Jackknife Sit-Up
Lying flat with arms overhead and legs straight, you close arms and legs together over your hips like a folding knife, then reopen to full length. The long-lever start is what makes it hard: the abs have to stop a fully extended body from arching before they get to fold it.
Own the movement
Set up first, then execute. Every cue below is something you can check on your next set.
Before the first rep
- Lie flat on a mat with legs straight and arms stretched overhead, biceps beside your ears.
- Press your lower back into the floor before the first rep and note how that feels — it is the position you are defending.
- Point your toes lightly so the legs stay one rigid lever rather than folding at the knee.
- Clear about a metre of space behind your head; the arms travel further than people expect.
Every rep
Breathing & tempo — Exhale hard as you fold, inhale as you extend. One second up, one second held at the top, two seconds down.
Sets, reps & load
Where to start, and what to change when it stops being hard.
Bodyweight is the starting point and stays the point for most people. Fifteen strict reps with a two-second lowering phase is a solid standard. Progress by slowing the descent and pausing at the bottom before you progress by adding reps; only once 20 clean reps feel easy is holding a 2-4 kg plate or light dumbbell overhead worth adding.
What goes wrong
The errors that show up most often, and the cue that fixes each one.
Take two full seconds on the lowering phase and pause a beat at the bottom with everything hovering. If you cannot, cut the reps in half and rebuild.
Stop the legs higher — at 45 degrees rather than at the floor — and only extend lower once you can keep the lumbar spine pinned for every rep.
Squeeze your quads and point your toes throughout. If you genuinely need bent knees, do them bent on purpose and log it as a regression.
Keep your arms next to your ears and your gaze on your knees. The neck should travel with the ribcage, not lead it.
Why bother
Most floor ab work starts already folded — knees bent, hands on the chest, spine half-flexed. This one starts at maximum length, arms behind your head and legs straight out, which is the position your abs have the least leverage in. The first ten degrees of every rep are the honest part.
Both ends move, so the hip flexors pull the legs up while the abs pull the ribcage down and hold the pelvis from tipping. That shared job is why it feels harder than the reps suggest: the set usually ends because your lower back starts arching off the floor, not because your abs burned out.
Where it fits
End of a session, after the compound work, two or three times a week. Pair it with an anti-rotation or anti-extension drill rather than with more crunches — it already covers spinal flexion thoroughly, and stacking three flexion exercises in one session buys nothing.
If this one isn't it
No kit, too hard, or too easy — here's where to go instead.
Same job, different kit
Start here instead
When this gets easy
Asked and answered
Is this the same thing as a V-up?
Effectively yes — the V-up is the same fold, usually described with the hands reaching to the toes. The difference is emphasis: here the target is legs at 35-45 degrees with the arms parallel to them, which keeps tension on the abs rather than turning the rep into a race to touch your feet.
My hip flexors do all the work. How do I feel my abs more?
Reduce the leg lever. Bend the knees to about 90 degrees so the legs weigh less at the hip, and concentrate on pulling the ribcage toward the pelvis first, legs second. Also press the lower back down before the rep starts — an arched lumbar spine hands the whole job to the hip flexors.
Will these give me visible abs?
They will thicken the rectus abdominis, which helps once body fat is low enough for it to show. No amount of folding burns fat off your midsection specifically — spot reduction is not a thing. Train the abs for strength, get lean through diet and overall training.
Is it bad for my lower back?
Repeated loaded spinal flexion is not free, and if you have a diagnosed disc problem, this is one of the first movements to drop. For most healthy trainees, done with the lumbar spine pinned to the floor and no bouncing, it is fine. Pain in the back rather than fatigue in the abs means stop the set.