The floor laying on side oblique crunch, commonly called a side jackknife, is a lying lateral-flexion exercise that targets the obliques by lifting the torso and legs simultaneously toward one another. Propping on the forearm keeps the shoulder stable while the working side of the waist performs the entire movement. It isolates the obliques more directly than standard crunches because the resistance comes from lifting the legs and torso against gravity in the frontal plane rather than the sagittal plane. This makes it a useful accessory movement for building lateral core stability.
Muscles Targeted
- Primary: Obliques (internal and external, working side)
- Secondary: Rectus abdominis, quadratus lumborum
- Tertiary: Hip abductors, latissimus dorsi (supporting arm)
Starting Position
Lie on your side on a mat with your legs extended and stacked, feet together. Prop yourself up on your forearm, positioning the elbow directly beneath your shoulder so your forearm sits perpendicular to your torso. Rest your top hand lightly behind your head or across your chest. Keep your hips stacked and your body in a straight line from head to feet before you begin.
Execution Steps
- Engage your obliques on the down side to lift your legs a few inches off the floor
- Simultaneously curl your upper torso toward your hips, leading with your top shoulder
- Bring your rib cage and hip bone closer together on the working side, reaching peak contraction
- Hold briefly at the point of maximum contraction
- Lower your torso and legs back down together under control to the starting position
- Complete all reps on one side, then switch sides and repeat
Form Cues
- Initiate the lift from the waist, not by swinging the legs with momentum
- Keep the supporting forearm planted and stable - it's a pivot point, not a lever for lifting
- Move your ribs and hip toward each other in one smooth arc rather than two separate motions
- Keep your neck neutral; don't pull your head forward with the top hand
- Exhale as you crunch up, inhale as you lower
Common Mistakes
- Swinging the legs up with momentum: reduces oblique tension and can strain the hip flexors; lift slowly and deliberately
- Rolling the hips forward or backward: shifts work off the obliques onto the hip flexors or lower back; keep hips stacked throughout
- Lifting only the legs or only the torso: cuts the range of motion and the oblique contraction in half; move both together
- Pushing off the floor with the supporting arm to gain height: turns the movement into a push-up variation instead of an oblique crunch; keep the forearm fixed
- Doing uneven reps on each side: creates a left-right strength imbalance; match reps and effort on both sides
Variations
- Standard side plank: an isometric hold version that builds oblique endurance without the crunching motion
- Weighted side crunch: holding a light dumbbell against the top hip increases resistance
- Side crunch with knee tuck: bending the knees instead of keeping legs extended reduces the lever arm for beginners
- Cable side bend: standing cable version that allows progressive loading through a similar lateral-flexion pattern
- Stability ball side crunch: performed with the hips supported on a ball for an added balance challenge
Tips for Progression
- Master the strict bodyweight version for 12-15 reps per side before adding load
- Add a light dumbbell or plate held against the top hip for extra resistance
- Slow the tempo to 2-3 seconds on the lowering phase to increase time under tension
- Progress toward the stability ball variation once balance and control improve
Training Notes
Include the floor laying on side oblique crunch 2-3 times per week in a core routine, performing 2-3 sets of 12-15 reps per side with 30-45 seconds rest. It pairs well with standard crunches and planks to hit the obliques from a lateral angle that front-facing ab exercises miss.