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.
In This Exercise:
- Target Muscle Group: Iliopsoas, rectus abdominis
- Type: Hypertrophy, function
- Mechanics: Compound
- Equipment: Exercise mat
- Difficulty: Intermediate, advanced
Jackknife Sit-Up Muscles Worked
There are several muscle groups involved in the jackknife sit-up. Here are descriptions of each one.
Iliopsoas
The iliopsoas is the primary hip flexor; made up of the major and minor psoas muscles and the iliacus muscle. It can function as a unit or separately to stabilize and facilitate movement. These muscles are important for proper standing and posture, the coxofemoral joint, and walking and running.
The iliopsoas is very much involved in every part of the hanging straight leg raise, especially when lifting the legs upward.
Strong hip flexors are crucial for just about any total body activity.
Rectus Abdominis
The rectus abdominis is fancy for abdominals, also known as the six-pack” muscles. These core muscles are responsible for trunk flexion (e.g., curl up during a crunch), and they assist during lateral trunk flexion (bending to the side).
When done properly, this exercise really activates the main abdominal muscles and is one of the best movements for building an impressive core.
Adductor Brevis
The adductor brevis is one of the adductor muscles that helps to pull the thigh medially (inward). It also helps to stabilize the trunk during certain movements, maintaining balance, and flexion of the thigh.
Adductor Longus
The adductor longus is another hip adductor muscle of the inner thigh. Therefore, it also pulls the thigh inward. Along with the adductor brevis and magnus it’s one of the strongest muscles of the thigh.
Obliques
The obliques (external and internal) are core muscles located on either side of the rectus abdominis or “six-pack” muscles. They facilitate trunk rotation and contralateral rotation, and with the erector spinae and rectus abdominis, the obliques help with lateral flexion of the trunk.
While there’s not much rotation during the jackknife sit-up, the obliques still get an awesome workout.
Sartorius
The sartorius muscle is actually the longest muscle in the body, that is located across both the knee and hip joint. Consequently, it can also function to flex both joints. It also externally rotates the hips.
Tensor Fasciae Latae
The tensor fascia latae (TFL) is a thigh muscle that works with several muscles to assist in the movement and stabilization of the hip and the knee. Along with the gluteus medius and gluteus minimus, the TFL internally rotates and abducts the hip. It also works with the gluteus maximus to abduct the hip via the iliotibial (IT) band.
The TFL also assists the rectus femoris in the flexion of the hip, and assists in pelvis stability while standing and walking.
Quadriceps
The quadriceps femoris or quads for short is a very large muscle located on the front of the upper leg. It consists of four muscle bellies that include the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. The rectus femoris flexes the hip while the other three muscles extend the knee.
The quads are important for many daily activities, posture, the gait cycle, and patellar stability.
It’s important to note that while involved in this movement, you will not benefit much in the way of leg size and strength.
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.
- Lie on your back with your arms extended overhead and legs straight out.
- 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.
- 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.
