Sit-ups and crunches: the difference, how to do each, and what the studies measured

A crunch is a short curl: you lift your head, shoulders and upper back off the floor while your lower back stays down. A sit-up carries on until your trunk is upright. Both start the same way, on your back with your knees bent, and both work your abs by curling your trunk forward.

This page covers the difference, the steps for each, what the studies measured, what holding your feet down changes, the harder versions, and the sets. The crunch is the curl-forward move in the core workout; the loaded gym version is the cable crunch.

Sit-up or crunch: what's different

Those are working definitions. The studies below also say "curl-up" and differ in how far the trunk comes up and what the feet do, so each result is described as the study ran it.

What the studies measured

Most of these studies put electrodes on the skin and measured muscle activity during an exercise. Activity during a set is a snapshot, not muscle growth or strength over weeks.

• The crunch against the bent-knee sit-up. In 14 people who also tested seven commercial ab machines, the crunch was among the exercises with the most activity in the upper and lower rectus abdominis, the long muscle down the front of the trunk, and in the obliques. The bent-knee sit-up was among those with the most oblique activity too, and with several machines it had the most activity in the rectus femoris, a hip flexor.¹
• Which sit-up. In 18 trained men, the traditional U.S. Army sit-up produced more rectus femoris activity and more hip bend than a modified sit-up that focused on bending the trunk. The modified sit-up produced more average activity in the rectus abdominis.²
• Feet held, or free. In 14 people holding sit-up and curl-up positions without moving, holding the feet down lowered activity at every abdominal site measured and raised it in the rectus femoris; bent knees against straight legs made no significant difference.³ In 23 British Army personnel doing two minutes of reps, the curl-up with arms crossed and feet held produced the highest activity in every abdominal muscle measured, and feet-restrained sit-ups and curl-ups both produced more rectus femoris activity than curl-ups with the feet free.⁴ The two disagree about the abs when the feet are held, and agree about the hip flexor.
• Six weeks of training. In 102 active, healthy college-aged adults, the group that trained with sit-ups for six weeks improved its maximum number of sit-ups in two minutes; the group that trained with curl-ups did not, and neither did a group that didn't train. The authors explain it as specificity of training: you get better at what you practice.⁵ That is a sit-up count, not a measure of anything else.

Read together: both exercises work the front abs; the sit-up adds more hip-flexor work, and holding your feet down adds more again. The activity studies are small and short, and none measured muscle growth. This page makes no claim about how your midsection will look.

How to do a crunch

1. Lie on your back with your knees bent and your feet flat, about hip-width apart.
2. Cross your arms over your chest, or rest your fingertips lightly by your temples. Don't pull on your head.
3. Breathe out and curl your head, shoulders and upper back off the floor, ribs toward hips, with your lower back staying down.
4. Pause at the top, chin slightly tucked, neck in line with your trunk.
5. Lower slowly to the floor. That's one rep.

How to do a sit-up

1. Start as for the crunch, arms crossed over your chest.
2. Curl up head first, then shoulders, then upper back, and keep going until your trunk is upright.
3. Lower under control, upper back first, then shoulders and head, until you're flat again. That's one rep.

Keep your feet free if you want less hip-flexor work. Holding them down, as the studies above show, raised activity in the rectus femoris, a hip flexor.

The common faults, and a fix for each

• Pulling on your neck. Cross your arms or keep your fingertips light, and keep your chin tucked slightly.
• Swinging up on momentum. If you throw your arms to get up, slow down and shorten the lift.
• Yanking with your feet. If you push into the floor to come up, shorten the lift.

Easier and harder versions

• Easier: a shorter lift, or the dead bug first.
• Harder: a slower lowering (three seconds), a pause at the top, or in a gym the cable crunch, which adds weight in steps.
• The next step up: in 21 healthy men and women aged 23 to 43, a hanging knee raise with arm straps and a reverse crunch on a bench inclined 30 degrees were among the exercises with the most muscle activity in the abs and the internal obliques.⁶ The core workout has the hanging version.

Sets, reps and where they go

• Two or three sets of 10 to 15 slow reps, stopping with a rep or two left, the range the core workout gives for crunches. (How to judge reps left.) These numbers are practical, not from a study.
• At the end of a session, after your main lifts, two or three days a week on days you already train, as in the core slot of the full-body workout plan.
• One curl-forward exercise a session is enough: the crunch, a sit-up or the cable crunch, beside a plank for the arch and a carry for the lean.
• Progress one thing at a time. (Progressive overload covers the options.)

Common questions

Sit-ups or crunches: which should I do?

It depends on the job. The crunch is the curl-forward move the core workout uses, and in the 14-person study it was not among the exercises with the most rectus femoris activity. If you are practicing for a sit-up test, the six-week trial found that sit-up training, not curl-up training, improved the sit-up count.⁵

What if my neck or back hurts?

If you have pain, an injury or a medical condition, a doctor or a physical therapist should set the limits before you train; this page makes no claim about what either exercise does to the spine.

If you'd rather have the week planned

Where a crunch goes in a week that already has squats, presses and rows is the part a page can't do. Coaching is the version where your workouts live in Trainerize, built around the days you actually have, and a weekly call looks at the week you just had and sets the next one. Here's how coaching works, and apply is the first step. Otherwise: at the end of your next session, do two sets of ten slow crunches.