Jackknife

Equipment

No Equipment Needed

Muscle Group

Core

Setup

Use a mat on a flat surface with enough space to extend your legs. This is an oblique-focused “side crunch” variation—move slowly and prioritize control over speed.

Starting Position

Lie on your side with your hips stacked and legs extended (or slightly bent if needed). Place your top hand lightly behind your head (don’t pull on the neck) and your bottom arm either on the floor for balance or across your body. Brace your core and keep your neck neutral with your gaze forward.

Execution

Exhale and crunch by bringing your ribs toward your hip on the same side while simultaneously lifting your legs slightly toward your torso (like a “jackknife” closing). Keep the movement small and controlled—focus on squeezing the obliques at the top. Pause briefly, then inhale and lower your shoulders and legs back down under control without fully relaxing. Repeat for the desired reps, then switch sides.

Tips on doing this exercise

Think “ribs to hip” and squeeze hard at the top for 1 second.

Keep your hips stacked—use the floor and your bottom arm for balance.

Control the lowering (2–3 seconds) to keep tension on the obliques.

If hip flexors dominate, bend your knees slightly and reduce leg lift height.

Progress by slowing tempo, adding a pause, or holding a light plate against your chest.

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Common Mistakes

Pulling on the neck or yanking the head forward

Using momentum and swinging the legs instead of controlled oblique contraction

Rotating the torso forward/back instead of staying stacked on the side

Letting hips roll or feet kick to finish reps

Rushing the lowering phase and losing tension

Doing a huge range that causes the low back or hip flexors to take over

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Recommended Ranges

Core/oblique endurance: 2–4 sets of 12–20 reps per side

Hypertrophy focus: 3–4 sets of 10–15 reps per side (pause at the top)

Beginner: 2–3 sets of 8–12 reps per side with shorter range
Rest 30–60 seconds between sets.

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