What muscles do sit-ups work?

The abs curl your ribs toward your hips, which is the first part of the rep. Your hip flexors lift the rest of your torso up around the hip joint, and your obliques assist. The abs are the target and the hip flexors are doing more here than in most abdominal exercises.

Are sit-ups bad for your back?

The concern rests on a modelled figure: a biomechanical model put lumbar compression at roughly 3,350 newtons per rep for a bent-knee sit-up, a little above the 3,300 newton occupational action limit for repetitive loading. That threshold is about high volume over time, not about a single set. If your back already dislikes bending forward under load, there are trunk exercises that do not require it.

Sit-ups or crunches, which is better?

For training the abs specifically, the crunch. Sit-ups move through a larger range and recruit the hip flexors significantly more, which is why the shorter curl-up became the standard in fitness testing. Neither is wrong, they just cover different amounts of the movement.

Should I hold my feet down when doing sit-ups?

Anchoring your feet lets your hip flexors pull against a fixed point, so you finish more reps and more of each rep comes from your hips. Free feet keep more of the work in the curl. Use the anchor if you need it, and know what it is doing.

How many sit-ups a day to lose belly fat?

No number does it. Six weeks of daily abdominal exercise on a matched diet improved endurance significantly and left abdominal fat unchanged. Fat over a muscle is not decided by training that muscle.

Why did the army stop doing sit-ups?

The US Army replaced the sit-up with a plank in its fitness test, citing concerns about repetitive lower back loading across large numbers of people tested at high volume. Several other services made similar changes. That is a testing policy decision, and it is not the same as a finding about one person doing a set.

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Bodyweight · beginner · waist

Sit ups

Also called Situp, Full sit-up, Bent-knee sit-up.

The short answer

A sit-up curls your torso off the floor until your elbows reach your knees, then lowers it back under control. The demonstration lowers only as far as the shoulder blades leaving the floor and starts the next rep from there, which keeps the tension on the whole set. It has two halves and only the first belongs to your abs: curling your ribs toward your hips is abdominal work, and lifting the rest of the way is your hip flexors swinging your torso up around the hip joint. That second half is why the sit-up moves further than a crunch and why it is not simply a bigger version of one.

How to do it

Step oneLie flat on your back with your knees bent and feet flat on the floor.
Step twoPlace your hands behind your head with your elbows pointing outward.
Step threeEngage your abs and curl your torso all the way up until your elbows approach your knees.
Step fourPause briefly at the top, then lower your torso under control, either all the way back to the floor or stopping just short of it to keep the tension on.

What usually goes wrong

Each starts with what you would notice.

You feel it across the front of your hips, not your stomach.

WhyYour feet are anchored under something, so your hip flexors have a fixed point to pull against and they take over the second half of every rep. Hip flexor activity in a sit-up is measurably higher than in a curl-up, and anchoring the feet is one of the things that raises it.

FixTake your feet out from under the bar or the couch and let them stay flat on the floor. If your feet lift when you sit up, the rep needed the anchor, and that is worth knowing.

Your neck aches afterward and your abs do not.

WhyYour hands are pulling your head toward your knees, so the rep starts at the neck instead of the ribs and your arms give the abs a head start they did not earn.

FixRest your fingertips at the sides of your head without gripping, and start each rep by curling the ribs down toward the hips before anything else moves. If the neck still leads, cross your arms over your chest instead.

You bounce off the floor and the reps get faster as the set goes on.

WhyThe bottom of the descent turns into a rebound, so the elastic recoil of dropping onto the mat launches the next rep instead of the abs starting it.

FixLower over two seconds and start the next rep from a dead stop, whether you set your shoulder blades down or hold them just clear of the floor. The set will end sooner and it will be a real one.

Your numbers keep climbing and nothing about your middle has changed.

WhyAdding reps to an exercise you can already do many of trains endurance. It was tested: six weeks of abdominal exercise, five days a week, on a matched diet, significantly improved abdominal endurance and did not change abdominal fat or any other body composition measure.

FixTreat the sit-up as one trunk exercise among several and stop measuring it by count. Holding a plate on your chest, slowing the lowering, or working on a decline all raise the demand without buying more repetitions.

Sit-up or crunch

A crunch stops where your shoulder blades leave the floor. A sit-up carries on until your torso is upright. The extra range is not extra abs.

Curling the ribs toward the pelvis is what the abdominal muscles do, and both exercises cover it. Everything after that is your torso rotating about the hip joint, which the hip flexors produce. That is the measured difference: sit-ups move through a larger range and activate the hip flexors significantly more, and the curl-up became the standard in fitness testing because it keeps that contribution small.

So the choice depends on what you are after. If the point is training the abs specifically, the crunch is the more targeted rep. If the point is a movement pattern that includes getting your body off the floor, the sit-up is the whole action, hip flexors included.

Should your feet be anchored

Hooking your feet under a bar, a bench or someone else lets you finish reps you could not otherwise finish. That is the entire mechanism, and it is also the objection.

With the feet fixed, the hip flexors have something to pull against and can drag the torso up around the hips. Free feet remove the anchor, so more of the rep has to come from curling. Foot fixation is one of the variables researchers change deliberately when they want to shift work between the abs and the hip flexors.

If you can only do them with your feet anchored, that is not a reason to stop. It is a reason to know that some of what you are training is the front of your hips, and to have something else in the week that trains the abs without them.

What the compression number actually means

The sit-up is the exercise most often called dangerous, and the reason is a real number. Biomechanical modeling put lumbar compression at roughly 3,350 newtons for a bent-knee sit-up and 3,500 for a straight-leg one, both a little above the 3,300 newton occupational action limit set by the US National Institute for Occupational Safety and Health, and repetitive loading above it is associated with higher injury rates in workers.

Read that carefully, because both the people who defend sit-ups and the people who condemn them tend to misread it. It is a threshold for repeated loading over a working life, not a per-rep breaking point. A set of ten sit-ups is not the thing that number was measured about. Hundreds a day, every day, for years, is closer to it.

It is also why several armed forces, including the US Army, dropped sit-ups from their fitness tests and replaced them with a plank. That is a decision made by those organizations about high-volume repeated testing across large populations, and it is worth knowing. It is not by itself proof about what a sit-up does to any individual.

What it does not do is reduce the fat over your abdominal muscles. That was measured against a control group on a matched diet, and six weeks of abdominal training changed the endurance and nothing else.

What it is for

The sit-up is a bodyweight core exercise targeting the abs, with the hip flexors and obliques assisting. It curls the torso off the floor and carries it up until the elbows reach the knees, and the demonstration stops short of the floor on the way back down rather than setting the shoulders on it.

Questions people ask

What muscles do sit-ups work?: The abs curl your ribs toward your hips, which is the first part of the rep. Your hip flexors lift the rest of your torso up around the hip joint, and your obliques assist. The abs are the target and the hip flexors are doing more here than in most abdominal exercises.
Are sit-ups bad for your back?: The concern rests on a modelled figure: a biomechanical model put lumbar compression at roughly 3,350 newtons per rep for a bent-knee sit-up, a little above the 3,300 newton occupational action limit for repetitive loading. That threshold is about high volume over time, not about a single set. If your back already dislikes bending forward under load, there are trunk exercises that do not require it.
Sit-ups or crunches, which is better?: For training the abs specifically, the crunch. Sit-ups move through a larger range and recruit the hip flexors significantly more, which is why the shorter curl-up became the standard in fitness testing. Neither is wrong, they just cover different amounts of the movement.
Should I hold my feet down when doing sit-ups?: Anchoring your feet lets your hip flexors pull against a fixed point, so you finish more reps and more of each rep comes from your hips. Free feet keep more of the work in the curl. Use the anchor if you need it, and know what it is doing.
How many sit-ups a day to lose belly fat?: No number does it. Six weeks of daily abdominal exercise on a matched diet improved endurance significantly and left abdominal fat unchanged. Fat over a muscle is not decided by training that muscle.
Why did the army stop doing sit-ups?: The US Army replaced the sit-up with a plank in its fitness test, citing concerns about repetitive lower back loading across large numbers of people tested at high volume. Several other services made similar changes. That is a testing policy decision, and it is not the same as a finding about one person doing a set.