Reverse Crunch: How to Do It, Muscles Worked and Variations

The reverse crunch is a crunch performed from the other end. Instead of lifting your shoulders towards your hips, you curl your hips and pelvis towards your ribs, which shifts the emphasis to the lower portion of the abdominal wall. It needs no equipment, takes up a square metre of floor, and is one of the few ab exercises that a beginner can learn in a couple of minutes and still find useful five years later.

It's also one of the most commonly butchered movements in any gym. Watch a room full of people doing them and most are swinging their legs, arching their back off the floor and working their hip flexors while their abs go along for the ride.

How to do a reverse crunch

Lie on your back on a mat with your arms flat by your sides, palms down. Bring your knees up so your thighs are vertical and your knees bent to roughly 90 degrees.

1. Set your back. Press your lower back firmly into the floor and hold it there. If you can slide a hand into the gap under your lumbar spine, you haven't set it properly. This position is the whole exercise; everything else is detail.
2. Curl, don't lift. Pull your knees towards your chest by rolling your pelvis upwards and tucking your tailbone, so your hips come an inch or two off the mat. The movement is small. If your hips are travelling a foot into the air, you're throwing your legs.
3. Pause at the top. Hold for a beat with your abs fully shortened. This is the position people skip, and it's where the muscle does the most work.
4. Lower slowly. Take three seconds to return, and stop the moment your lower back starts to peel off the floor. That point is your current range; it'll improve.
5. Breathe out on the way up. Exhaling hard helps the abdominal wall contract and stops you bracing against a held breath.

Muscles worked

The prime mover is the rectus abdominis, the long paired sheet running from your ribs to your pubic bone. It contracts as a single muscle, so nobody is switching off the top half and firing only the bottom. What does happen is a shift in emphasis, and that shift is measurable: research using diagnostic ultrasound found that the uppermost segment of the rectus abdominis thickened most during a crunch, while the lowest segment thickened most during a leg raise, with muscle recruitment tracking the segments closest to the load being moved (Diagnostic Ultrasound Shows Preferential Activation of Rectus Abdominis Segments, International Journal of Exercise Science, 2023).

That's the honest version of the "lower abs" argument. You can't isolate a region, but you can bias it, and the reverse crunch biases the bottom.

Alongside the rectus abdominis, the internal and external obliques work to stop your pelvis rotating, and the transversus abdominis contracts to hold the whole cylinder rigid. The hip flexors, mainly the iliopsoas and rectus femoris, assist throughout, which is unavoidable since your legs have to move. The goal is to keep them as assistants rather than promoting them to management.

Common mistakes

Swinging the legs. The most frequent one by far. Momentum does the work, the abs get a rest, and the rep count goes up while the results stay flat. Slow everything down until each rep takes about four seconds.

Letting the lower back arch. When the lumbar spine lifts off the mat, the pelvis has stopped tucking and the hip flexors have taken over. Shorten the range rather than chasing depth.

Straightening the legs too early. Keeping the knees bent shortens the lever and keeps the load manageable. Straighter legs make the exercise dramatically harder, which is a progression, not a starting point.

Pulling with the arms. Hands flat on the floor, not gripping the edge of a bench and heaving. If you need to hold something to complete the rep, the rep is too hard.

Doing 50 of them. High-rep ab work mostly trains endurance in a muscle that already has plenty. Fifteen hard, slow reps beat fifty sloppy ones.

Variations and progressions

Feet-elevated dead bug is the regression if the reverse crunch reproduces back pain or you can't keep the lumbar spine flat. Our dead bug guide covers it.

Straight-leg reverse crunch extends the lever by taking the knee bend out. Expect the reps to drop sharply, and don't attempt it until you can control 15 bent-knee reps with a three-second lowering phase.

Bench reverse crunch has you lying on a flat bench holding the top edge behind your head, which gives your hips room to travel further and lets you work through a longer range. A sit-up bench with an incline setting adds resistance without adding weight.

Hanging knee raise is the same movement pattern performed from a bar, and it's a genuine step up in difficulty because your abs now have to control your whole lower body against gravity. See our hanging leg raise guide once you're ready, and you'll need a pull-up bar.

Weighted reverse crunch holds a light dumbbell between the feet or a medicine ball between the knees. Start with 2kg. It gets hard faster than you'd expect.

Where it fits in a programme

Two or three ab sessions a week is plenty, and each one only needs to be about ten minutes. A sensible pairing runs a rotation-resistance move, an anti-extension move and a flexion move: side plank, dead bug, reverse crunch. That covers the abdominal wall's actual jobs rather than just doing three variations of the same curl.

If your goal is visible abs rather than strong ones, the training is the smaller half of the equation. Body fat determines whether the muscle shows, and that's decided by your overall energy balance and weekly activity, not by ab work. The NHS recommends 150 minutes of moderate activity a week alongside strength training on two days, and our guide on what exercise burns the most belly fat goes into why crunches aren't it.

All you need for the basic version is a decent mat. Ours are covered in the best yoga mat guide, and a thicker one is worth having if you're doing this on a hard floor, since a bony sacrum against laminate ends sessions early.

Recommended reads

• Lower ab exercises that actually work
• The hanging leg raise: technique and progressions
• Dead bug: the safest core exercise for a sore back
• The best sit-up benches in the UK
• All our exercise and workout guides