Side Jackknife
The side jackknife has you lie on your side and fold your upper body and legs toward each other at the same time, closing the gap between your ribs and hips like a jackknife closing. It's a dynamic, bodyweight way to train the obliques through a full range of motion, rather than just holding them isometrically like a side plank does.
Why it works
The obliques run diagonally along your sides, and their main job is bending your torso sideways and rotating it. Lying on your side and folding your ribcage and legs toward each other puts those muscles through their actual shortening action against gravity, rather than just asking them to resist movement the way a static hold does. That's what separates the side jackknife from an isometric side bridge: it trains the obliques to actively produce the bending motion, not just resist it. Most core training leans heavily on movements that flex the spine forward or hold it still, so a side-bending exercise like this one fills a gap that a typical crunch or plank routine leaves untouched, which matters since real-world twisting and side-bending tasks, reaching sideways for something, catching yourself off balance, rely on exactly this muscle action.
Set up
- Lie on your side with your legs stacked and fully extended, body in a straight line.
- Rest your bottom arm along the floor for light support, or place your hand lightly behind your head.
- Keep your top arm resting along your side or lightly touching your top leg.
How to do it
- Simultaneously lift your legs and your upper torso off the floor, folding your ribs toward your hips.
- Reach your top hand toward your feet as you fold, if that's comfortable, to help you feel the contraction.
- Pause briefly at the top of the fold, feeling the squeeze through your obliques.
- Lower back down with control until your body is straight again.
- Complete the full set on one side before switching to the other.
Form cues
- Fold from the ribs and hips together, not just the legs.
- Keep the movement in the same plane, no rolling forward or back.
- Pause and squeeze at the top of each fold.
- Lower with control, don't just drop back down.
Breathing: Exhale as you fold your torso and legs together, inhale as you lower back down.
Common mistakes
Rolling slightly onto the back or front to make the fold easier.
Fix: Stay strictly on your side throughout the movement; rolling takes the load off the obliques and onto other muscles.
Only lifting the legs or only lifting the torso instead of folding both together.
Fix: Focus on closing the distance from both ends at once, ribs coming down to meet the hips as the hips lift to meet the ribs.
Using momentum to swing up rather than a controlled fold.
Fix: Slow the tempo down and pause briefly at the top; a controlled fold trains the obliques far more than a fast, swinging one.
How to program it
- Since this is a bodyweight core movement, two or three sessions a week is a reasonable frequency.
- Two or three sets of 8 to 12 reps per side is a solid target once you can complete the fold with good control.
- Pair it with a front-facing core move like a crunch or jackknife sit-up to balance oblique and rectus abdominis work in the same session.
Safety notes
- If your lower back rounds or strains during the fold, shorten the range slightly until your oblique strength catches up.
- Keep the hip you're lying on cushioned with a mat; the bony point of the hip can get uncomfortable on a hard floor.
Exercise profile
- Force
- Pull
- Mechanics
- Compound
- Category
- Strength
Frequently asked questions
How many side jackknives should I do?
Two or three sets of 8 to 12 reps per side is a solid range once your form is controlled. Prioritize a clean fold over a high rep count.
What's the difference between a side jackknife and a side bridge?
A side bridge is a static hold, your obliques resist movement the whole time. A side jackknife is dynamic, your obliques actively shorten to fold your ribs and hips together and then lengthen back out, training a different part of their job.
Can I do the side jackknife if I have lower back pain?
This isn't medical advice, but if your lower back rounds or aches during the fold, that's usually a sign to shorten the range or build oblique strength with a static side bridge first before adding the dynamic version.
Why do side jackknives feel harder on one side than the other?
It's common for one set of obliques to be stronger than the other, often the side opposite your dominant hand. Train both sides for equal reps rather than matching effort, and the strength gap will usually close over time.