Jack knife Floor
- Target muscle
- Rectus Abdominis
- Synergist muscles
- Adductor Brevis, Adductor Longus, Iliopsoas, Obliques, Pectineous, Sartorius, Tensor Fasciae Latae
- Equipment
- Body weight
- Body part
- Waist
- Type
- Strength
The jack knife floor is a bodyweight core exercise that targets the rectus abdominis by simultaneously folding your upper and lower body toward each other from a lying position. The hip flexors, adductors, obliques, and tensor fasciae latae assist throughout the movement. It fits well in any core or ab-focused session and builds midline control with no equipment.
How to do the Jack knife Floor
- 1Lie flat on your back with your legs extended and your arms reaching overhead on the floor, close to your ears.
- 2Press your lower back into the floor and brace your core before you begin.
- 3In one controlled motion, lift your legs and torso at the same time, reaching your hands toward your feet.
- 4Keep your knees as straight as possible and your arms close to your ears as both halves of your body rise.
- 5Meet your hands and feet above your hips at the top, folding your body into a V-shape.
- 6Pause for a second at the top with your abs fully contracted.
- 7Lower your legs and torso back down over 2–3 seconds, resisting gravity the whole way.
- 8Return your arms overhead and your heels to the floor under control, and end the set once your lower back can no longer stay flat.
Form tips
- Exhale as you fold up and inhale as you lower, using your breath to reinforce the core contraction.
- Initiate the rep by drawing your ribs toward your pelvis rather than throwing your limbs — this keeps the abs, not momentum, doing the work.
- If you can't keep your legs straight, set a fixed slight knee bend for the whole set while you build hamstring flexibility and strength, then straighten them as you progress.
- Squeeze your inner thighs together throughout the lift so your adductors keep both legs travelling as one unit instead of splaying apart.
Common mistakes
- Using momentum to swing the legs and torso up instead of contracting the abs, which reduces the training stimulus and can strain the lower back.
- Letting the lower back arch excessively off the floor at the start, which shifts stress away from the abs and onto the lumbar spine.
- Letting the knees creep into a deeper bend as the set gets hard, which shortens the lever and quietly offloads the abs — a fixed slight bend chosen up front is a regression, drifting into one mid-set is cheating the rep.
- Dropping the legs and torso back to the floor too quickly, which eliminates the eccentric phase where much of the core strengthening occurs.
Frequently asked questions
What muscles does the jack knife floor exercise work?
The primary muscle is the rectus abdominis. The hip flexors (iliopsoas and sartorius), adductors (adductor brevis and adductor longus), obliques, pectineus, and tensor fasciae latae all assist with the movement.
How is the jack knife floor different from a crunch?
A crunch only lifts the upper body, isolating the abs. The jack knife floor raises both the legs and the upper body simultaneously, engaging the hip flexors and adductors more heavily alongside the rectus abdominis.
Can beginners do the jack knife floor exercise?
Yes, but it requires baseline core and hip flexor strength. Beginners can make it easier by bending the knees slightly or reducing the range of motion until they build enough control to perform the full movement.
How many sets and reps should I do for the jack knife floor?
For core strength, 3 sets of 10–15 controlled reps is a solid starting point. Prioritize quality over volume — a slow rep through a full range of motion beats a fast, sloppy one.
Where should I feel the jack knife floor?
You should feel it mainly in your abs, with your hip flexors at the front of your hips also working hard to raise your legs. Sharp pain or strain in your lower back means you are arching your spine or relying on momentum — reset and move more slowly.
