Jackknife Sit-Up
The jackknife sit-up, also known as a V-up, has you lying flat on your back and lifting your torso and straight legs at the same time, meeting in the middle so your hands reach toward your feet. Unlike a regular sit-up, which only moves your torso, this asks your upper and lower abs to fold toward each other simultaneously, which is a bigger coordination and strength demand for the same amount of floor space. It suits anyone with a decent base of core strength already, since the full-body folding motion is harder to control than it looks.
Why it works
Your rectus abdominis runs the full length of your torso, and most ab exercises only ask it to shorten from one end, either the ribs curling toward the pelvis in a crunch or the pelvis tilting toward the ribs in a reverse crunch. The jackknife sit-up asks both ends to move toward the middle at once, which means your upper and lower abs contract together rather than one end staying relatively still. Your hip flexors also do real work here lifting your straight legs, so this is closer to a compound core movement than a pure abdominal isolation exercise, similar to the coordination reflected in its compound mechanics rating.
Set up
- Lie face up on the floor, reaching your arms straight past your head and keeping your legs fully extended.
- Keep your legs together and pointed toward the ceiling as you prepare to lift.
- Press your lower back gently into the floor before you start moving.
- Pick a fixed point ahead of you to keep your neck neutral through the rep.
How to do it
- Lift your torso and your straight legs off the floor at the same time.
- Reach your hands toward your feet as your body folds into a V shape.
- Meet in the middle with your hands close to your shins or ankles if your flexibility allows.
- Lower your torso and legs back down together under control.
- Stop just before your shoulders and heels touch the floor to keep tension on your abs.
- Repeat for your target reps, keeping the same tempo on the way up and down.
Form cues
- Lift your torso and legs at the same speed so you meet cleanly in the middle.
- Keep your legs as straight as your hamstrings allow rather than bending your knees to cheat the range.
- Lead with your chest reaching up, not just your arms flailing forward.
- Control the lower on the way down instead of dropping back to the floor.
Breathing: Exhale as you fold up into the V, inhale as you lower back down with control.
Common mistakes
Using momentum to jerk the torso and legs up instead of controlled strength
Fix: Slow the rep down and lift through your abs deliberately, even if that means fewer clean reps instead of more sloppy ones.
Letting the legs drop faster than the torso lifts, breaking the V shape
Fix: Practice each half separately first if needed, then focus on lifting both halves at the same tempo once you combine them.
Bending the knees significantly to make the range easier
Fix: Keep a slight, natural bend at most, and regress to a tuck crunch instead if straight legs feel out of reach right now.
Slamming the torso and heels back to the floor between reps
Fix: Lower under control and stop just short of the floor to keep your abs working through the whole set.
How to program it
- Three sets of 8 to 12 reps work well as a core finisher once you can do a handful with clean form.
- Because it's a demanding compound ab movement, two or three times a week with a day of rest between sessions is enough for most people.
- Master a strict sit-up and a reverse crunch separately before combining them here, since jackknife sit-ups ask for both movements' strength and coordination at once.
- If your form breaks down partway through a set, stop there rather than finishing the set with momentum.
Safety notes
- Keep the movement controlled rather than using momentum, since a fast, jerky V-up puts unnecessary strain on your lower back.
- Stop if you feel pinching or strain in your lower back rather than working through it.
- Build up your rep count gradually rather than jumping straight to high volume.
Exercise profile
- Force
- Pull
- Mechanics
- Compound
- Category
- Strength
Frequently asked questions
How many jackknife sit-ups should I do per set?
8 to 12 clean reps for three sets is a solid target once you've built the coordination for the movement.
What's an easier version if I can't do a full jackknife sit-up yet?
A tuck crunch trains the same folding motion but only moves your knees toward your chest, which is a good regression while you build strength and coordination.
Do I need my legs completely straight for this exercise?
As straight as your hamstring flexibility allows is the goal, but a slight natural bend is fine, and bending more to make it easier means it's time to regress to an easier variation instead.
Is a jackknife sit-up the same as a V-up?
Yes, jackknife sit-up and V-up are two names for the same exercise, lifting your torso and straight legs together to meet in a V shape.
Why does this feel so much harder than a regular sit-up?
A regular sit-up only lifts your torso, while a jackknife sit-up lifts your torso and legs at the same time, which demands more coordination and strength from your entire abdominal wall at once.