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 while lying on the ground. The hip flexors, adductors, obliques, and tensor fasciae latae assist throughout the movement. It is well suited for building core strength and midline control without any equipment.
How to do the Jack knife Floor
- 1Lie flat on your back on the floor with your legs extended and your arms reaching overhead, close to your ears.
- 2Keep your lower back in contact with the floor and brace your core before you begin.
- 3In one controlled motion, lift your legs and your upper body simultaneously, reaching your hands toward your feet at the top.
- 4Keep your knees as straight as possible and your arms close to your ears as both halves of your body rise.
- 5Aim to bring your hands and feet close together above your hips at the peak of the movement, forming a V-shape with your body.
- 6Hold the top position for a brief moment, feeling the contraction in your abs.
- 7Lower your legs and upper body back to the starting position in a slow, controlled manner.
- 8Allow your arms to return overhead and your back to return to the floor before initiating the next rep.
Form tips
- Exhale as you fold up and inhale as you lower back down to use your breathing to reinforce the core contraction.
- Keep your gaze on your hands and your chin off your chest so your neck stays neutral as your torso rises.
- If you struggle to keep your legs straight, a slight bend at the knee is acceptable while you build flexibility and strength.
- Engage your inner thighs throughout the lift to help stabilize the movement and support the work of the hip flexors.
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.
- Bending the knees too much, which shortens the lever and makes the exercise significantly easier than intended.
- 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 reps should I do?
For core strength, 3 sets of 10–15 controlled reps is a solid starting point. Prioritize quality of movement over volume — a slow, controlled rep with full range of motion is more effective than a fast, sloppy one.
