A side jackknife is a crunch done lying on your side. Propped up on your bottom arm with your top hand behind your head, you lift your top leg up and away from the other one while you bend your upper body sideways, so the top elbow and the top leg move toward each other, then lower both. The muscles along the side of your waist (obliques) do the side bend, and the muscles at the side of your top hip lift the leg.

How to do the side jackknife

Lie on one side on a mat with your legs straight and stacked. Prop your upper body up on your bottom arm, with the arm straight and the hand flat on the mat under your shoulder.
Put your top hand lightly behind your head, with the top elbow pointing up toward the ceiling.
Tighten your stomach. Lift your top leg up and away from the bottom one while you bend your upper body sideways, so your top elbow and top leg move toward each other.
Lower the leg and your upper body slowly until your legs meet again.
Finish all your repetitions, then turn over and work the other side.

What you should feel

Work along the side of your waist on the top side, and in the outer hip of the leg that lifts. The shoulder you lean on works to hold you up.

Common mistakes

Rolling back onto your buttock, or rolling your chest toward the floor.
Pulling your head sideways with your top hand.
Kicking the leg up fast and letting it drop.
Letting the shoulder you lean on hunch up toward your ear.
Holding your breath.

How often

Many programs start with 2 to 3 sets of 8 to 12 repetitions on each side, a few days a week. Your physio will adjust this.

Stop and check with your physio if

Pain in the wrist, elbow or shoulder of the arm you lean on.
Sharp pain in your lower back or at the side of your hip, or pain spreading down a leg.
Neck pain that builds as you lift.
Pain or tingling spreading down an arm.
Pins and needles in your arm or hand that do not go away after you stop, or part of your arm or hand goes numb. Get medical advice the same day. If one side of your face or body suddenly goes numb or weak, or your speech becomes slurred or muddled, call emergency services, even if it goes away.

From the clinic

The voice guide says to rest on your elbow with it under your shoulder, and to hold. In the video the bottom arm is straight with the hand on the floor, and the top leg lifts and lowers again and again rather than holding still, so this page follows the video. If your wrist or shoulder complains, lean on your forearm instead, with the elbow under your shoulder. That base is steadier and easier on the wrist.

Lift and lower at a steady pace. If the leg swings, the work leaves your waist. With a painful shoulder, or after shoulder surgery or a dislocation, ask your physio before you lean your weight on one arm. If you have high blood pressure, breathe steadily through every repetition and never hold your breath.

If lifting the leg and bending sideways together is too much for now, side-lying hip abduction trains the leg lift on its own. The kneeling side plank and side plank work the same side of the waist as a hold.

If you have had a hip replacement or other hip surgery, get the go-ahead from your surgeon or physio before you try the side jackknife. Some operations limit this kind of work for the first months, so the timing depends on your surgery. After some surgical approaches, lifting the leg out to the side has to wait until the tissues at the side of the hip have healed.

Osteoporosis programs often limit repeated side bending of the spine. With osteoporosis or low bone density, ask your physio whether side jackknives suit you. If you have had a fracture in your spine, check with your physio before you begin. If you have had surgery on your spine, chest or heart in recent months, get the go-ahead from your surgical team or physio first.

If you are pregnant, have a hernia or have had stomach surgery, ask your physio or doctor before you start. If you have recently given birth, including by cesarean section, or you have a gap down the middle of your stomach muscles (diastasis recti), ask your physio, midwife or doctor first.

Written and checked by the PocketPhysio editorial team. Last updated 2026-09-28.

This page is general education, not a diagnosis or a personal treatment plan. If you have pain, an injury or a health condition, see a physiotherapist or doctor before starting new exercises.

Side jackknife