Crunches: How to Do Them, Muscles Worked and Better Variations

The crunch has spent two decades being dismissed as outdated, usually by people selling something else, and it has quietly outlasted most of what replaced it. It needs no kit and no space, it puts tension straight through the abdominal wall without dragging your hip flexors into the party, and ultrasound measurement backs that up: the crunch produced greater thickening of the upper rectus abdominis than a leg raise in one study, by more than a third. What it does not do is any of the things the infomercials promised. Get the technique right and it earns a place in your week; do it badly and you will mostly train your neck.

How to do a crunch

Lie on your back on a mat or carpet, knees bent at roughly 90 degrees, feet flat and hip-width apart. Nobody needs to hold your feet down; if someone does, you are about to do sit ups instead.

1. Set your hands. Fingertips resting lightly behind your ears, elbows wide and in your peripheral vision. Crossing your arms over your chest works too and removes the temptation to pull.
2. Flatten your lower back. Press your lumbar spine gently into the floor before you move. This is the position you are trying to hold throughout, and losing it is what turns a crunch into a hip flexor exercise.
3. Curl, don't lift. Peel your shoulder blades off the floor by shortening the distance between your ribcage and your pelvis. Your lower back stays down. The movement is small, maybe 30 degrees of trunk flexion, and it should look unimpressive.
4. Breathe out at the top. Exhale hard as you curl up and squeeze your abs for a beat at the top of the rep. That forced exhale is what gets the deeper abdominal muscles involved.
5. Lower slowly. Take two or three seconds coming down, and stop just before your shoulders touch the floor to keep tension on the muscle. Then go again.

Muscles worked

• Rectus abdominis. The prime mover, and specifically its upper portion. Trunk flexion is its job, and the crunch gives it a short range with constant tension.
• Obliques. The external and internal obliques work as stabilisers on a straight crunch and become prime movers the moment you add rotation, which is why the bicycle crunch feels so different across your waist.
• Transversus abdominis. The deep corset muscle gets some involvement, mostly through the forced exhale rather than the curl itself.
• Hip flexors. Present but quiet, assuming your lower back stays pinned. As soon as it lifts, they take over.

If you want the lower abdominal region worked harder, the crunch is the wrong tool and the reverse crunch or a hanging leg raise is the right one. Same muscle, different end doing the moving.

Crunches vs sit ups

A sit up takes you all the way to sitting, which means your hip flexors haul your torso through most of the range while your abs go along for the ride. It also flexes the spine further and, in the classic anchored-feet version, pulls hard on the lower back. The crunch stops at the top of the abdominal contraction and stays there.

For abdominal training, that makes the crunch the better exercise for nearly everyone. Sit ups still have a use if you are training for a military or emergency services fitness test that counts them, since specificity wins in that situation, and they load the hip flexors in a way some athletes want. Choose deliberately rather than by habit.

Common mistakes

Pulling on your head. The classic. Laced fingers behind the skull turn into a lever the moment the set gets hard, and your neck pays. Fingertips behind the ears, or arms crossed on your chest.

Going too fast. Bouncing off the floor at two reps a second uses momentum and elastic recoil rather than muscle. Three seconds down, one second up, pause at the top.

Chasing rep counts. A hundred sloppy crunches trains stamina in a muscle that mostly wants tension. When 20 clean reps stop being hard, hold a dumbbell or a weight plate on your chest and drop back to 12.

Letting the lower back arch off the floor. That is the signal that your hip flexors have taken over. Reset, press the lumbar spine down, and shorten the range until you can hold it.

Treating them as fat loss. Abdominal work adds muscle underneath; it does not selectively strip fat from above. A randomised trial that added abdominal exercise to treadmill running did find a modest reduction in trunk fat compared with running alone, so the effect exists, but it is small and it arrived on top of a proper aerobic programme rather than instead of one.

Variations worth doing

• Weighted crunch. Hold a plate or dumbbell across your chest, or extended overhead for a longer lever and a harder set. This is the single best progression and the one most people skip.
• Cable crunch. Kneel facing a stack, rope by your ears, and curl your ribs towards your knees. Progressive load in small increments, which is why it earns a place in most gym programmes. Our cable crunch guide covers the setup in detail, and a home cable machine makes it available without a gym membership.
• Stability ball crunch. Draping your back over a gym ball lets you start from a slight extension, which lengthens the range and makes the same rep count considerably harder.
• Decline crunch. On a sit up bench set to a shallow decline, gravity fights you throughout. Add the incline gradually because it gets brutal fast.
• Bicycle crunch. Rotation brings the obliques in properly. Slow it down; almost everybody does this one at double the useful speed.

Sets and reps

Three sets of 12 to 20 reps, two or three times a week, sits in the right range for most people. Rest 45 to 60 seconds. Put them at the end of a session rather than the start, because fatigued abs make your compound lifts wobble.

Progress by adding weight, not reps. Once you are curling with a 10kg plate for 15 clean reps, you are training the abs like any other muscle group, which is the whole point. And build the rest of the middle around it: planks for anti-extension, dead bugs for control, hollow body holds when you want a real test of the whole front.

Recommended reads

1. Reverse crunch technique
2. Lower ab exercises that work
3. The best ab roller in the UK
4. The best exercise ball in the UK
5. Workout and exercise guides