Core · Bodyweight
Half Jackknife
How to do the half jackknife, the muscles it works, and the mistakes to avoid. Pulled straight from the Kovo app's exercise library.
- Equipment
- Bodyweight
- Difficulty
- Beginner
- Logged as
- Reps
- Movement
- Flexion
- Mechanic
- Isolation
How to do it
Lie flat on your back with your legs extended and arms reaching overhead. Simultaneously lift your torso and your bent knees toward each other, reaching your hands toward your shins at the top. Lower under control back to the floor without fully relaxing.
- 1Your abs and hip flexors folding your torso and knees together
- 2A strong contraction in the middle of the movement at the top
- 3Steady control as you lower back toward the floor
Common mistakes
- Yanking up with momentum instead of crunching deliberately
- Letting your lower back arch off the floor at the bottom
- Straining your neck; lead with your core, not your chin
Muscles worked
The half jackknife mainly works the lower abs, upper abs, and hip flexors, with the obliques assisting.
- Lower AbsPrimary
- Upper AbsPrimary
- Hip FlexorsPrimary
- ObliquesSupport
- QuadsMinor
Kovo programs the half jackknife for you and tracks it every session.
We build your whole program, drop the right lifts into each workout, and adjust your weights as you get stronger. Free on iOS and Android.
More abs & core exercises
- How to do the ab wheel rollout
- How to do the ankle tap plank
- How to do the ankle taps
- How to do the resistance band dead bugs
- How to do the resistance band pallof press
- How to do the rollout
Last reviewed June 2026.