What Are Jack Knife Sit Ups?

There are lots of ways you can improve your exercise routine. Adding jackknife sit-ups is a brilliant way to spice up your regime without going too far outside the box.

To perform this move, you must maneuver your body into a V shape (a jackknife). It works by growing and strengthening core muscles.

Jackknife sit-ups are a brilliant way to challenge yourself and make your abs sing. It is a type of move that requires significant flexibility in addition to a very strong core.

However, simultaneously raising your legs and arms to meet in the middle isn’t always the easiest thing if you lack the requisite strength. There are some important things you must know.

If you’d like to learn more about these sit-ups, this article has got you covered.

What Is A Jackknife Sit-Up?

When you think of brilliant core exercises, you likely think of planks, hanging knee raises, and crunches.

Unfortunately, doing the same exercises over and over again can become tiring and frustrating, and there’s only so much satisfaction to be had when doing a sit-up.

Thankfully, there are plenty of other engaging exercises that do wonders for your core. One of these is jackknife sit-ups, an advanced version of the sit-up maneuver that we’re familiar with.

These provide a low-impact, full-body workout that works to build abs, increase strength, and improve stability. 

Muscle Areas Targeted By The Jackknife Sit-Up

Jackknife sit-ups are great for hitting muscle groups all over your body. The primary muscles that are targeted during the jackknife sit-up are as follows:

  • Rectus abdominis (abs)
  • Transverse abdominis
  • Obliques
  • Lower back
  • Glutes

How To Do A Jackknife Sit-Up

Learning how to do these sit-ups is pretty straightforward if you’re already working on your core. You must follow a certain number of steps which we have outlined below.

  1. Lie flat on the floor on a mat with your legs extended and arms behind your head. Your legs, heels, upper back, head, buttocks, and arms should all be in contact with the floor. 
  2. Take a deep breath and engage your abdominal muscles by drawing your belly button toward your spine.
  3. Inhale and then exhale. As you keep your legs extended, slowly start to raise your legs until they hit a 45-degree angle with your hips. Simultaneously bring your arms upward so they are parallel to your legs, slowly lifting your shoulder blades, head, and torso a few inches off the mat. This is the jackknife position.
  4. Make sure that your chin is not tucked into your chest at this stage, and don’t allow your head to fall backward, otherwise, you risk causing serious injury. Hold for around three seconds to feel the full abdominal benefits of the position.
  5. Extend your body to your starting position without placing your hands or feet back on the floor.
  6. Take a deep breath and then repeat the movement as many times as you want.

If you find it tough to hold this position or move in this way, then you can perform the exercise by bending your knees. This is commonly known as a jackknife crunch.

How Many Reps?

The best way to do this is to start with 10 to 12 reps per set and aim to complete a total of 2 to 3 sets.

Allow yourself a rest time of around 30 seconds after your first set of reps to make sure that your muscles have some time to recover.

Try to do 2-3 sets of this exercise around 4 days a week for at least 6 weeks to experience the maximum benefits.

When To Progress To The Jackknife Sit-Up

When To Progress To The Jackknife Sit-Up

The jackknife sit-up is a highly effective core exercise. Yet if you are new to ab training, you may not be able to do it. 

If this is the case then there’s nothing to worry about. This is because you have the time to dedicate to working up to an official jackknife sit-up, and there is no rush to get to that point.

Begin by strengthening all the core muscles that are involved in the jackknife sit-up.

You can do this by introducing a well-balanced strength training routine into your life. In the majority of cases, this will be more than enough.

Focusing on your leg muscles and midsection via lots of different exercises will improve your flexibility and make it much easier to complete the movement.

Benefits Of Jackknife Sit-Ups

Many benefits are associated with jackknife sit-ups, but only a handful truly stand out. We’ve outlined a couple of these below for you to take a look at.

Target Multiple Areas At Once

When you do a general jackknife sit-up, you’re using nearly a dozen muscles at once. However, your rectus abdominis is the main targeted muscle. 

During this exercise, you will also end up using your obliques, thigh muscles, and additional muscles in your chest, calves, and gluteus to achieve the movement.

If you want to target more muscles, try doing a jackknife sit-up with a stability ball or a power wheel.

Easy On The Joints

The best part about this movement is that it does not cause extra strain on your joints which means you can do them often without long-term negative effects.

You also don’t need to buy special equipment to do this exercise which is great news for your wallet!

Increased Flexibility

As your aching abdomen will no doubt testify, jackknife sit-ups are a true test for abs.

You’ll also find that your lower back benefits significantly from the maneuver, and you will find yourself becoming increasingly more flexible as time goes on.

One day, you will be able to achieve the optimum jackknife position — keeping your legs straight throughout the entire maneuver.


Jackknife sit-ups are an advanced version of a regular sit-up, requiring considerable focus and strength.

The physical benefits of using these sit-ups are very prominent, and you can expect a strengthened core, improved flexibility, and more controlled movements as a bare minimum.