Buzz
If you’ve prepared for a military fitness exam or endured the presidential fitness test in gym class, you’re familiar with sit-ups. Someone secures your feet, and you perform as many sit-ups as possible within the time limit. However, you might have also heard that sit-ups can strain your back, and that gym instructors were mistaken—crunches or planks might be better alternatives. So, what’s the truth?
The origin of sit-ups
When you think about it, it’s odd that sit-ups became a standard exercise. Why would we need to repeatedly lie down and sit up until our abdominal muscles ache? What purpose does this movement serve in real life?
Sit-ups were included in military and similar fitness tests due to concerns about the back health of service members. Studies suggested that individuals with weaker abdominal muscles were more prone to back pain and injuries. (This notion has since been challenged, but that’s a separate discussion.) The focus wasn’t just on abdominal strength but on endurance: How long could your core muscles sustain activity without fatigue?
The sit-up test was designed to address this concern. If a recruit could repeatedly engage their abdominal muscles, it indicated strong abdominal endurance. Rising from a lying position indeed places significant demand on the abdominal muscles. Problem solved, or so it seemed.
The downside of sit-ups
Sit-ups can cause back discomfort. This doesn’t necessarily mean sit-ups are harmful to your back—more on that later—but many people experience back pain after performing numerous sit-ups.
Why does this happen? Let’s take a quick look at the anatomy involved.
The primary muscle targeted by sit-ups is the rectus abdominis, commonly known as the six-pack muscle. This muscle stretches vertically from your ribcage to your pelvis, and when it contracts, it pulls the front of your ribcage toward your pelvis. It also plays a role in stabilizing your torso in various positions. (Other muscles in your back and sides contribute as well.)
Additionally, we have hip flexor muscles, which are responsible for bringing your thighs closer to your torso. Picture curling into a fetal position; your hip flexors are the muscles that draw your knees toward your chest.
During a typical gym-class sit-up, both muscle groups are engaged. Your rectus abdominis lifts your shoulders off the ground, while your hip flexors assist in bringing your torso closer to your knees.
This presents two issues: First, sit-ups involve both your abs and hip flexors, rather than isolating the abs. While this isn’t necessarily a problem for you—since both muscle groups can be strengthened simultaneously—it means the test doesn’t accurately measure what it’s intended to. Second, and more relevant to you, is the potential for back discomfort.
Why sit-ups may cause back pain
Sit-ups engage both your abdominal muscles and your hip flexors. One of the hip flexors is located along the front of the thigh (it also functions as a quad muscle), but there’s a lesser-known muscle group that can contribute to back pain during sit-ups.
This muscle group is called the iliopsoas. These muscles link your pelvis to the front of your lower spine. In simpler terms, during a sit-up, when your pelvis remains relatively stationary, these muscles exert force on your spine.
Under normal circumstances, this wouldn’t be an issue. When you use your hip flexors for activities like bending at the hip in daily life or during other gym exercises, your core muscles help stabilize your torso and spine.
However, during a timed, high-repetition sit-up test, the focus is on completing as many reps as possible within the allotted time. Proper bracing or engaging your abs more than your hip flexors isn’t rewarded or easily measurable. Your score depends solely on the number of valid reps performed. As your rectus abdominis tires, your hip flexors increasingly take on the workload, potentially leading to back pain or even injury.
Why sit-ups aren’t inherently the issue
So, are sit-ups harmful? The answer isn’t so straightforward.
A study frequently cited claims that sit-ups account for 56% of injuries linked to the Army Physical Fitness Test. However, the study’s findings are more nuanced. While sit-ups were more likely than running or push-ups to result in soldiers reporting injuries, nearly all were minor and didn’t interfere with their duties or require medical care. (Reading between the lines, these were likely cases of temporary back soreness that resolved quickly.)
One argument against sit-ups is that repeated spinal flexion over time can be harmful. However, the study found no difference in injury rates between new recruits and those who had performed sit-up tests for years.
Injury rates were highest among soldiers with prior training injuries and those who trained the least and scored poorly on the test. This strongly implies that weakness, not sit-ups, increases injury risk. The authors also referenced an earlier study showing that soldiers with the lowest fitness test scores were twice as likely to sustain back injuries during duty compared to higher-scoring peers.
Unsurprisingly, people prepare for the sit-up test by performing hundreds of sit-ups weekly. (The study noted an average of 300 sit-ups per week during training.) If high-repetition sit-ups under fatigue cause back pain, the issue lies not in the exercise itself but in the test and the excessive training volume. There’s no evidence suggesting that a few sets of sit-ups in a regular workout routine pose a risk of injury.
How to perform sit-ups without risking back pain
The problems with sit-up tests have been recognized for decades. In the 1990s, several alternatives were proposed, some of which gained popularity.
Looking back to World War II-era fitness tests, sit-ups were often performed with legs straight on the ground. (An illustration from an army guide can be found here.) The bent-leg sit-up was an early modification designed to better target the abdominal muscles.
In the 1990s, there was significant pushback against sit-ups, and if you were around then, you might remember the shift from sit-ups to crunches. This variation involved lying on your back with knees bent and hands behind your head. Instead of sitting up fully, you only needed to engage your abs, lifting your head and shoulders slightly off the ground. When performed slowly and with control, this movement isolates the abs while minimizing hip flexor involvement.
A comparable exercise is the McGill curl-up. Here, one knee is bent while the other remains straight, but the concept is similar. The movement is slow and controlled, with attention paid to preventing excessive arching of the lower spine. When the iliopsoas pulls on the spine, a noticeable hollow forms under the lower back, which is why you’re advised to press your back against your hands or a mat placed beneath it.
Many military tests that include sit-ups have modified how they are performed, though they still resemble traditional sit-ups. Someone holds your feet, your knees are bent, you start lying on the ground, and each rep is completed when your arms, crossed over your chest, touch your thighs. This is a 1990s update from earlier sit-up styles, and some military branches refer to them as “curl-ups” or “crunches.” However, they are closer to sit-ups than standard crunches.
A more recent trend is to phase out these movements entirely, acknowledging (albeit belatedly) that they share the same issues as older sit-up tests. The Army and Navy now use a plank test, and the Marines are transitioning from crunches to planks. (Currently, recruits can choose between the two.) The Air Force allows a choice between planks, sit-ups, or a cross-leg reverse crunch, which resembles bicycle crunches.
So, how should I train my abs?
This discussion has focused on physical fitness tests. Gym classes and military tests face the challenge of evaluating hundreds of people quickly, with minimal equipment and clear scoring standards. They started with high-repetition sit-up tests, adjusted the sit-up form, and eventually replaced them with plank tests, which aim to meet the same criteria while reducing complaints of lower back pain. (Though, if your back sags during a plank test, don’t be surprised if it hurts afterward.)
None of this is particularly relevant to our personal workouts. You can do crunches or curl-ups if you prefer. But sit-ups are also fine if done correctly. Sit-ups aren’t an issue if you:
Maintain a braced core throughout the exercise
Cease the movement once your abdominal muscles feel tired
Stop immediately if you experience back pain
Back injuries can also occur during other exercises, so these guidelines are useful for crunches, planks, or any activity—even yard work or daily movements. These form tips not only make sit-ups a viable option again but also enable you to explore variations like Swiss ball sit-ups, GHD sit-ups, V-ups, boat pose, and more.
While strengthening your core (or chasing that six-pack for aesthetic reasons—no judgment here), remember there are more options than just lying on the floor contracting your abs. Functional exercises like farmer’s carries and heavy holds are excellent for core development. Movements such as squats, deadlifts, and rows also engage your core effectively. And don’t overlook our comprehensive guide for crafting a routine that targets your entire core with a variety of exercises.
