Crunches vs. Sit-Ups: What's *Really* The Difference?

If you had to name the most well-known, classic abdominal exercise, you’d probably say the sit-up or the crunch. These old-school moves occupy a similar place in fitness lore (think: painful memories of gym class) and often get used synonymously. 

While crunches and sit-ups might look similar, start in the same position, and both work your core, they’re two distinct exercises. “A crunch is done by contracting your abdominal muscles to cause your torso and spine to concave (think: trying to make your shoulders get closer to your hips),” explains Peloton instructor Logan Aldridge. “A sit-up is done by taking your torso (upper body) from lying on the ground on your back to a vertical position (sitting upright).” 

Is one any better than the other? And how do you choose which to add to your ab routine? We’re breaking down all the differences between crunches and sit-ups ahead.

Crunches vs. Sit-Ups: What Are the Main Differences?

There are many similarities between crunches and sit-ups, but perhaps the most important one is that they’re both effective at targeting your core, Logan says. They mainly target the rectus abdominis, the top layer of ab muscles in your core and the one you might know as the “six pack.” Both moves tend to work the rectus abdominis with a bias towards the upper half, adds Tanner Neuberger, a physical therapist at Athletico Physical Therapy. 

As for the differences between crunches and sit-ups, here are the main ones to note.

Range of Motion

As Logan noted, the most significant difference between crunches and sit-ups is how far you lift your torso. With a crunch, you’re moving through a small range of motion, curling just your head and shoulders off the floor. With a sit-up, that range of motion is much larger as you lift your entire torso off the ground until it’s perpendicular to the floor and you’re sitting up (hence, the name).

Muscle Engagement

Both crunches and sit-ups work your rectus abdominis, and research shows that the moves activate the muscle to a similar extent. However, the crunch is better at isolating the rectus abdominis, while the sit-up involves a few more muscles. For example, because you’re hinging at the hips to lift your entire torso, sit-ups also recruit your hip flexor muscles, including the rectus femoris (one of your quad muscles), more than crunches do, Neuberger says. Research shows that sit-ups involve the oblique and back muscles more than crunches, though to a smaller degree than the hip flexor.

Endurance vs. Strength

Crunches have a short range of motion, allowing for faster reps and a higher volume of muscular contractions, Logan says. This makes crunches a better option for improving muscular endurance, which can, in turn, help you create more core stability. Sit-ups, on the other hand, are more challenging since you have to lift your entire torso off the ground. They may offer a better opportunity to build maximal strength, especially when they’re weighted, Logan says. This also means crunches may be easier for exercisers just starting.

The Pros and Cons of Crunches

Pros: strengthens rectus abdominis, builds muscular endurance in the abs, more accessible for beginners or those with limited abilities

Cons: weak activation of other core muscles, relatively low intensity

The main perk of crunches (also called curl-ups) is that they’re spectacular at targeting your rectus abdominis. They also recruit your transverse abdominis (your deepest core muscle) and obliques to a lesser degree. For example, one small study published in the Journal of Strength and Conditioning Research found that, during crunches, the transverse abdominis peaked at only 40 percent of its maximum contraction level, meaning it’s not the best for strengthening this muscle (moves like a dead bug or plank variation are much better). Still, crunches can undoubtedly contribute to overall core strength, which can help you move more efficiently, maintain better posture, and reduce your injury risk.

As Logan mentioned above, since the range of motion is small and you can crank out quicker reps, crunches can help build muscular endurance. One study on untrained teens found that doing crunches just once a week was enough to significantly increase their ab muscle endurance. Because crunches require a smaller range of motion than sit-ups, they also tend to be more manageable for beginners or those with limited abilities.

Over the years, some experts and researchers have voiced concern over the fact that both crunches and sit-ups involve lumbar flexion (i.e., curving the lower back) and load the spine in a way that potentially increases the risk of a back injury or lower back pain. Others maintain that, for people without spinal injuries or issues, the benefit of increased core strength from regular crunching supersedes any risk. 

Neuberger, for one, doesn’t think you should worry about getting injured doing crunches. “[There's] lots of fear-mongering around sit-ups and crunches and lumbar flexion, in general, but it’s a consistent movement pattern we use on a day-to-day basis,” he says. Prepping the body for that motion—rather than avoiding it altogether—may better equip it for those movements. Neuberger even lists exposure to lumbar flexion and shear forces in the lumbar spine as a benefit of both crunches and sit-ups, since it increases your body’s resilience in this movement pattern.

“It’s a good idea to include it in a training program at low frequency and intensity to start, and once you adapt and improve, you can start to load more and more frequently,” Neuberger says. So it could make sense to start with crunches and progress to sit-ups as you get stronger. As long as you’re doing the moves properly and don’t have any pain or spinal injuries, you should be benefiting your back, not putting it at risk. “Bigger, stronger abs are a good way to improve overall spine health,” Neuberger says.

How to Do a Crunch Properly

Speaking of doing them right—here’s how to do crunches with proper form. Neuberger recommends bracing your core by bringing your belly button towards your spine to get the transverse abdominis involved, and going at a slow and steady pace.

The Pros and Cons of Sit-Ups

Pros: strengthens rectus abdominis, strengthens hip flexors, higher-intensity

Cons: hip flexors can take over, may pose additional injury risk 

Sit-ups are also a solid exercise for the rectus abdominis and pose a significant strength challenge, making this move great for higher-intensity ab workouts. They also seem to activate more muscle groups than crunches, including the obliques, back muscles, and hip flexors.

The hip flexor activation in sit-ups is a double-edged sword, though. Let’s start with the good: Many of us who sit all day develop hip flexors that are both tight and weak; for healthy hips, you need to mobilize and strengthen. By working your hip flexors, sit-ups may help with the latter. The downside? If you’re doing sit-ups with the intention of targeting your abs, it’s possible that your hip flexors take over and become the primary muscle used to complete the movement, cheating you out of the ab burn.

If your hip flexors get too involved during sit-ups, it may also increase your risk of an injury by encouraging hyperextension of the lower back, which may result in unsafe compression of the lumbar spine. 

Focusing on correct form can prevent this and ensure you work your abs. One small study published in the Journal of Strength and Conditioning Research tasked 18 young men with doing sit-ups and found that when participants were coached to use their abs (not hip flexors) and to control the movement by raising and lowering their torso one vertebra at a time, the exercise activated their rectus abdominis and obliques more than their hip flexors. The other group, who did traditional sit ups with no such coaching, experienced the opposite; their hip flexors did more of the work.

But there’s a catch. Doing sit-ups with proper form introduces spinal flexion (aka rounding), which is behind the controversy over crunches, as explained above.

Does all this mean you should skip sit-ups completely? Not necessarily. As Neuberger mentioned above, lumbar flexion happens all the time in our day-to-day movements, and training for that during your workouts can help increase your body’s resilience in that movement pattern. Ultimately, researchers believe the rewards override the risks as long as you don’t have any existing spinal injuries or associated contraindications, such as a herniated disc. But if you are unsure if sit-ups or crunches are right for you, always speak to your doctor, physical therapist, or other medical professional before doing these exercises.  

How to Do a Sit-Up Properly

To do sit-ups correctly, a slow, methodical movement is best, Neuberger says. Here’s how to do a sit-up, step by step.

Crunches vs. Sit-Ups: What's the Best Exercise?

There’s no one-size-fits-all answer here. “Neither is better or worse. They both have a purpose and benefit in any individual's training plan,” Logan says. Overall, he prefers sit-ups because of their functional nature. “They have a more relevant and diverse application to total strength and function,” Logan says. For example, the strength built in sit-ups transfers to real-work tasks and activities like getting up from bed or sitting up from laying down. 

That said, sit-ups can be difficult for many people. If you’re just starting to exercise, can’t make it to the top of a sit-up, feel like your hip flexors are taking over, or have any pain, a crunch could be a better ab move for you. Likewise, if you want to dial in on your rectus abdominis, crunches could help you better isolate that muscle.

Takeaway

Crunches and sit-ups are iconic ab moves, and both can help you build stronger abs. But neither one is perfect, nor does either one strengthen your whole core. Feel free to incorporate crunches and sit-ups in your workout routine; just make sure you’re laser-focused on your form and include a variety of other core moves—like planks, hollow holds, bird-dogs, glute bridges, and more—in your rotation, too.