What Are Jack Knife Sit Ups?

We’ll be real with you: Working out is incredibly difficult and consistently will take sacrifice and commitment. It never gets easier. Obviously in our busy lives after work, we don’t want to go to the gym and sweat. Eating a pizza and sitting at home sounds much more appetizing. But if you want the body of your dreams and to feel great in your own skin, you have to make a change and schedule going to the gym.

Focus on abs

If you are a beginner at the gym, we recommend first focusing on the ab region simply because it is integral to all forms of working out. The core is a pivotal point for all types of exercises and if it is weak that can lead to injury especially while lifting weights.

There are a lot of core exercises you can do including using ab rollers, ab machines, planking, or doing sit-ups. The regular sit-up does get old after a while so why not vary it up a bit and try Jack knife sit ups?

The Jack Knife Sit-ups

So now for the main topic of this article: the Jack knife sit ups. This is one of the most popular sit-ups as it targets the abdominal muscles and all the obliques in a great way. Also known as the V-Up, this is a hard sit up, but once you get the hang of it you will become addicted.

What makes the jack nife sit ups so great is that you don’t need any equipment, it is for the body only, and it works on building strength and toning that core. Whether you are a beginner or a more advanced gym-goer, the jack knife sit ups are definitely going to work for you. To begin the jack knife sit ups, lie completely flat on the floor. We recommend placing an exercise or yoga mat under you to make yourself more comfortable. Put your arms extended straight back right behind your head. Extend your legs out on the ground. This is known as your starting position for the jackknife sit up. Next, breathe out, and bend at the waist on the floor while raising your legs and arms to meet in what is called the “jackknife position.” This is where they meet at the highest point in front of you while lying on the floor. Your legs should be about at a 40° angle from the floor, and keep your arms parallel to the legs. Engage that core and lift your upper torso off the floor. Breathe in and lower the arms and legs back to the original position on the floor. Continue to do however many reps of this Jackknife sit up.

Flexibility is important

The Dangers

Now, this is a difficult sit up, way more difficult than your normal sit up simply because of the amount of strength it takes to get both the legs and arms and torso off the ground. You are working both your upper and lower oblique muscles at the same time and it can become fatiguing. You will run into some dangers that we want to make you aware of.

Many people quickly get tired doing the jack knife sit up, and they begin to swing their arms to gain momentum to hit their legs. This then becomes more of an arm and shoulder exercise, rather than an ab exercise. You should not be swinging your arms at all, and it is the motion of the torso moving up that brings the arms to meet the legs. Flexibility is important in this sit up, especially in the legs, and you may need to modify depending on where you are. If you have to bend at the knee, that is completely ok. For any sort of exercise modification is important, especially if you have a past injury, so do what you need to do. The main key to the jack knife sit up should always engage your core; it is the reason you are doing all of this in the first place.

The moment you can’t engage the core anymore, simply stops doing the exercise, as you may throw out your back or use muscles you shouldn’t be using. Exercises are to be done properly, and we would rather you do 5 jack knife sit ups with the proper form than 50 with an improper form.


Working out is all about consistency so continue to push yourself to exercise at least four times a week.

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