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July 04, 2023 8 min read

The Jackknife sit-up is a popular abdominal exercise that's great for  strengthening the ab muscles. It's a tough one that really tests your core strength and stability. Athletes of all levels can use this exercise to up their fitness game, whether at home or at the gym.

In this article, we'll dive deep into the Jackknife sit-up, also known as V-Ups, covering its benefits, proper form, and different versions. Whether you're a newbie or a seasoned fitness pro, the Jackknife sit-up is a killer addition to your workout routine. Get ready to build a rock-solid, ripped core by adding this exercise to your fitness arsenal.

Muscles Worked When you do Jackknife Sit-Ups

The Jackknife sit-up is mostly about the abs, but it also engages other muscles to a lesser extent.

These are some of the main muscles worked in Jackknife sit-ups:

1. Rectus Abdominis

The rectus abdominis, also known as the abs, is the muscle in the front of the belly that gives you that coveted “six-pack” look. When you do Jackknife sit-ups, this muscle is the star of the show as it contracts and brings your upper body closer to your legs by flexing the spine.

2. Oblique Muscles

The muscles on the sides of your waist, known as the obliques, help you twist and bend your trunk. When you do Jackknife sit-ups, these muscles kick in to keep your body steady and help you move.

3. Transverse Abdominis

The transverse abdominis, which is the deepest layer of our ab muscles, plays a crucial role in keeping our core stable and supported. When we do Jackknife sit-ups, it kicks into action, helping to stabilize our spine and keep our form in check.

4. Hip Flexors

So, there are these muscles called the hip flexors, specifically the iliopsoas and rectus femoris, which are in the front of the hip joint. When you do Jackknife sit-ups, these muscles contract to flex your hip joint and move your legs toward your upper body.

5. Adductors

The adductor muscles include the adductor brevis, magnus, and longus. They do what their name suggests—they bring the thigh towards the opposite leg or inward. To keep your legs together while lifting them up and down, make sure to engage these muscles.

6. Tensor Fasciae Latae

We've got these muscles that most people wouldn't even know exist, and one of them is in the lower body, called the tensor fascia latae. It may not be well-known, but it plays a crucial role in our body. Along with the glutes, the TFL helps rotate and move our hip and also supports the flexion of the hip with the rectus femoris. It even helps keep our pelvis stable when we're standing or walking. Pretty important little muscle, huh?

7. Quadriceps

The muscles at the front of your thigh, known as the quadriceps, play a role in straightening your knee. When you do Jackknife sit-ups as part of your ab workout, every time you lift your legs, these muscles get activated.

8. Chest and Shoulders

Although they are not the primary muscles targeted, the chest muscles (pectoral muscles) and shoulders (deltoids) also come into play to some extent for support and help during the movement.

It's worth mentioning that how much your muscles get involved can differ from person to person due to factors like how you do the exercise, your form, and how strong you are. Plus, there might be some other supporting muscles that are also involved, but to a lesser extent.

How to do Jackknife Sit-Ups with Proper Form

To perform Jackknife sit-ups with proper form, follow these steps:

  1. Lie flat on your back on a mat or comfortable surface. Extend your arms straight overhead and keep your legs fully extended, close together, and flat on the ground.

  2. Engage your core muscles by contracting your abdominal muscles and pulling your navel in towards your spine.

  3. As you exhale, simultaneously lift your upper body and legs off the ground. Keep your arms straight and reach towards your toes with your hands. Imagine folding your body in half, bringing your upper body and legs towards each other at a 45-degree angle.

  4. At the top of the movement, your body should resemble a “V” shape, with your torso and legs forming the two sides. Try to touch your toes with your hands, or get as close as possible while maintaining control and proper form.

  5. Inhale as you slowly lower your upper body and legs back down to the starting position, keeping them hovering slightly above the ground.

  6. Repeat the movement for the desired number of repetitions or as prescribed by your workout routine.

Here are some extra tips to keep your form on point:

— Instead of relying on momentum or swinging, focus on using your abs to start the movement.

— Keep your neck in a neutral position by looking straight ahead or slightly up. Avoid tucking your chin in, as it can strain your neck.

— Don't arch your lower back too much during the exercise. Engage your core muscles to maintain stability and protect your spine.

— Take it slow and concentrate on the mind-muscle connection to make sure your abs are working properly.

— If you're new to this or finding the full Jackknife sit-up difficult, you can modify it by bending your knees slightly and reaching for your shins instead of your toes.

Remember to listen to your body, start with modifications or progressions that work for you, and talk to a qualified fitness pro if you have any concerns or special needs.

Common Mistakes Made with Jackknife Sit-Ups

If you want to make the most of the Jackknife sit-ups exercise, it's important to do it correctly. Here are some common mistakes to watch out for and be careful with when performing this exercise.

  • The Jackknife sit-up can be a bit tricky to master. It's common for people to overlook the importance of breathing, which can affect their performance. To avoid this mistake, try to slow down your movements and take a breath before each new repetition. Remember to exhale as you reach the top and begin lowering your legs and torso back down to the floor.

  • One common mistake people make when doing Jackknife sit-ups is rushing through the reps. This can cause instability and limit the range of motion. To avoid this, it's important to do the repetitions slowly, focusing on engaging your midsection muscles with each rep. You can also make the exercise more challenging and enhance core engagement by adding a brief pause at the top position.

  • One more important error to steer clear of when doing Jackknife sit-ups is not fully extending your range of motion. This often happens because your core muscles are not strong enough. If you're unable to lift your legs and torso at the same time, it's best to focus on simpler exercises like leg raises and sit-ups.

It is never a bad idea to get a personal trainer to help you with using the proper form of any new exercise to ensure you target the correct muscles and avoid injuries. A CPT can also help you with the number of reps to suit your fitness level.

Benefits of Doing Jackknife Sit-Ups

Including Jackknife sit-ups into your workout routine offers the following benefits:

1. Strengthening your Core

When you do the jackknife exercise, you're mainly working on your abs. This includes the rectus abdominis (those muscles that give you a six-pack), the obliques (the ones on the sides of your waist), and the transverse abdominis (the deep core stabilizers). By regularly doing this exercise, you'll be able to strengthen and tone your core muscles, which will help improve your stability, posture, and overall strength.

2. Full-Body Engagement

The main thing to pay attention to with Jackknife sit-ups is the core, but they also work other muscles like the hip flexors, quadriceps, chest, and shoulders. This exercise targets multiple muscle groups, giving you a well-rounded workout and the chance to burn some calories.

3. Mobility and Flexibility

When you stretch towards your toes while doing Jackknife sit-ups, it helps make your hamstrings, hip flexors, and lower back more flexible and mobile. This can ultimately enhance your ability to move around and relieve any stiffness you may feel in these areas.

4. Balance and Coordination

Doing jackknife sit-ups can really test your balance and coordination since you're lifting both your upper body and legs together. If you make it a habit to do this exercise often, it can really improve your body awareness and proprioception.

5. Overall Core Stability

Having a strong core is super important for all kinds of physical activities and exercises, like lifting weights, going for a run, playing sports, and even just doing everyday movements. If you want to boost your core stability and improve your overall performance while reducing the chances of getting injured, give jackknife sit-ups a try. They're a great way to level up your core muscles.

6. Defined Abs

If you want those abs to really pop, don't forget to add Jackknife sit-ups to your workout routine. Combine them with a healthy diet and overall fat reduction, and you'll be on your way to a more chiseled and defined six-pack.

It's important to keep in mind that everyone's results may differ, so it's crucial to incorporate Jackknife sit-ups into a well-rounded exercise routine. This should include cardio, strength training, flexibility exercises, and a healthy diet to achieve maximum fitness and overall health benefits.

Variations of the Jackknife Sit-Up

The following core exercises are variations of the Jackknife sit-up that will bring some variety into your workout routine.

1. Side Jackknife Sit-Up

Instead of lying flat on your back, try the jackknife variation where you lie on your side. Extend your legs, press your hip into the mat, and place your hand behind your head. Take a breath and lift your upper torso and leg towards each other at the same time. This variation targets your internal and external obliques.

2. Jackknife Crossover

The jackknife crossover is a cool ab exercise move where you lift one arm toward the opposite leg. It's a pretty easy exercise to pick up and really targets those obliques.

3. Jackknife Knee Raises

If you're looking to build up to doing a full sit-up, the jackknife knee raise is a solid option. The key difference is that instead of fully extending your legs, you keep your knees bent as you lift them toward the ceiling. This tweak makes the exercise easier to do and helps you get the hang of the movement.


To sum it up, Jackknife sit-ups are a super effective bodyweight exercise for building core strength and boosting overall fitness. They target both the upper and lower abs, as well as the hip flexors, which help tone and strengthen the muscles crucial for maintaining good posture, balance, and stability.

When done right and consistently, Jackknife sit-ups can improve flexibility, increase range of motion, and lower the risk of back pain and injury. Whether you're a beginner or a seasoned athlete, adding Jackknife sit-ups to your fitness routine is a fun way to challenge yourself and reach your fitness goals. So, why not give them a shot and witness the results firsthand?

If you're looking for more in-depth core training, you can find a list of 10 sit-up variations from easiest to hardest here.