Jackknife Sit-Ups Will Level Up Your Core Workout—Here's How to Do Them

Woman performing a jackknife sit-up

Studio Firma/Stocksy

Core exercises are great for sculpting our abs, but they also have day-to-day benefits: The stronger your core is, the more stable your spine and overall body can be. But sit-ups and crunches get boring after a while, leading some of us to abandon ab exercises altogether.

Jackknife sit-ups are a different take on the classic sit-up move and, as such, serve as a great alternative when you're tired of the same old, same old for core work. To help you get started on trying them, we decided to delve deep into what jackknife sit-ups are, how to do them, and how to modify them for your specific needs, with the help of personal trainers Mallory Fry and Steve Stonehouse. Read on for everything you need to know about jackknife sit-ups.

Meet the Expert

  • Mallory Fry, ACSM, is a master coach for Row House and an American College of Sports Medicine-certified exercise physiologist.
  • Steve Stonehouse is a NASM-certified personal trainer, a USATF-certified run coach, and the director of education for Stride Fitness.

What Are Jackknife Sit-Ups?

Jackknife sit-ups are an ab/core exercise in which you contract your arms, legs, head, and shoulders in toward your center. You may be familiar with the jackknife's other name, which is a V-up; that term might actually help you to visualize them a little more easily (unless you're well-versed in the various types of knives out there). Stonehouse tells us that jackknife sit-ups "are core movements in which you're moving all four limbs together each repetition." Fry adds, "a jackknife is an advanced abdominal exercise moving in the sagittal plane of motion." The sagittal plane refers to thinking of your body in two halves, and moving those halves together or apart.

The Benefits of Jackknife Sit-Ups

  • Improved core strength: "Strengthening the core not only helps our daily lives in movement, but it also helps us create a stable torso to protect our spine," Fry says.
  • Efficiency: Stonehouse says that "any time you can challenge movement from both ends of your abdominals, you're maximizing your time [efficiency] and efforts."
  • Building strength using bodyweight: "Keeping your arms and legs extended increases the amount of weight you're moving," Stonehouse tells us.
  • Primary and secondary muscle engagement: "This specific movement targets the rectus abdominis and the transverse abdominis, with the secondary muscles being the obliques, glutes, and lumbar back," Fry says. "The rectus abdominis is what the general public refers to as the 'six-pack,' whereas the transverse abdominis is our deep core muscles that are typically skipped in everyday exercise. Secondary muscles are active muscles to support a specific muscle action. These muscles specifically help control and protect the body."

How to Do Them

  1. "Lie flat on the floor with your arms and legs extended," Stonehouse instructs. Fry recommends you be "on a hard flat surface;" we suggest a mat over the floor, for comfort's sake.
  2. Next, Fry says to "extend the arms, raising them up along the head (think biceps to ears)." She notes that "keeping the lats engaged will allow the shoulders to drop away from the ears."
  3. To get into the jackknife position, exhale and engage your core muscles while bringing your arms and legs up as close together as possible. Make sure to bring your head, shoulders, and part of your torso up along with your arms—your upper body should be raised up off the ground at the top of the position.
  4. Once in this position, you want to hold it briefly. Stonehouse suggests two to three seconds for a hold and stresses the importance of not holding your breath during this time or when you release. To prevent you from holding your breath, he recommends you "count your reps out loud as you perform this movement. You can't hold your breath if you're speaking."
  5. Slowly release to your starting position, then repeat.

"Once in the starting position, we contract the rectus abdominis to pull the legs and upper body together into a seated position," Fry says. "Remember the goal is to have the legs and arms pointed straight up."

Variations

There are numerous ways to make this exercise more or less challenging, depending on your needs. Here are some options.

More Challenging Variations

  • Try a slow, controlled release: "During the downward movement, think of taking three to five seconds to complete" the move, Fry recommends. 
  • Do the move continuously, with no hold: Fry tells us this is "ideally the same movement without pause or resets in the resting position," which she describes as "a continuous up and down movement with an alternating leg movement." She says that "both legs will stay elevated off the floor at all times. The rib cage will stay tucked and core connected at all times."
  • Alternate sides: Stonehouse instructs us that for this variation, you should "try alternating sides so that your right arm and left leg move together, and then your left arm and right leg."

Easier Modifications

  • Keep your back fully on the floor, and move your limbs only as far as a tabletop position: "In this position, focusing on the connection of the core as a contraction of the abdominis creates a crunch reaching the arms toward the legs," says Fry. "This will help to strengthen the upper abdominus."
  • Lift your legs, but not your back or arms: In this variation, "the legs will be straight in the air focusing on the connection, then lower toward the ground and raise back to starting position," says Fry. "This will help strengthen the lower abdominis along with the lumbar back and deep core."
  • Keep only your back on the ground: Do the full movements with your arms and legs, but do not lift your shoulders. This makes the move easier on your back.

Jackknife Sit-Ups vs. Regular Sit-Ups

A standard sit-up is an exercise that most people have performed at one time or another, even if it isn't part of their regular fitness routine. It involves keeping your feet firmly planted on the floor, with your knees bent. You lift your head, neck, and shoulders in an upward motion until they meet your legs, then you relax back down into your starting position and repeat. This move mainly works your upper abs.

Conversely, jackknife sit-ups use your lower abs, as well, because you also move your legs. With jackknives, you also extend your arms, which uses your oblique muscles in a way that regular sit-ups do not. Jackknives are a more advanced version of a sit-up, so they're better for people who are advanced in their workout routines. On the other hand, regular sit-ups are better for novices or anyone who hasn't acquired a lot of abdominal strength yet.

Safety Considerations

Place a mat underneath you for jackknife sit-ups, as the move relies on the comfort of your back in order to be successful. "There is not a lot of cushion on your lower back, so this movement can be uncomfortable without it," Stonehouse says.

To avoid hurting your back, Fry tells us to "keep in mind the low back should always stay in connection with the surface underneath. This connection from back to floor stems from the deep core (transverse/obliques)." Allow your legs to drop slowly to avoid injury, and don't drop them all the way unless you're confident in your strength. "The deeper the legs drop, the more connection and strength the body needs to control the movement in a safe manner," Fry tells us.

If you have any back, neck, or shoulder injuries, consult with your practitioner before trying this advanced move. And if you experience any pain at all when performing the exercise, stop right away and seek medical attention.

The Final Takeaway

Jackknife sit-ups are an advanced ab exercise. They involve just part of your back staying on the floor while you lift your arms, legs, head, and shoulders up toward your center in a contracting motion. They're also known as V-ups, which describes the shape your body contracts into at the peak of the movement.

Because they are an advanced exercise, there are modifications available to make them easier. There are also numerous variations that can increase their difficulty level. You should avoid doing jackknives if you have an injury to your back, and always be cautious when performing this move even if you don't. If you've been wanting to level up your sit-up game, jackknives are the perfect move to try.

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