If you’ve ever trained for a military fitness test or been made to do the presidential fitness test in gym class, you know the sit-up. Somebody holds down your feet, and you sit up as many times as you can before the test is over. But then again, you may have also heard that sit-ups are bad for your back, and that all those gym teachers were wrong and we should be doing crunches or planks instead. So what’s the deal?
Where sit-ups come from
It’s kind of strange, if you think about it, that the sit-up should be a thing in the first place. In what world would we have to lie down and sit up repeatedly, until our bellies are sore? What are we even training for?
Sit-ups were incorporated into military and quasi-military fitness tests out of concern for service members’ backs. Research seemed to show that people with weak abdominal muscles were more likely to have back pain and injuries. (This idea has since been called into question, but that’s another story.) Specifically, the issue wasn’t with abdominal strength but with abdominal endurance: How long could your core muscles work without giving out?
The sit-up test was the answer to this question. If a recruit can contract their abdominal muscles repeatedly, they must have good abdominal endurance. Sitting up from a lying-on-the-ground position does, indeed, ask a lot of the abdominal muscles. Problem solved, right?
The problem with sit-ups
Sit-ups can hurt your back. This isn’t the same as saying that sit-ups are bad for your back—we’re getting to that—but people do often report that their back hurts after doing a lot of sit-ups.
How can this happen? Well, let’s have a quick anatomy lesson.
The main muscle that sit-ups are supposed to work is the rectus abdominis, the six-pack muscle. It runs lengthwise from your ribcage to your pelvis, and when it contracts, the front of your ribcage moves closer to the front of your pelvis. It also does part of the job of holding your torso steady in a variety of positions. (Muscles in your back and sides do their part, too.)
But we also have muscles called hip flexors, which serve to bring your thighs closer to your torso. Imagine curling up into the fetal position; your hip flexors are the muscles that bring your knees to your chest.
During a standard gym class sit-up, you use both. Your six-pack muscle brings your shoulders off the ground, and your hip flexors help to bring your torso closer to your knees.
So there are two problems here: One is that sit-ups use your abs and your hip flexors, instead of being just a test of the abs. That’s not really your problem; you can make your abs and hip flexors stronger at the same time. It just means the test isn’t testing what it’s supposed to. But the second problem is about you personally: You might get an achy back.
How sit-ups can hurt your back
So sit-ups work your abs, and they also work your hip flexors. One of our hip flexors runs along the front of the thigh (it’s also a quad muscle), but there is a lesser-known muscle group that can potentially cause some back pain during sit-ups.
This is the muscle group known as the iliopsoas. These muscles connect your pelvis to the front of your lower spine. Or to put it another way: In a sit-up, when your pelvis is relatively fixed in position, these muscles are pulling on your spine.
Now, normally this wouldn’t be a problem. When you use your hip flexors to bend at the hip in everyday life (or in other gym lifts), you also use your other core muscles to brace your torso and stabilize your spine.
But in a high-repetition, timed sit-up test, the goal is to get as many reps as possible in the given timeframe. No prizes are given for bracing well or for activating your abs more than your hip flexors; those aren’t even easy to measure. You are scored only on the number of legal reps you accomplish. So your abs get tired, but you keep going. As your rectus abdominis fatigues, your hip flexors take over more and more of the work. This can cause back pain and, arguably, injury.
Why sit-ups aren’t really the problem
So sit-ups are bad, right? Well, it’s more complicated than that.
There’s a study that I found cited in many places to back up the statistic that sit-ups cause 56% of the injuries associated with the Army Physical Fitness Test. What the study actually found was a bit more complex. Yes, the sit-up was more likely than the other components of the test (running and push-ups) to result in the soldier saying they had an injury. But nearly all of these were minor “injuries” that didn’t affect their duties and didn’t require medical attention. (Reading between the lines, it sounds like these were people who had a sore back after the test, but recovered quickly.)
One idea about sit-ups being dangerous holds that repeated flexion of the spine over the years is bad for you. But the authors of this study didn’t find any difference in injury rates between new recruits and people who had been doing the sit-up test for years.
Injury rates were greatest among soldiers who had previously been injured while training for the test, and among soldiers who trained the least and who got the lowest scores on the test. That strongly suggests that being weak makes you more likely to get hurt, not that sit-ups are wrecking soldiers’ backs. The authors also cited a previous study that found soldiers with the lowest scores on the fitness test were twice as likely as their higher-scoring peers to get a back injury while on duty.
Unsurprisingly, people train for the sit-up test by doing lots of sit-ups. (The injury study here found that people averaged about 300 sit-ups per week in training.) If sit-ups hurt your back when done for high reps past the point of fatigue, then the problem isn’t any given sit-up—it’s the test, and the thousands of reps under fatigue that you do while training for the test. So far, there is no evidence to suggest that you’ll injure yourself just doing a few sets of sit-ups as part of your normal exercise routine.
How to do sit-ups without hurting your back
These issues with sit-ups tests have been understood for many years. In the 1990’s, a number of alternatives were proposed. Some of them caught on.
First, if we look way back to World War II-era fitness tests, the sit-up of the time was often done with legs straight on the ground. (There’s an illustration from an army guide here.) The bent-leg sit-up was one of the early modifications meant to get the test to focus more on the abdominal muscles.
In the 1990’s, there was a wave of sit-up backlash, and if you were around back then, you may have recalled that abdominal training shifted from sit-ups to crunches. This version of the crunch involved lying on your back, with knees bent and hands behind your head. Instead of sitting up, you just had to contract your six-pack muscles, bringing your head and the tops of your shoulders off the floor. If done slowly and under control, this movement works your abs with minimal involvement from the hip flexors.
A similar movement is the McGill curl-up. You have one knee bent and one straight, but otherwise it’s similar. Slow, controlled, and paying attention to your lower spine to make sure it doesn’t arch too much. When the iliopsoas pulls on your spine, you’ll get more of a hollow under your lower spine; this is why you’re told to press your back onto your hands or onto a mat placed under your lower back.
Several of the military tests that use sit-ups changed how they have members do them, but they still look a lot like a sit-up. Somebody holds your feet down, your knees are bent, you start by lying on the ground, and each repetition is considered to be finished when your arms, crossed over your chest, touch your thighs. This is a 1990’s-era modification from earlier styles of sit-up, and some branches of the service call them “curl-ups” or “crunches.” But they look a lot more like sit-ups than they do standard crunches.
There’s a more recent trend to phase out even these movements, after a recognition (several decades too late, but hey) that they have the same problems as the older styles of sit-up tests. The Army now uses a plank test, as does the Navy, and the Marines are currently in the process of switching from crunches to planks. (For now, recruits can choose which to do.) The Air Force offers your choice of a plank, sit-ups, or a cross-leg reverse crunch, which looks a bit like bicycle crunches.
So how should I work my abs?
So far, this has been a story of physical fitness tests. Gym classes and military fitness tests share the problems of needing to judge hundreds of people, quickly, with little to no equipment and clear grading criteria. They began with high-repetition sit-up tests, modified the sit-ups, and eventually started swapping them out for plank tests, which meet the same grading criteria with hopefully fewer complaints of lower back soreness. (If your back starts sagging during a plank test, though, don’t be surprised if your back hurts afterward.)
None of this really matters when it comes to our own workouts. Sure, you can do crunches or curl-ups if you want. But you can also do sit-ups if you want. Sit-ups aren’t a problem if you:
Keep your core braced during the movement
Stop when your abs feel fatigued
Stop if your back hurts
It’s also possible to hurt your back doing other exercises, so these are good rules to follow when you’re doing crunches, planks, or anything else—including yard work and other movements you might do in everyday life. Not only do these form tips bring sit-ups back into your repertoire, they also allow you to do sit-up variations like Swiss ball sit-ups, GHD sit-ups, V-ups, boat pose, and more.
As you try to make your core stronger (or to aim for that six-pack for vanity’s sake—it’s OK, I hear ya), don’t forget that there are more exercises you can do than just lying on the ground tensing your abs. Heavy carries and holds, like farmer’s carries, are arguably one of the most functional ways to work your core. Exercises like squats, deadlifts, and rows will work your core as well. And don’t forget our comprehensive guide to putting together a routine that will work your entire core, with your choice of a wide variety of exercises.