit-up is a must-have bodyweight core exercise. It doesn’t require equipment which means you can do it anywhere but it’s also very challenging once you do enough and if you have the mobility and flexibility. 

You can include it in your workouts any way you like but just make sure to do it correctly. 

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