Sit Ups: How to Do Them, Muscles Worked and Are They Bad for You?

The sit up has been declared dead roughly once every five years since the 1990s, and it keeps turning up anyway, because it is free, it needs no equipment, and it does something no plank does: it moves your spine through a full range under your own control. The criticisms are not wrong, exactly. They are just aimed at the version where someone does three hundred a day with their feet wedged under a sofa and their hands yanking on the back of their neck.

Done sensibly and in reasonable volume, sit ups earn their place. Here is how to do them, what they actually train, and when to pick something else.

How to do a sit up

No kit needed, though a mat saves your tailbone.

1. Lie down. Back flat, knees bent to roughly 90 degrees, feet flat on the floor about hip-width apart and not anchored under anything.
2. Set your hands. Cross them over your chest, or rest your fingertips lightly at your temples. Do not lace them behind your head and pull; that is how necks get strained.
3. Brace. Take a breath, gently tighten your abs, and tuck your chin slightly so your neck stays in line with your spine.
4. Curl up. Peel your spine off the floor one segment at a time, starting with your head and shoulders, then your upper back, then your lower back, until your torso is upright. Exhale as you rise.
5. Lower with control. Reverse the same sequence and take two to three seconds on the way down. The lowering phase is where most of the useful work happens and most people just drop.

The tempo test: if you can do twenty in twenty seconds, you are throwing yourself up with momentum rather than lifting yourself with your abs. Slow it down and the same set becomes far harder.

Muscles worked

• Rectus abdominis. The main mover, and the muscle people mean when they say abs. It flexes the spine, curling your ribcage towards your pelvis.
• Obliques. Internal and external obliques stabilise through the movement and take on much more work in any twisting variation.
• Hip flexors. The iliopsoas and rectus femoris pull your trunk up once your lower back leaves the floor. This is the defining feature of a sit up compared with a crunch, and it is not automatically a bad thing.
• Transverse abdominis. Works isometrically to keep your midsection tight and your lower back supported.
• Neck flexors. Hold your head in position. Overworked and sore if you tug on your head instead of using your abs.

There is a longstanding argument about whether different abdominal exercises can bias different parts of the rectus abdominis. Ultrasound work published in 2023 found that they can: a crunch produced the greatest thickness change in the upper segment, leg raises hit the lower segment hardest, and the sit up landed in between (Diagnostic Ultrasound Shows Preferential Activation of Rectus Abdominis Segments). Useful to know if you are building a session: pairing sit ups with hanging leg raises covers more ground than either alone.

Sit ups versus crunches

They are not competing for the same job.

A crunch stops when your shoulder blades clear the floor. Small range, abs isolated, hip flexors barely involved, low spinal load. Best for building the rectus abdominis without much else going on.

A sit up goes all the way to upright. Bigger range, hip flexors heavily involved, more overall muscle at work, and more compression through the lower spine at the bottom of each rep. Better for general strength through a movement you actually perform in daily life.

If your goal is purely visible abs, crunches and cable crunches do the job with less fuss. If you want a midsection that works, sit ups belong in the mix alongside anti-movement work like the plank and dead bug.

Are sit ups bad for your back?

The honest answer is: for some people, sometimes, at high volumes.

The mechanism critics point at is real. Repeated full spinal flexion under load compresses the intervertebral discs, and repeat it thousands of times and you are testing the limits of that tissue. Military and emergency service fitness tests demanding hundreds of reps have a documented association with lower back complaints, which is why several have replaced the sit up outright.

That is an argument against sit up abuse, not against sit ups. Two or three sets of fifteen to twenty-five reps, three times a week, is nowhere near the volume where these problems show up. If you already have back pain, a diagnosed disc issue, or you feel a pinch in your lower back during the movement, skip them and build your core with planks, side planks, dead bugs and bird dogs instead. Those train your midsection to resist movement, which is what it does most of the time anyway.

The hip flexors deserve a mention too. If you sit at a desk all day and then do high-volume sit ups every evening, you are shortening and hammering muscles that are already tight, and the usual complaint is a dull ache at the front of the hip. Balance it with hip flexor stretches and some glute work.

Sets, reps and progression

Sit ups are a strength exercise, so treat them like one.

• Building strength: 3 to 4 sets of 15 to 20 controlled reps, three times a week, with 60 seconds of rest.
• Endurance or circuits: 2 to 3 sets of 25 to 30, shorter rest, faster tempo but still no flopping.
• Adding load: once 25 clean reps is easy, hold a weight plate or dumbbell against your chest and drop back to sets of 10 to 12. This is far more productive than piling on reps.

Progression options, in rough order of difficulty: slow the lowering phase to four seconds, add a pause at the top, hold a plate, do them on a decline bench, then move to weighted decline sit ups. An ab roller is another good next step, though it demands a lot more shoulder and lat control.

Abdominal training counts towards the muscle-strengthening component of the NHS activity guidelines, which ask for work covering all major muscle groups on at least two days a week.

Variations worth adding

• Butterfly sit up. Soles of the feet together, knees splayed out. Reduces hip flexor involvement noticeably and forces the abs to work harder.
• Twisting sit up. Rotate at the top to bring one elbow towards the opposite knee. Brings the obliques in properly.
• Decline sit up. On a bench set to a decline, so gravity works against you through more of the range. A big jump in difficulty.
• Weighted sit up. Plate held on the chest or overhead. Overhead is harder and much stricter on form.
• Janda sit up. Actively pull your heels towards you against the floor as you rise, which inhibits the hip flexors and puts almost everything on the abs. Brutal, and a good humbling exercise if you think sit ups are easy.

Recommended reads

1. Crunches: how to do them properly
2. Plank: technique and progressions
3. Best sit up bench UK
4. Best ab roller UK
5. Lower ab exercises