Training guide
A crunch curls your head and shoulders off the floor until your shoulder blades lift, while your lower back stays down. Keep your feet free, let your fingertips rest at the sides of your head, leave a fist-sized gap under your chin and breathe out as you curl: that spares your neck and keeps the work in your six-pack muscle, which a crunch trains hard.
Crunches build ab endurance and strength, but they don't burn the fat over your stomach. The case that they damage healthy backs rests mainly on pig spines bent thousands of times in a lab, and soldiers who trained with sit-ups had no more injuries than those doing core exercises. With osteoporosis, a prolapse, later pregnancy or the first weeks after a birth, change them.
This guide shows you the movement step by step on the 3D figure, how to protect your neck, what the evidence says about your lower back, how many to do and how to make them harder, with the moves and workouts that go with them and the warning signs.
On your back with your knees bent and your feet flat and free, breathe out and curl your head and shoulders up until your shoulder blades leave the floor, ribs towards your pelvis, lower back staying down. Pause, then lower slowly.
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In 18 adults, crunches drew 53% of maximum in the upper six-pack muscle, against 40% in a sit-up and 34% in a plank, with very little hip flexor work. In 14 people, the crunch was among the top exercises for the six-pack muscle, beside ab machines.
Rest your hands rather than pulling, keep a fist-sized gap under your chin and breathe out as you curl: in small studies, these lowered neck-muscle activity and raised ab activity. Holding your feet down lowered ab activity and let the hip flexors take over.
Partial curl-ups gave the most ab work for the load on the spine of 12 exercises. Fears of damage rest on pig spines bent thousands of times under load, no study has shown harm in healthy human spines, and soldiers training with sit-ups had no more injuries than those doing core exercises. Change them if bending flares your back.
In your first two weeks, 2 sets of 10 to 15 with 3 or 4 reps to spare; then 2 or 3 sets of 12 to 20, stopping 1 to 3 reps short, 2 or 3 days a week (our sit-up guide's starting point). Keep all your bending reps to about 60 a session, one expert review's untested suggestion, and make them harder rather than doing more.
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Crunches strengthen the muscle under the fat; they don't burn the fat over it. Six weeks of ab exercise changed no fat measure in a trial, and across 13 studies training a body part didn't reduce the fat over it.
Lying on your back with your knees bent and feet flat, you curl your head and shoulders up until your shoulder blades leave the floor, your ribs drawing towards your pelvis, while your lower back and hips stay down. The bend is mostly in your upper back: about 3 degrees of bending in the lower back in a crunch against about 30 in a bent-knee sit-up, as one study reports from earlier work. Research often calls it a curl-up or partial curl-up; the Pilates chest lift is a close cousin.
Mainly the rectus abdominis, the six-pack muscle down the front, with the obliques at your sides; the hip flexors barely join in (6% of maximum, against 23% in a sit-up). The deepest layer, the transversus abdominis, works at most about 40% of its maximum in curl-ups, so crunches aren't its best exercise. Your neck muscles hold your head up, which is why the hand and chin cues matter.
These numbers come from EMG (electromyography: electrodes measuring how hard a muscle works), which shows effort in one session, not what a muscle will build.
A sit-up keeps going until you're upright, so the hip flexors pull you the rest of the way: sit-ups worked the deep psoas at 15 to 35% of its maximum against under 10% in a curl-up, and compressed the lower spine by more than 3,000 newtons. Our sit-up guide covers sit-ups and sit-up tests.
A steady McGill curl-up hold, as the crunch's move page says, or the Pilates chest lift: both lift your head and shoulders only a little.
From our move page and ACE's crunch (breathe out as you curl, ribs towards the pelvis, hold briefly at the top), with the fist-sized chin gap from our sit-up guide and the free feet from a muscle activity study. ACE's version puts your hands behind your head with the elbows drawn back; either way, the hands only support. For a harder crunch, put your arms overhead, lie back over a ball with it under your lower back, or hold a light weight on your chest.
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The movement on the figure: Crunch
In 13 men, a curl-up with neck bending restricted by their own hand lowered activity in the main neck muscle (the sternocleidomastoid) and raised ab activity. Keep a fist-sized gap under your chin, and think of lifting your chest rather than your chin.
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In 12 adults, breathing out slowly during a curl-up lowered neck-muscle activity and raised activity in the deeper ab muscles.
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A review of 87 muscle activity studies advises against pulling with your hands behind your head. Rest your fingertips at the sides of your head or cross your arms on your chest; if you prefer ACE's hands-behind-the-head style, let them only cradle your head.
Rest your head on the floor between reps, or swap to reverse crunches or dead bugs, which keep your head down, as our sit-up guide suggests. See a GP if neck pain or stiffness hasn't gone after a few weeks, or comes with pins and needles or a cold arm. Our neck pain after a workout guide has more.
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In 18 adults, the crunch drew 53% of maximum in the upper six-pack muscle and 39% in the lower, 28% in the external oblique and 33% in the internal oblique, with only 6% in the main hip flexor at the front of the thigh. In 14 people, it was among the highest of 9 exercises for the six-pack muscle. In ACE's study of 13 exercises, the basic crunch was the yardstick: the ball crunch scored 139% of it and the long-arm crunch 119%, though only 125% or more counted as notably better, and the study wasn't peer reviewed.
Holding the feet down lowered activity at every ab site and raised hip flexor activity in 14 people. Moving the arms from your chest to overhead added load and raised activity in every ab muscle in 10 women measured with fine-wire electrodes.
Six weeks of ab exercise left 24 sedentary adults doing 47 curl-ups on a test, against 32 without training. Four weeks of slow curl-ups raised curl-up counts and the size of the trunk muscles on scans. Twelve weeks of curl-ups thickened the six-pack muscle by 0.7 mm and raised its strength in 70 women. Soldiers who swapped sit-ups for core work for 12 weeks kept their sit-up scores.
In 30 women, crunches raised the pressure inside the tummy no more than getting up from a chair; coughing and jumping raised it most in 20 adults. In 17 women after a birth, a curl-up raised it to 43% of a cough's rise, which the authors judged little risk to the pelvic floor. Squeezing the pelvic floor first didn't outstrip the rise in a curl-up in 11 people.
Most crunch numbers come from single-session muscle activity studies, which a methods review found don't predict which exercise builds more muscle. Treat the rankings as clues.
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Sit-ups compressed the lower spine by more than 3,000 newtons in a modelling study of 12 young men, with no real difference between bent-knee and straight-leg versions. Of 12 ab exercises in 9 volunteers, partial curl-ups gave the most ab work for the load on the spine, and no single exercise trained every ab muscle at a low load. An expert review reports about 2,000 newtons for a basic crunch.
Pig neck-spine segments herniated when bent and straightened under load up to 86,400 times in a row. An expert review points out that this was dead animal tissue bent far beyond any workout, that living tissue adapts, and that no study has shown flexion exercise harms healthy human spines; it calls the harm theory highly speculative and limits bending only for people with disc problems or back pain that's worse bending forward.
In 1,141 soldiers over 16 weeks, sit-up training caused no more injuries than core stabilisation exercise, though soldiers with a back injury lost more days in the sit-up group (8.3 against 4.2). In 4,147 soldiers over 2 years, core stabilisation prevented no more back pain than traditional exercise.
For most healthy backs, crunches in sensible doses are reasonable but not essential. If bending flares your back, or you have osteoporosis, train your trunk with dead bugs, bird dogs, planks and the McGill curl-up instead, as our sit-up guide says. With back pain, aim for pain no more than about 5 out of 10 that settles within an hour or so and is no worse the next morning, and stop anything that sends pain down a leg.
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The same expert review suggests no bending exercises within an hour of getting up, when the discs hold the most fluid. It's opinion based on lab data, so treat it as optional.
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Move one step along when every set reaches 20 clean reps (our sit-up guide). If bending is new or your neck tires, start with McGill curl-ups, 2 to 3 sets of 5 holds of 8 to 10 seconds (our 30-day ab challenge), or the chest lift, 8 to 10 reps (its move page's dose). Then crunches, 2 sets of 10 to 15 for two weeks and then 2 or 3 sets of 12 to 20 (our sit-up guide and crunch and sit-up workout); the long-arm crunch, 1 to 2 sets of 10 to 15 (our crunch and sit-up workout uses 1); the ball crunch with the ball under your lower back, 2 to 3 sets of 10 to 15 (our editorial starting point); the weighted crunch, 2 to 3 sets of 8 to 15, starting light (our editorial starting point); the decline crunch, 2 to 3 sets of 8 to 12 (our editorial starting point); and in the gym, the kneeling cable crunch, 3 sets of 10 to 12 (our gym ab workout), or the ab crunch machine, 2 to 3 sets of 8 to 12 (its move page's dose).
3D demoMcGill curl-upBeginner · No equipment
3D demoChest lift (Pilates curl-up)Beginner · No equipment
3D demoCrunchBeginner · No equipment
3D demoLong-arm crunchBeginner · No equipment
3D demoExercise ball crunchBeginner · Exercise ball
3D demoWeighted crunchIntermediate · Medicine ball
3D demoDecline crunchIntermediate · A bench or box
3D demoKneeling cable crunchIntermediate · A cable machine
3D demoAb crunch machineBeginner · Strength machine2 sets of 10 to 15 crunches, stopping each set with 3 or 4 reps to spare, as our plans start every move.
2 or 3 sets of 12 to 20, stopping 1 to 3 reps short of failure, with 60 to 90 seconds' rest: our sit-up guide's starting point. NASM gives 2 to 3 sets of 8 to 15 for moving ab exercises. Going to failure adds little muscle, and people misjudge the reps they have left by about one, so stop while your form is clean.
2 or 3 days a week, with a day between: the expert review suggests about 48 hours between bending sessions. With weekly sets matched, how often you train a muscle made little difference to growth.
About 60 bending reps a session at most, crunches and reverse crunches included, starting with 2 sets of 15: one expert review's suggestion, untested, which our crunch and sit-up workout keeps to.
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When every set reaches 20 clean reps, move one step along the ladder above or add a little weight, as our plans' double progression does. The expert review suggests 6 to 15 reps with added weight for strength; for endurance, the ACSM suggests light loads for more than 15 reps with short rests.
Your neck does the work. Let your fingertips rest at the sides of your head, or cross your arms.
Your hip flexors take over. Leave your feet free.
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Momentum does the work. Curl up and lower slowly: ACE warns that going too fast brings in the hip flexors and tilts the pelvis.
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Tuck it slightly and keep a fist-sized gap to your chest.
Breathe out as you curl and in as you lower.
Once your lower back leaves the floor, it's becoming a sit-up. Stop when your shoulder blades lift.
They don't burn belly fat, and one expert review suggests about 60 bending reps a session at most. Make the move harder instead.
Stop if crunches make your back worse or send pain down a leg, and use the dead bug or McGill curl-up while it settles.
Six weeks of ab exercise on 5 days a week changed no fat measure, waist size or skinfold in 24 adults; 12 weeks of ab training added to a diet took off no more belly fat than the diet alone in 40 women; and across 13 studies of 1,158 people, training a body part didn't reduce the fat over it. Fat comes off your whole body with your overall activity and eating habits, as our belly fat guide explains.
A crunch strengthens the six-pack muscle; how flat your stomach looks depends mostly on the fat over it, and bodies differ in where they store it. Crunches are light exercise, about 2.8 METs (2.8 times the energy you use at rest), as our sit-up guide notes.
Build ab endurance and strength: curl-up scores rose within 6 weeks in one trial, and curl-ups made the six-pack muscle thicker and stronger over 12 weeks in another. Our get abs guide puts them into a plan.
Both: they do different jobs. A crunch bends your trunk and a plank holds it still. In the same 18 adults, the crunch drew more upper six-pack activity (53% against 34%) and the plank on the toes more external oblique activity (40% against 28%). If bending flares your back, lean on planks, side planks and bird dogs; our plank guide teaches the plank.
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Start with crunches: they work the six-pack muscle at least as hard as sit-ups, with less hip flexor pull and less load on your lower back. Our sit-up guide covers sit-ups, anchored feet and sit-up tests.
For your obliques, the side-lying oblique crunch (2 to 3 sets of 10 to 15 slow reps each side, its move page's dose) and the cross-body crunch (1 set of 8 to 10 each side, as our crunch and sit-up workout uses) add a sideways or turning curl. Our bicycle crunch guide covers the twist in detail.
The NHS advises avoiding sit-ups, planks and high-impact exercise to start with, and contacting a GP if the gap between your tummy muscles is still obvious 8 weeks after the birth. Start with the gentle tummy exercises the NHS describes and the early stages of our diastasis recti guide.
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In 70 women with a gap 6 to 12 months after a birth, 12 weeks of head lifts, curl-ups and twisted curl-ups on 5 days a week (registered as 3 sets of 8 to 12, close to the most they could do, about 10 minutes a session) neither widened nor narrowed the gap, and made the six-pack muscle thicker and stronger. Our diastasis recti workout follows that programme.
A head lift or twisted curl-up narrowed the gap above the belly button by about 10 mm in 38 women, while drawing the tummy in as hard as possible widened it a little below. A crunch narrowed it most but distorted the tissue down the middle most in 54 women, and curl-ups and sit-ups narrowed it about equally in 19. What these short-term changes mean over months is unknown.
If the middle of your tummy bulges into a ridge as you curl, make the move easier, as our diastasis recti guide advises, and ask to see a specialist physiotherapist if your tummy bulges with exercise.
These train the jobs crunches don't: dead bugs, 1 set of 6 each side as a warm-up, side planks, 2 holds of 15 to 30 seconds a side (or on your knees), and bird dogs, 1 to 2 sets of 6 each side with 3 to 5 second holds (our crunch and sit-up workout); forearm planks, 3 holds of 20 to 45 seconds (our plank hold guide); and half-kneeling Pallof presses, 2 sets of 8 to 10 each side (our sit-up guide's no-bend moves). Cat-cow, 8 slow waves, and child's pose, 1 to 3 minutes, are their move pages' doses.
3D demoDead bugBeginner · No equipment
3D demoSide plankIntermediate · No equipment
3D demoBird dogBeginner · No equipment
3D demoForearm plankIntermediate · No equipment
3D demoHalf-kneeling Pallof pressBeginner · A band + An anchor point
3D demoCat-cowBeginner · No equipment
3D demoChild's Pose (Balasana)Beginner · No equipmentOur crunch and sit-up workout keeps you to about 60 bending reps, with side planks and bird dogs for balance. Core session A of our 30-day ab challenge brings crunches into its curl block, the beginner core workout uses McGill curl-ups, the diastasis recti workout follows the curl-up trial for after a birth, and the 7-minute workout has crunches in a circuit.
A 15-minute ab workout with a crunch track and a sit-up track, about 60 bending reps at most, plus a side plank and bird dog to balance it.
15 minBeginner10 movesNo equipmentA 12-minute core session for the front of the trunk: dead bugs, curl-ups or crunches and front planks, each with steps from beginner to harder.
12 minBeginner11 movesNo equipmentA 15-minute beginner core workout on the floor: dead bugs, bird dogs, knee planks, side planks and curl-ups.
15 minBeginner6 movesNo equipmentA 12-minute tummy routine copied from a trial in women with diastasis recti: head lifts, curl-ups and twisted curl-ups, 5 days a week for 12 weeks.
12 minBeginner5 movesNo equipmentThe 12-move circuit from a 2013 fitness journal article: 30 seconds per move and 10 to switch, with a short warm-up and an easier swap for every move.
12 minIntermediate13 movesA wall, A stepThe 30-day ab challenge brings crunches in on its curl days; the postpartum plan includes the diastasis recti workout once the early stages are easy.
A free 30-day ab challenge: four 12-minute core sessions a week, from dead bugs to hollow holds, with rest days and an honest word on belly fat.
4 training days12 minBeginner5 days a weekA postpartum workout plan from the first days after birth to running and lifting again, week by week, with a caesarean track and readiness checks.
5 training days25 minBeginnerIn later pregnancy, the NHS advises avoiding lying flat on your back for more than a few minutes, which can make you feel dizzy. Our rule starts after the first trimester: swap crunches for side-lying, all-fours or standing core work, as our pregnancy guide sets out.
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The NHS says sit-ups and high-impact exercise put pressure on your pelvic floor and can increase leaks, and an NHS pelvic health physiotherapy leaflet advises people with a prolapse to avoid abdominal crunches. Leave them out and follow our pelvic floor and prolapse guides.
The UK consensus advises modifying or avoiding repeated or end-range rounding of the spine, and the Royal Osteoporosis Society names sit-ups among the exercises that put pressure on the spine. In a small, old study, new spinal fractures followed in 8 of 9 women given bending exercises. Train your trunk with dead bugs, bird dogs and planks instead, as our osteoporosis guide sets out.
After abdominal or bowel surgery, a hernia repair or a caesarean, or with a stoma, leave crunches out while you heal and build up through gentler core work, stopping if you notice bulging around a scar or stoma, as the move page says. With a hernia, crunches needn't be banned, but leave out any move that makes it bulge, as our hernia guide advises.
Swap to the dead bug, bird dog or McGill curl-up while it settles, as our back pain guides do.
Keep your head resting between reps, or swap to reverse crunches for now, and see our neck pain guide.
If you get chest pain or pressure, feel faint or dizzy, are unusually breathless, or your heart races or beats irregularly during exercise, as our plans advise. The American Heart Association says these need immediate medical evaluation.
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For chest pain that feels tight, heavy or squeezing, or spreads to your arms, neck, jaw or back; a sudden, extremely painful headache, especially while straining; someone who faints while exercising; or palpitations that don't go away, or come with chest pain, breathlessness or feeling faint. Don't drive yourself to A&E.
Call 999 or go to A&E for loss of feeling or numbness around your genitals or bottom, new trouble peeing or controlling your bladder or bowels, changes in genital sensation during sex or problems with erections or orgasm, or pain, tingling, weakness or numbness in both legs. These are signs of cauda equina syndrome, which needs treating in hospital urgently; don't drive yourself. Ask for an urgent GP appointment or get help from NHS 111 for back pain with a high temperature or feeling hot, cold or shivery, or severe pain that starts suddenly or gets worse quickly. See a GP if back pain or sciatica isn't improving after a few weeks.
If you haven't exercised for some time or have a medical condition, speak to your GP first, as the NHS advises. Breathe out on the effort and never hold your breath, especially with high blood pressure, heart disease, glaucoma, a hernia or recent tummy surgery, as our breathing guide explains.
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See a GP if you think you have a hernia: a bulge or lump in your groin, upper thigh or tummy that may get bigger when you cough or strain and smaller when you lie down. Get help from NHS 111 if you have a hernia and pain in or around it, a bloated tummy, sickness, constipation, a high temperature or sudden confusion. Call 999 or go to A&E for a femoral hernia (a lump at the top of the thigh) with very bad tummy pain, sickness, being unable to fart or poo, a high temperature or sudden confusion.
See a GP about any leaking, which is common and treatable, or a lump in or around your vagina, or a feeling of heaviness or a bulge, as our pelvic floor guide says.
A sudden sharp pain in your tummy wall during a set may be a strained muscle. Get help from NHS 111 or an urgent treatment centre if it's very painful or getting worse, badly swollen or bruised, or not getting better with self-care. Our abdominal strain guide covers recovery.
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See a GP if neck pain or stiffness hasn't gone after a few weeks, or comes with pins and needles or a cold arm.
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Aching abs for a day or two after a new session are normal. Sharp pain isn't: the NHS says to stop exercising straight away if you feel any pain or become unwell, and to get help from 111 if you're worried or it doesn't go away.
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Ab exercises strengthen the muscle but don't burn the fat over it: 6 weeks of ab exercise changed no fat measure, adding ab training to a diet did no better than the diet alone, and across 13 studies training a body part didn't reduce the fat over it.
The case rests on pig spine segments bent up to 86,400 times under load. No study has shown harm in healthy human spines, partial curl-ups gave the best ab work for the load on the spine, and soldiers training with sit-ups had no more injuries. With osteoporosis, or a back that flares with bending, change them.
Pulling on your head does. Resting your hands, keeping a chin gap and breathing out as you curl lowered neck-muscle activity and raised ab activity in small studies.
Fixed feet lowered ab activity and raised hip flexor activity, and a review of 87 studies advises against them.
High reps build endurance. Muscle grows from hard sets, more with 10 or more sets a week in the ACSM's overview of 137 reviews, and visible abs depend on body fat. One expert review suggests about 60 bending reps a session is plenty.
In 70 women with a gap 6 to 12 months after a birth, 12 weeks of curl-ups didn't widen it and made the muscles stronger. In the first weeks after a birth, the NHS advises avoiding sit-ups to start with.
The six-pack works as one muscle: its upper and lower parts worked alike in two muscle activity studies. On ultrasound, a crunch emphasised the upper part a little, but it worked the whole muscle more than a leg raise did.
Lie on your back with your knees bent and your feet flat and free, fingertips resting at the sides of your head. Breathe out and curl your head and shoulders up until your shoulder blades leave the floor, with your lower back staying down. Pause, then lower slowly.
Let your hands rest rather than pull, keep a fist-sized gap between your chin and chest, and breathe out as you curl. Lift your chest, not your chin, and rest your head between reps. If it still aches, swap to reverse crunches or dead bugs for now.
You can, as long as they only support your head: pulling is what strains your neck. Fingertips lightly at the sides of your head, or arms crossed on your chest, make pulling less likely. Arms stretched overhead make the crunch harder.
Not for most healthy backs in sensible doses: the evidence of harm comes mainly from pig spines bent thousands of times in a lab. If bending flares your back or sends pain down a leg, switch to the dead bug or McGill curl-up. With osteoporosis, leave crunches out.
Mainly the six-pack muscle, the rectus abdominis, with the obliques at your sides and very little of your hip flexors. In one study, crunches worked the upper part of the six-pack muscle harder than sit-ups or planks.
You don't need a daily number. Start with 2 sets of 10 to 15 for two weeks, then 2 or 3 sets of 12 to 20, stopping 1 to 3 reps short, 2 or 3 days a week. Keep all your bending reps to about 60 a session, one expert's untested suggestion.
No. Six weeks of ab exercise didn't change belly fat in a trial, and training a body part doesn't remove the fat over it. Crunches build the muscle; seeing it depends on your overall body fat.
Start with crunches: they work the six-pack muscle at least as hard, with less hip flexor pull and less load on your lower back. Our sit-up guide covers sit-ups and sit-up tests.
Both. Crunches train your trunk to bend and planks to hold still, and each worked a different part harder in one study. If bending flares your back, lean on planks, side planks and bird dogs.
No. Holding your feet down lowered ab activity and let your hip flexors take over in a study. Leave them free.
Once 2 or 3 sets of 20 are clean, put your arms overhead, crunch on a ball with it under your lower back, hold a light weight on your chest, or use a decline bench or a cable. Make the move harder rather than doing more reps.
After the first trimester, avoid lying flat on your back for more than a few minutes: swap crunches for side-lying, all-fours or standing core work. After a birth, the NHS advises avoiding sit-ups to start with; later, curl-ups didn't widen a gap between the tummy muscles in a trial, as our diastasis recti guide explains.
Exercise education, not a personal prescription, and clinical review of these guides is pending. Check with a GP before starting hard training if you have a heart condition, chest pain, dizziness or breathlessness when active, and stop for sharp or increasing pain. Health disclaimer.
