Are Crunches Bad for You? The Risks and Alternatives

Crunches are not inherently dangerous, but the repeated spinal flexion they require does create measurable stress on the intervertebral discs and may not be worth the trade-off given how many alternatives exist. The concern isn’t that a single crunch will injure you. It’s that hundreds or thousands of them, performed over weeks and months, place the lumbar spine in a position that laboratory and clinical research consistently links to disc damage. Whether that matters for you depends on your training history, spinal health, how many you do, and what you’re actually trying to accomplish with your core training.

What Happens to Your Spine During a Crunch

Every time you curl your torso off the floor, your lumbar spine flexes forward. That flexion compresses the front of each intervertebral disc and stretches the back. Research using both human imaging and cadaveric specimens shows that this motion pushes the disc’s nucleus (the gel-like center) toward the rear of the spine, thinning the posterior wall of the disc in the process. Over time, this creates conditions that favor disc bulging and herniation.

A widely cited laboratory study using porcine spines found that disc herniation occurred even with relatively modest compressive loads, as long as flexion-extension cycles were highly repetitive. Increasing the compressive force made the injuries more frequent and more severe.

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A follow-up study examining the mechanics of acute disc herniation confirmed that flexion facilitates the flow of nuclear material outward, promotes radial tears in the disc wall, and makes the endplate-vertebra junction vulnerable to failure.

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It’s worth pausing here. These studies used thousands of repetitions under controlled lab conditions, which is obviously different from doing three sets of fifteen crunches a few times a week. But the mechanism is real: spinal flexion under load is the movement pattern most consistently associated with posterior disc damage. A 2024 randomized trial discussion summarized the concern plainly: repetitive lumbar flexion can lead to posterior displacement of the nucleus with thinning of the posterior annulus, and that pattern is associated with herniated disc disorders.

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What Crunches Actually Do Well

If crunches carry spinal risk, why have they dominated gym floors for decades? Because they’re genuinely effective at one specific thing: activating the rectus abdominis, the “six-pack” muscle that runs down the front of your abdomen. A comparative electromyography (EMG) study found that the crunch exercise was superior to the sit-up for activating both the upper and lower portions of the rectus abdominis.

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Another study looking at two minutes of continuous curl-ups found that doing them with arms crossed and feet restrained produced the highest integrated EMG across all abdominal muscles.

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For pure rectus abdominis hypertrophy, crunches work. A study comparing two different crunch protocols (four heavy sets versus one long lighter set) found that both produced similar increases in abdominal muscle thickness.

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So the exercise delivers on its narrow promise: it makes the front of your abs bigger and stronger.

The problem is that for most people, a thicker rectus abdominis is not actually what they need. The rectus abdominis is a “global” mover muscle. It generates torso flexion. But the deeper stabilizing muscles, particularly the transversus abdominis and multifidus, are the ones that protect the spine during everyday movement and athletic performance. These local muscles work together to stiffen the lumbopelvic region, acting as what researchers have described as “the natural brace.”

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Core Strength Versus Actual Performance

There’s a persistent assumption that stronger abs mean better athletic performance. The evidence for this is surprisingly thin. A systematic review and meta-analysis looking at the relationship between trunk muscle strength and physical fitness found only a small correlation between core strength and actual performance measures. Core stability training did produce large improvements in trunk strength itself, but those gains translated into only small-to-medium improvements in physical performance tasks compared to doing no training or regular training alone.

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This matters because it reframes the cost-benefit question. If hammering your abs with crunches produces large gains in ab strength but only marginal improvements in the things you actually care about (running faster, jumping higher, throwing harder, reducing injury), then the spinal cost of all that flexion buys you very little functional payoff. This doesn’t mean core training is useless. It means the type of core training matters more than the volume, and exercises that build stiffness and stability tend to transfer better to real-world tasks than exercises that build flexion strength.

Short-term isometric core training, for instance, has been shown to increase both passive and active core stiffness, which researchers believe may influence injury resilience and performance.

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A Concern for Postpartum Women

One risk that rarely gets mentioned in general fitness advice is the effect crunches have on the pelvic floor. A study in postpartum women found that while the crunch generated internal squeeze pressure similar to a Kegel exercise, ultrasound imaging told a different story: the levator hiatus (the gap in the pelvic floor muscles) was significantly wider during a crunch, and the pelvic floor muscles were longer than during a Kegel.

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In plain terms, a crunch pushes downward on the pelvic floor while also stretching it open. For someone who has recently given birth and whose pelvic floor is already weakened, this pattern could worsen symptoms like urinary incontinence or pelvic organ prolapse rather than help them. Postpartum women are often told to “get their core back” through crunches and sit-ups, when the evidence suggests those are precisely the wrong exercises for that population during early recovery.

When Crunches Become Genuinely Risky

For some people, the question isn’t “are crunches suboptimal?” but “should I avoid them entirely?” Evidence-based guidelines for physical activity in special populations are clear on at least one group: people with osteoporosis should avoid trunk flexion exercises. The recommendation is rated as strong evidence (Level 2, Grade A), putting it among the more confident clinical guidance in the musculoskeletal world.

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The reasoning is straightforward: osteoporotic vertebral bodies are already weakened, and the compressive-flexion load of a crunch can cause vertebral fractures. The same guidelines note that powerful twisting of the trunk should also be avoided. For people with acute low back pain, the picture is more nuanced. The evidence supports doing movements that don’t provoke pain, including both flexion and extension, as long as they’re guided by what the individual can tolerate.

Anyone with a known disc bulge or herniation sits in tricky territory, too. The same mechanism that makes crunches problematic for healthy discs over thousands of repetitions is amplified when the disc is already compromised. If the posterior annulus is already thinned or torn, repeated flexion is pushing nuclear material toward the weakest point in the structure.

How the Military Changed Its Thinking

Perhaps the most telling institutional shift happened in the U.S. Army. For decades, the sit-up was a cornerstone of military fitness testing. A cluster randomized trial involving over 1,100 soldiers compared the injury outcomes of soldiers doing traditional sit-up-based training versus those doing a core stabilization exercise program. The overall injury rates were similar between the two groups, but among soldiers who did experience a low back injury, those in the traditional sit-up group had roughly twice as many days of work restriction: an average of 8.3 days compared to 4.2 days for the core stabilization group.

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That finding matters. It suggests that while sit-ups don’t necessarily cause more injuries overall, they make the back injuries that do occur worse and harder to recover from. This was one of several data points behind the Army’s eventual decision to replace the sit-up with the leg tuck (and later the plank) in its Army Combat Fitness Test. When an institution with millions of participants and a strong incentive to avoid lost duty days moves away from an exercise, that carries weight.

Spine-Sparing Alternatives That Work

If you want a strong, functional core without the flexion cost, the research points toward exercises that challenge the trunk to resist movement rather than produce it. The concept is sometimes called “anti-movement” training: anti-extension (resisting your back from arching), anti-rotation (resisting twisting), and anti-lateral flexion (resisting side-bending).

The plank family of exercises is the most studied spine-sparing option. EMG data show that prone planks, especially variations performed on a Swiss ball or with hip extensions, activate the internal obliques significantly more than crunches or sit-ups do.

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Side planks and side crunches on a ball drove significantly higher external oblique activity than standard crunches. For the rectus abdominis specifically, crunches still had an edge over side-position exercises, but prone exercises (planks and their variations) were comparable. In other words, planks and their progressions can hit most of the same muscles while keeping the spine in a neutral position.

The Pallof press, a cable or band-based anti-rotation exercise, has gained popularity in strength and conditioning for training lumbopelvic stability without any spinal flexion at all.

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Loaded carries (walking with a heavy weight in one hand or both), farmer’s walks, and suitcase carries also train the core to stabilize under load in a way that transfers directly to real-life movement. These aren’t flashy, but they demand serious core engagement while the spine stays neutral.

Other options include dead bugs, bird dogs, and various rollout exercises. All of these challenge abdominal bracing without cycling through the flexion pattern that stresses the posterior disc.

Flexion Exercises for Back Pain Patients

Here’s where it gets counterintuitive. Despite the theoretical risk of spinal flexion, clinical trials comparing flexion-based exercise programs to extension or stabilization programs for people with chronic low back pain have found remarkably similar outcomes. A randomized trial of patients with degenerative spondylolisthesis found no significant differences in pain, disability, or flexibility between lumbar stabilization exercises and flexion exercises.

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A preprint review of the McGill approach to core stabilization, which emphasizes spine-sparing exercises, found that the controlled trials testing it showed mixed statistical significance and little clinical significance in treating low back pain.

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This doesn’t mean spine-sparing exercise is useless. It means that for pain reduction specifically, the type of exercise may matter less than simply doing some kind of exercise consistently. The spine-sparing argument is stronger for injury prevention and disc health over years than it is for treating existing pain in clinical settings.

Your Discs Are Not as Fragile as You Think

A narrative that sometimes accompanies anti-crunch advice is that intervertebral discs are passive, fragile structures that can only degenerate over time. The reality is more nuanced. Discs respond to mechanical loading, and some types of loading appear to make them healthier. A study of young athletes found that basketball and soccer players showed evidence of disc hypertrophy (larger, plumper discs) and, in soccer players, greater disc hydration compared to non-athletes.

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Running appears to have a similar effect. A study comparing long-distance runners, joggers, and sedentary individuals found that runners had significantly higher disc hydration and taller discs relative to their vertebral bodies. The effect was strongest in the disc nucleus, with long-distance runners showing roughly 15% higher hydration in the central region compared to non-athletic controls.

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The key distinction seems to be between cyclic loading in relatively neutral positions (running, walking, jumping) and repetitive end-range flexion under compression (crunches, sit-ups). Both are “loading” the spine, but they affect the disc very differently. The former appears to stimulate beneficial remodeling; the latter pushes the disc’s anatomy in a direction associated with damage. Researchers have proposed multiple models to describe how continuous mechanical loading affects disc metabolism, noting that the disc’s response depends on the type, magnitude, and duration of the forces applied.

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When the Equipment Doesn’t Fit Your Body

There’s one more practical wrinkle that gets overlooked. Abdominal crunch machines, which are standard in most commercial gyms, are designed around an average-sized male body. A three-dimensional musculoskeletal modeling study found that individuals with smaller body dimensions, including many women and younger users, couldn’t be properly accommodated on the machine. The resulting poor posture and misaligned pivot point meant that hip flexor contribution increased substantially, reducing the exercise’s ability to isolate the abdominals in the first place.

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So for a significant portion of gym-goers, the crunch machine isn’t just loading the spine in a problematic pattern; it’s also failing to do the one thing it’s supposed to do well. If your frame doesn’t match the machine’s design, you’re getting more hip flexor work and less abdominal activation, along with all the same disc stress. Floor crunches avoid this particular problem, but they introduce their own form issues: most people pull on their neck, don’t control the range of motion, and end up driving the movement from momentum rather than muscular contraction. Poor form compounds the flexion risk without adding training benefit.

Bodyweight and free-standing alternatives like planks, Pallof presses, and loaded carries don’t have this sizing problem. They scale naturally with your body because your body is the equipment. That’s one more practical reason they tend to be recommended over machine-based or floor-based crunch variations for general populations.