Advanced

Straight-Leg Jackknife

The straight-leg jackknife has you lie flat, then simultaneously lift straight legs and a straight torso so the hands reach toward the toes at the peak. Both ends of the body fold over the hips at once, forcing the entire rectus abdominis to shorten hard through a large range. It is a demanding full-length crunch that rewards good hamstring flexibility and a strong core.

Equipment
Body Weight
Body part
Abs
Type
Strength
Browse by movement
Spinal Flexion
Muscles worked
Target
Upper Rectus Abdominals, Lower Rectus Abdominals
Secondary
Rectus Femoris
Tertiary
Abdominal External Oblique

How to do it

  1. 1

    Lie flat on your back with legs straight and arms extended overhead.

  2. 2

    Brace the abs and press the low back lightly toward the floor.

  3. 3

    Exhale and lift the straight legs and torso at the same time, reaching the hands to the toes.

  4. 4

    Balance briefly on the hips in the folded V position.

  5. 5

    Inhale and lower the arms and legs together under control.

  6. 6

    Stop just short of the floor to keep continuous tension.

Tips

  • Reach the hands toward the feet rather than jerking the head forward.
  • Bend the knees slightly if tight hamstrings pull the low back off the floor.
  • Lower under control instead of slamming back down to protect the spine.

Variations

Common mistakes

  • One half of the fold moves before the other, so the legs shoot up while the torso lags behind, losing the simultaneous fold that makes this move demanding.
  • The hands slap against the shins instead of continuing to reach for the toes, cutting the top of the range short.
  • Momentum from a fast torso swing replaces the ab contraction, so the rep looks finished but the abs barely worked through the fold.
  • Once the legs and torso separate near the bottom, the low back lifts off the floor instead of staying lightly pressed down through the whole rep.
  • Holding the breath through the fold instead of exhaling on the way up makes the lift feel heavier than the bodyweight actually is.

Frequently asked

What is a good starting dose for the straight-leg jackknife, and what should I be able to do first?
Two sets of 5 to 6 controlled reps is a sensible opening dose, stopping well before the lower back lifts off the floor. Before attempting it, you should already own a strict hollow-body hold and a straight-leg toe touch crunch, since both hamstring flexibility and lumbar control decide whether this move stays safe.
How do I regress or progress the straight-leg jackknife?
To make it easier, bend the knees slightly throughout or work the toe touch crunch with the legs already vertical until your hamstrings and low back control improve. To make it harder, add a 1-to-2 second pause balanced on the hips at the top of each rep.
What does the straight-leg jackknife train, and when is it a poor choice?
It trains maximum-range abdominal flexion and is best used as an advanced finisher for lifters chasing a strong, well-developed rectus abdominis. It is a poor choice for anyone with tight hamstrings, a history of low back pain, or limited hip flexion, since the straight-leg lever puts real strain on the lumbar spine.

Related exercises

Other movements among FitRay's 2,000+ exercises that load the same target muscle — same equipment and level first.

All Abs exercises