What 30 days can and can't changeEmerging evidence
The likely gains are more reps, steadier and harder holds, and better control. In 24 adults, 6 weeks of abdominal exercises on 5 days a week left them able to do more curl-ups than a group that didn't train (47 against 32). In 22 inactive adults, a 5-minute daily routine for 4 weeks raised sit-up endurance by about half on average, with a very wide spread and no separate control group, and in 30 novice weightlifters, 8 core sessions over 4 weeks doubled their plank time, again with no control group. A 4-week programme of slow curl-ups enlarged the trunk muscles on MRI scans.
These studies were small and short, and part of an early gain is getting used to the test. In the thigh, muscle growth was measurable from about weeks 3 to 4 of hard training in 25 men, and early strength gains are partly your nervous system learning the move. A flatter stomach isn't on the list.
Sources 11, 12, 13, 14, 15, 16
Ab exercises don't burn belly fatStrong evidence
In 24 adults, 6 weeks of ab exercises on 5 days a week didn't change belly fat or waist size. In 40 women, adding 12 weeks of ab training to a diet took off no more fat than the diet alone. Across 13 studies of 1,158 people, training one body part didn't burn the fat over it, and when 104 adults trained one arm for 12 weeks, MRI scans showed fat loss across both arms.
Two small trials found some local fat loss, but with far more work than this challenge: 57-minute sessions of running with weighted crunches and torso rotations, 4 days a week for 10 weeks, in 16 men (whose total fat loss was the same as with running alone), and an 8-week circuit in 14 adults.
Fat comes off the whole body. Strength training for the whole body took about 1.5 percentage points off body fat across 58 studies, and each extra 30 minutes a week of aerobic exercise took about half a centimetre off the waist across 116 trials. Our weight loss plan pairs both with eating a little less.
Sources 11, 17, 18, 19, 20, 21, 22, 23
Why five kinds of moveEmerging evidence
Your trunk curls, resists arching, resists bending sideways, resists twisting and carries loads, and no single move covers them all: in a lab study of 12 abdominal exercises in 9 adults, none trained every abdominal muscle while keeping the load on the spine low, and the authors advised a variety. Front and side planks and bird dogs worked most trunk muscles at under half their maximum, which suits endurance and control, and side planks worked the muscles at the side of the trunk and hip harder (30 adults). Endurance differs by direction too: in 75 young adults, side planks were held for much less time than a back-extension test, especially by the women.
For the harder steps: reverse crunches, rollouts and hanging knee raises drew the most six-pack activity in two studies of 21 and 18 adults, and a long-lever plank with the pelvis tucked worked the stomach muscles much harder than a standard plank in 19 adults. In a plank test, about a third of 471 students stopped because their legs tired and a third because their arms did, which is why the planks get harder rather than longer. Carries load the trunk differently from lifts, in a lab study of strongman events.
Upper and lower abs work together as one muscle in curls and leg raises (11 adults), and muscle activity in a single session isn't a validated guide to growth. So these studies support variety and steps, not one best ab exercise.
Sources 24, 25, 26, 27, 28, 29, 1, 30, 31, 32
Back painStrong evidence
Core exercise helps long-term low back pain about as much as other exercise, not more (29 trials), and exercise in general cut pain by about 15 points out of 100 against no treatment, usual care or placebo (249 trials). Exercise with education reduced new episodes of low back pain by about 45% (21 trials, 30,850 people).
Replacing sit-ups with core stability exercises didn't change injury rates in 1,141 soldiers, though those who hurt their low back lost fewer days. Stay active through a flare, and see our back pain guide.
Sources 33, 34, 35, 36
After a birth: the gap down the middle and the pelvic floorStrong evidence
After a birth, the NHS advises avoiding sit-ups, planks and high-impact exercise to start with, beginning with gentle tummy and pelvic floor exercises, and seeing a GP if the gap down the middle of your tummy is still obvious 8 weeks after the birth.
The gap is common: 60% of 300 first-time mothers had one 6 weeks after the birth and a third a year later, and it wasn't linked with back or pelvic pain at a year. Exercise narrows it only a little, by about 0.4 cm in 6 trials of 161 women. Curl-ups aren't banned later: in 70 women 6 to 12 months after a birth, 12 weeks of 10-minute curl-up sessions on 5 days a week made the six-pack muscle thicker and stronger without widening the gap.
If you leak, pelvic floor training comes first: women with stress incontinence who did it were about 8 times more likely to report a cure (56% against 6%). Start with our postpartum, diastasis recti and pelvic floor guides.
Sources 37, 38, 39, 40, 41, 42
Change it if this applies to youStrong evidence
Our suggested changes, from the guidance below:
- Osteoporosis or a past spinal fracture: the UK consensus advises avoiding a lot of forward bending, especially repeated or loaded, so leave out crunches, reverse crunches and hollow holds, and keep the McGill curl-up small. Dead bugs, planks, side planks, bird dogs, the band press and carries stay in.
- A hernia: see a GP if you think you have one, and stop any move that makes it hurt or bulge. Get help from NHS 111 for pain in or around a hernia, a bloated tummy, being sick or not being able to poo.
- A prolapse: the NHS advises avoiding activities that strain the pelvic floor, and our prolapse guide leaves out crunches.
- Pregnancy: start anything new gently, and later on keep time flat on your back to a few minutes. See our pregnancy guide.
- Heart failure: ask your heart team first. The British Heart Foundation advises avoiding holding your breath and exercise that uses your whole body as a weight, such as planks.
- High blood pressure: keep holds short and keep breathing. Across 270 trials, exercise lowered resting blood pressure over time, but holding your breath raises it during a hold.
Sources 43, 44, 45, 46, 47, 48, 49
Check first if this applies to youStrong evidence
Before day 1, answer the PAR-Q+ (the Physical Activity Readiness Questionnaire for Everyone): a yes to any of its 7 questions leads to follow-up questions that may send you to your doctor or a qualified exercise professional first. The World Health Organization notes that people without such problems generally don't need a medical check before light or moderate activity.
With ME/CFS (myalgic encephalomyelitis or chronic fatigue syndrome), NICE (the National Institute for Health and Care Excellence) advises against general exercise programmes and fixed increases, which is what a challenge is.
Sources 50, 51, 52
Warning signsStrong evidence
Call 999 (911 in the US) for chest pain that feels tight or squeezing or spreads to your arms, neck, jaw, stomach or back, or comes with sweating, feeling sick, light-headedness or breathlessness, and if you faint while exercising. In the US, call 911 for heart attack symptoms even if you're not sure.
Sources 53, 54, 55, 56
When exercise stops being good for youStrong evidence
Exercising too much is one of the signs of an eating disorder the NHS lists. Beat, the eating disorder charity, describes unhealthy exercise as rigid or punishing, often done in secret, done despite injury or illness, or used to make up for eating.
If that sounds familiar, talk to your GP, or call Beat's helpline (3pm to 8pm, Monday to Friday): England 0808 801 0677, Scotland 0808 801 0432, Wales 0808 801 0433 and Northern Ireland 0808 801 0434. Our eating disorders guide has more.
Sources 57, 58, 59
After the 30 daysEmerging evidence
A challenge is a start. Habits take longer than 30 days to form: in the four studies of a review that timed it, the typical time was 59 to 66 days, ranging from 4 to 335. In 389,481 fitness app users, training steadily in the first 28 days was the strongest sign of still training a year later, and after a push-up challenge in Australia, activity was a little higher 2 weeks later but no higher at 3 months.
So pick your next step before day 30: keep 2 or 3 core sessions a week at your new steps, move to the gym ab workout or the advanced core workout for harder moves, or fold core work into a full-body plan.
Sources 60, 61, 62