What Causes a Bulge in the Abdomen When Doing Crunches?

That ridge or dome-shaped bulge running down the center of your belly during crunches is almost always a sign of diastasis recti abdominis, a condition where the two halves of your rectus abdominis (the “six-pack” muscle) have separated along the strip of connective tissue between them. When you curl upward, the spike in pressure inside your abdomen pushes your organs and tissue outward through that weakened midline gap, producing a visible bulge that can range from a subtle ridge to a pronounced tent shape. The condition is far more common than most people realize, and the crunch itself is usually what makes it visible rather than what caused it in the first place.

How the Bulge Actually Forms

Your two rectus abdominis muscles run vertically from your ribcage to your pelvis, connected at the center by a band of fibrous tissue called the linea alba. In a healthy abdomen, this band is stiff enough to hold the muscles close together even under pressure. When you contract your abs during a crunch, intra-abdominal pressure rises. If the linea alba is intact and firm, the pressure is distributed evenly and the surface of your abdomen stays relatively flat or contracts inward.

When the linea alba has stretched or thinned, though, the story changes. The gap between the muscle halves widens, and the tissue in between can no longer resist the pressure. Research comparing women with and without diastasis recti found that those with the condition had lower linea alba stiffness and significantly more distortion of the midline tissue during tasks like a head lift or semi-curl-up. In women without the condition, the linea alba actually stiffened during the same movements, helping to stabilize the midline. In women with diastasis, stiffness did not increase during contraction, and distortion got worse.¹ That distortion is what you see as a bulge. The tissue between the muscles buckles outward rather than bracing flat.

Why the Linea Alba Weakens in the First Place

The most widely recognized cause is pregnancy. Hormonal shifts during pregnancy soften connective tissue throughout the body, and the expanding uterus places sustained mechanical stress on the abdominal wall for months. That combination of looser tissue and constant stretching allows the rectus muscles to drift apart.² Most pregnant women develop some degree of midline separation by late pregnancy, and for many it persists after delivery.

But pregnancy is not the only driver. Obesity raises intra-abdominal pressure chronically and contributes to abdominal fat accumulation, both of which can push the muscles apart over time. Improper exercise technique, particularly movements that repeatedly spike abdominal pressure without adequate core control, can worsen or even initiate the separation in people who were never pregnant.² Men are not exempt. While the condition is discussed far less outside of postpartum contexts, the same mechanical and lifestyle factors apply.

The Connective Tissue Angle

Some people seem to develop diastasis more easily than others, and part of that comes down to the raw material of the linea alba itself. A study examining tissue samples from women with and without diastasis recti found that both type I and type III collagen were significantly less abundant in women who had the condition.³ These are the two main structural proteins that give the linea alba its tensile strength. Lower levels mean the tissue is inherently less capable of resisting the forces that try to pull it apart.

The linea alba is also not uniform along its length. A meta-analysis of anatomical studies found that it is naturally widest at the level of the navel, and that the collagen fiber arrangement varies by region. Below the navel, the collagen tends to run more transversely, which may offer less resistance to longitudinal forces. Sex hormones also affect collagen metabolism, which helps explain why the condition is so closely tied to pregnancy and hormonal changes.⁴ In other words, you might be structurally predisposed to a weaker midline before you ever do a single crunch.

Diastasis Recti Versus a Hernia

A common worry when people see an abdominal bulge during exercise is whether they might have a hernia. The distinction matters because the two conditions are managed differently, even though they can sometimes coexist. In diastasis recti, the linea alba is stretched and thinned but still intact as a continuous sheet of tissue. Nothing has torn through it. The bulge you see is the fascial layer itself distending outward under pressure, with your abdominal contents pressing against it from behind.

A true ventral or umbilical hernia, by contrast, involves an actual defect or hole in the abdominal wall through which tissue (usually fat from the omentum, sometimes a loop of intestine) protrudes. Hernias can become incarcerated or strangulated, which is a medical emergency. Diastasis recti does not carry that same acute risk. However, the two can overlap: a person with diastasis may also have a small hernia within the weakened midline.⁵ If you feel a hard, painful lump that does not flatten when you relax, or if the bulge appears even at rest and worsens with coughing, that warrants a medical evaluation to rule out a hernia.

How to Tell If You Have It

A quick self-check gives a rough idea. Lie on your back with your knees bent. Place your fingers horizontally across your midline just above the navel. Slowly lift your head and shoulders off the floor as if starting a crunch. If you feel a gap between the two muscle ridges that you can sink your fingers into, and especially if you see the characteristic dome or ridge shape along that gap, you are likely dealing with some degree of diastasis. A gap wider than about two finger-widths is the traditional threshold most clinicians use.

For a more precise measurement, ultrasound imaging is the standard. Research has shown it to be highly reliable for measuring the inter-recti distance, the gap between the two muscle bellies. One study found excellent consistency in measurements taken at the level of the navel and above, with minimal detectable change values small enough to track meaningful improvement over time.⁶ Digital calipers are another option, though their agreement with ultrasound varies depending on where on the abdomen you measure. Above the navel, calipers tend to read slightly narrower than ultrasound.⁷ For tracking your own progress, picking one measurement method and sticking with it matters more than which one you choose.

Does the Bulge Mean Something Is Wrong?

Not necessarily. A visible midline bulge during crunches can look alarming, but diastasis recti does not automatically come with pain or functional problems. The relationship between the size of the gap and symptoms like back pain turns out to be murkier than you might expect. A systematic review pooling results from 13 studies found that most of them, about eight out of thirteen, reported no association between diastasis recti and low back pain.⁸ A smaller body of evidence has found a correlation, including one study that reported a moderately strong link between the severity of the separation and pain intensity in postpartum women.⁹ Similarly, an observational pilot study in women aged 30 to 45 who had given birth found no significant relationship between the degree of separation and lumbar pain sensitivity.¹⁰

The takeaway is that a big gap does not guarantee pain, and a small gap does not guarantee comfort. Some people with wide separations feel perfectly fine and function at a high level. Others with narrower separations experience bothersome symptoms because the quality of the tissue between the muscles matters as much as the raw width of the gap. What the research broadly suggests is that if you have the bulge but no pain, no pelvic floor issues, and no functional limitations, you are not necessarily dealing with a medical problem that needs fixing.

Pelvic Floor and the Bigger Picture

Because the deep core muscles, the pelvic floor, the diaphragm, and the abdominal wall all work together as a pressure-management system, there has been a lot of speculation about whether diastasis recti weakens the pelvic floor. The evidence is more nuanced than the popular narrative suggests. A prospective cohort study that tracked women from pregnancy through the first year postpartum found no significant difference in pelvic floor muscle strength, endurance, or urinary incontinence between women with and without diastasis at six weeks, six months, or twelve months after delivery.¹¹ Interestingly, earlier in pregnancy, women who already had diastasis actually showed stronger pelvic floor measurements than those without it, which may reflect compensatory muscle recruitment. The relationship between the abdominal wall and the pelvic floor is clearly interconnected, but the idea that a wider gap automatically means a weaker pelvic floor does not hold up cleanly in the data.

What to Do About the Bulge

If the bulge is bothering you cosmetically or functionally, exercise-based rehabilitation is the first line of treatment. The goal is not simply to close the gap (though that can happen) but to retrain the deep stabilizing muscles so that the linea alba can better resist pressure during movement. A scoping review of current clinical practice found that programs incorporating both deep and superficial core work, pelvic floor exercises, and breathing techniques showed promising results in reducing the inter-recti distance and improving related symptoms.¹² Neuromuscular-style training, which emphasizes motor control and deep core activation, appeared to offer greater functional benefits compared to traditional abdominal exercises alone.¹³

What about crunches themselves? This is where the conversation gets counterintuitive. Despite being the movement that makes the bulge visible, the crunch actually narrows the gap between the muscles while you are doing it. A study measuring the immediate effects of exercise during pregnancy and postpartum found that abdominal crunch movements consistently reduced the inter-recti distance, sometimes by as much as 20 mm, depending on the location and timing. By contrast, drawing-in exercises, where you pull your navel toward your spine, produced a small widening of the gap.¹⁴ Head lifts and twisted curl-ups similarly decreased the distance above and below the navel.¹⁵ The bulge, in other words, does not mean the exercise is making the separation worse. The muscles are pulling together; the tissue between them is just buckling under the pressure at the same time.

That said, closing the gap in the moment and improving long-term tissue integrity are different things. A randomized trial involving pregnant women found that the inter-recti distance increased in both exercising and non-exercising groups over the course of late pregnancy, and decreased for both groups by six weeks postpartum.¹⁶ The reassuring finding was that exercising did not make the separation worse. But whether crunches alone are enough to restore the tissue long term is a different question, and one that the research has not definitively settled.

Learning to Control the Deep Core

A major part of rehabilitation for diastasis recti involves relearning how to activate your transversus abdominis, the deepest layer of abdominal muscle that wraps around your torso like a corset. Many people lose the ability to recruit this muscle effectively after pregnancy, prolonged inactivity, or injury, and without it, every crunch or sit-up loads the outer muscles while the inner stabilizers stay dormant. That imbalance is what allows the midline to dome outward.

Ultrasound-guided biofeedback, where you watch your deep muscles contract on a screen in real time, has been explored as a tool for teaching this activation pattern. Research on the technique has produced mixed results. One study found that ultrasound feedback helped healthy subjects learn to correctly perform an abdominal hollowing exercise in far fewer attempts than verbal instruction alone.¹⁷ A separate pilot study in patients with low back pain, however, found no significant advantage of ultrasound-guided feedback over traditional hands-on cueing for improvements in pain and function, though all participants improved overall.¹⁸ The practical upshot is that the specific tool matters less than actually practicing the correct contraction pattern consistently. If you can find a physiotherapist who can teach you to engage your deep core, the method they use to give you feedback is secondary.

When Surgery Enters the Picture

Most cases of diastasis recti do not require surgery, but for people with large separations that have not responded to rehabilitation, or who have significant functional impairment or a coexisting hernia, surgical repair is an option. The standard approach involves plicating, or stitching together, the separated edges of the rectus muscles to narrow the linea alba back to its original width. This is often done during an abdominoplasty but can also be performed on its own. Techniques vary widely among surgeons in terms of suture material, whether mesh reinforcement is added, and the number of closure layers used.⁵

Long-term outcomes appear favorable for those who do proceed. One follow-up study, tracking patients an average of five years after combined plication and hernia repair during abdominoplasty, found that none reported recurrence of the diastasis or the hernia. The overall complication rate was roughly 12 percent, covering minor issues rather than major adverse events.¹⁹ Surgery is generally considered only after conservative rehabilitation has been given a fair trial, typically at least six to twelve months of consistent exercise-based work. For postpartum women, waiting at least a year after delivery before considering surgical options is standard practice, since natural recovery continues through that window.

Why the Bulge Looks Worse at the Navel

If your midline bulge is most prominent around the belly button, that is consistent with the anatomy. The linea alba is naturally at its widest at the level of the umbilicus, which makes that spot the most vulnerable to distension. The collagen architecture at and below the navel also tends to be less favorable for resisting separation, with a higher ratio of the more elastic type III collagen relative to the stiffer type I collagen, and a fiber orientation that is less effective at countering the longitudinal pull of a crunch.⁴ The umbilical ring itself creates a natural interruption in the tissue, adding another weak point. So even in people with relatively mild separation above or below, the area right around the navel is often where the bulge is most visible and where the tissue quality tends to be poorest.

This also explains why measurement location matters so much when clinicians assess diastasis recti. Checking only above the navel might miss a wider separation at the umbilical level, and vice versa. If you are tracking your own progress, checking all three zones, above, at, and below the navel, gives the most complete picture.