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Are Sit-Ups Bad for You? Sit-Ups vs Crunches vs Planks

Reading Time: 6 minutes 30 seconds


DATE: 2023-03-30

Most people have some experience with these two very common abdominal and core exercises. The sit-up is a classic, and considered by some to be old-fashioned, while the crunch is what you’ll see more people doing in the gym as an alternative. 

Is there still value in an old-school sit-up? Do crunches work the abs as well? Which will burn belly fat and give you a flat mid-section? In other words, which should you be using, if not both? The answer isn’t that simple, so we’ll break it all down for you here. 

Sit-Ups vs Crunches – What’s the Difference? 

Both sit-ups and crunches work the abdominal muscles by lifting the chest and back to contract the abs. That’s about where the similarities end. The set of muscles worked in each exercise include some significant differences. 

Form is different too, beyond the basic lifting movement. The primary difference is that during a sit-up, you lift your entire torso off the floor, including the lower back. During a crunch, you keep the lower back pressed into the floor and only lift the shoulders and chest.  

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Benefits of Sit-Ups

A sit-up is a classic strength training. But many have never done a single sit up again after elementary school gym class. A sit-up is a classic exercise for a reason, and there are several good reasons to you might want to consider including them in your core or ab day routine. 

Sit-Ups Build Overall Core Strength

One of the biggest benefits of doing sit-ups is that they target and strengthen much more than just the abdominals. If you want overall core strength and a single move that hits several muscle groups, a sit-up will do it. 

A sit-up done correctly recruits several abdominal muscles, including the rectus and transverse abdominals and external and internal obliques. Additionally, sit-ups hit the hip flexors and the lower back. 

Build Better Functional Movements

Functional exercises like sit-ups are so important because they support the kinds of movements your body makes on a daily basis. Every time you sit up from a vertical position, you’re using the muscles worked in a sit-up. The more functional exercises you incorporate into workouts, the easier you’ll move throughout the day with a lower risk of injuries. 

Improve Stability and Posture

Because sit-ups work the overall core area, they’re great for improving your body’s stability. Stability in other exercises and daily functional movements comes primarily from the core. 

Improved stability reduces injury risk but also gives you better posture. You need to engage all those core muscles to sit and stand straight and with good posture. 

Build Flexibility and Range of Motion

Sit-ups require significant range of motion in the hips and spine. Doing sit-ups regularly can improve flexibility in these areas, making it easier to do other exercises and avoid injuries. 

Greater flexibility in the hips and lower back help to counteract a sedentary lifestyle. If you sit at a desk most of the day, these areas of the body have few opportunities to move. They can get tight and painful, but sit-ups, when done correctly, offset this. 

How to Do Sit-Ups Correctly

Form is always important when exercising, but even more so when doing sit-ups. With poor form, a sit-up can do more harm than good. Sit-ups can cause lower back rounding, which can lead to pain and injury. Get form right and watch it every single time: 

  • Lie on a mat with knees bent at 90 degrees and feet flat on the floor. 

  • Put your fingers on your head behind your ears, elbows out to the sides. 

  • Exhale as you contract the abs and lift your torso up toward your knees. 

  • Inhale and slowly lower back down to the starting position. 

The movement should be slow and careful. Focus on not allowing the lower back to round. It’s easier to get this form right if you place your feet under something to hold them in place. 

Benefits of Crunches

The slight difference in movement between a crunch and a sit-up leads to a completely different set of benefits. 

Crunches Are Accessible

Proper form for a crunch is a lot easier and less technical than for a sit-up, so crunches are more accessible. A solid crunch requires a much smaller range of motion and is easier to do. It’s a great starting point for most people who are new to working out. 

A Crunch is a Good Foundation for Core Strength

Crunches do not target as many muscles or provide the same degree of flexibility and movement as sit-ups. However, they do provide a good foundation for adding core strength. A crunch is an easy, safe place to start and will build strength necessary for more complicated core exercises. 

If you or a client is looking for six-pack abs, this guide will help. 

Crunches are Great for Muscle Isolation

While sit-ups work a larger group of muscles, crunches provide a good opportunity for isolation. A crunch primarily engages the rectus abdominis, the line of muscles that lead to a classic six-pack look when well defined. If you want to isolate these muscles for strength and aesthetics, crunches are a good choice. 

How to Do Crunches with Good Form

Doing a crunch with good form is easier than a sit-up because you do not have to worry about the back rounding: 

  • Lie on a mat in the same starting position as a sit-up. 

  • Again, place your fingers behind your ears, elbows out. 

  • Contracting the abs, exhale and lift your head and shoulders off the mat. 

  • Inhale and lower back down.

The movement should only go so far as to lift your shoulders off the mat, no more. Lift with your abs and avoid pulling up with your arms on your head. 

So, Which is Better, Crunches or Sit-Ups? 

The ultimate answer is, of course, it depends. As a personal trainer, you can take this information and determine which exercise, if not both, is best for each of your clients. In addition to the benefits of each, there are a couple of other factors to consider: 

Sit-Ups vs Crunches – Injury Risk

One of the most important considerations in this debate is the risk of injury. Despite all the benefits of sit-ups, they carry a high risk of lower back injury. This is one of the reasons the U.S. Army began phasing out sit-ups (1). Crunches are better for newbies who might struggle with sit-up form but also for anyone with existing lower back issues. 

Burning Belly Fat 

People often turn to these kinds of exercises hoping they’ll lead to flat, toned abs. While crunches and sit-ups can certainly contribute to a flatter stomach, alone neither one is the best exercise for burning calories and fat. 

Any exercise that builds more muscle in the body will increase your body’s metabolism and ultimately help you burn more fat. However, you cannot spot burn fat. In other words, doing 100 crunches will not selectively burn fat from your belly. 

To burn belly fat and get a flatter stomach, you need several factors. Cardio and strength training together is a powerful combination for burning overall fat and toning the abs. Add a healthy diet with a small calorie deficit and you have the right recipe for results. 

What About Planks?

The plank improves not just abdominal but total core strength. A plank is an isometric exercise, which means the muscle contracts without any actual movement of the body or lengthening of the muscle.

An isometric plank is used to strengthen and improve endurance and stability in the abdominal muscles, while also including the obliques, gluteus muscles, and hamstrings. To some extent you even work the shoulders and arms.

There are some variations on the plank, but the most basic form is in a prone position with hands, forearms, and elbows on the floor, toes on the floor, and hips up and in a neutral position with the rest of the upper body. In other words, your body is straight like a plank.

Planks are typically held for a designated time period, such as 30 or 60 seconds at a time, or in shorter time periods with only a few seconds of rest in between, such as 20-second holds with 5-second rest periods.

Why Are Planks So Great?

The U.S. military made a switch to planks in their Army Combat Fitness Test. They likely discovered what a lot of trainers already knew: Planks beat sit-ups every time, in every way.

Planks have been proven both in the gym as well as in research (2) to be the most effective way to incorporate and activate the abdominal and trunk muscles, support proper posture, and encourage proper alignment of the spine.

Planks may seem boring and they might just look too easy to possibly be beneficial. But this exercise only looks deceptively simple. Staying still and as stiff as a board while on your elbows and toes is a lot more difficult than it looks. 

Get the Right Education

Ultimately, crunches and sit-ups each have their own benefits and drawbacks. Whether or not they have a place in your routine depends on your needs and goals. In general, it’s best to include a variety of core strengthening workouts to ensure you hit all muscles, avoid injury, and improve flexibility and range of motion. 

ISSA’s Certified Personal Trainer Program is a great place to start if you have a passion for fitness and working with people. Become a certified professional trainer on your time.  

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  1. US Army phasing out sit-ups due to spine injuries. Bonati Spine Institute. (2021, November 5). Retrieved March 10, 2023, from https://www.bonati.com/blog/us-army-phasing-sit-ups-due-spine-injuries/ 

  2. Gottschall, J. S., Mills, J., & Hastings, B. (2013). Integration core exercises elicit greater muscle activation than isolation exercises. Journal of Strength and Conditioning Research, 27(3), 590–596. https://doi.org/10.1519/JSC.0b013e31825c2cc7

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