Exercise guide
Half Jackknife
- Intermediate
- Compound
- Rep-based
- Waist
The Half Jackknife is a dynamic core exercise that isolates the rectus abdominis and obliques by combining trunk flexion with single-leg hip flexion. It improves unilateral core stability and coordination while engaging the hip flexors and quadriceps.
Reviewed by the Crucible Fit team · Updated June 2026
Muscles worked
Setup
- Lie flat on your back on a mat with your legs fully extended and arms stretched overhead.
- Press your lower back firmly into the floor to engage your deep core and eliminate any arch.
- Keep your feet together and your gaze directed toward the ceiling.
How to do it
- Exhale as you simultaneously lift one leg and your entire upper body off the floor, reaching your hands toward your rising foot.
- Pivot on your hips to create a 'V' shape, keeping the moving leg straight and the stationary leg pinned to the floor.
- Inhale as you slowly lower your torso and leg back to the starting position with a controlled 2-second tempo.
- Complete all repetitions on one side before switching, or alternate legs as specified in your program.
Form checklist
- Keep the moving leg as straight as possible to maximize quadriceps and lower ab engagement.
- Avoid using momentum or swinging your arms to pull yourself up; use your core to initiate the lift.
- Ensure your shoulder blades clear the floor at the top of the movement.
- Keep the non-working leg and heel pressed into the mat to stabilize your pelvis.
Pro tips
- Focus on 'folding' at the hips rather than just lifting the shoulders; think about bringing your ribcage toward your thigh.
- Pause for a split second at the peak of the movement to maximize the contraction in the upper and lower abdominals.
Make it harder
- Keep both feet hovering 2-3 inches off the ground throughout the entire set to maintain constant tension on the core.
- Hold a light weight or medicine ball in both hands to increase the resistance during the trunk flexion phase.
Frequently asked
- What muscles does the half jackknife work?
- The half jackknife primarily targets the abs and obliques, and also works the hip flexors as secondary muscles.
- What equipment do you need for the half jackknife?
- The half jackknife requires no equipment — just your body weight.
- Is the half jackknife good for beginners?
- The half jackknife is rated intermediate. Build a base with simpler variations first, then progress to it with light load and strict form.