Crunches are forboden, so sayeth the diastasis recti gods. Thou shalt not move out of neutral. Thou shalt not flex your spine whilst lying on the floor.
Why? Because pressure, pelvic floor, bigger gap, or something. The rationales vary. Indeed, I was once firmly anti-crunch. I talked about how postpartum women don’t know how to control intra-abdominal pressure, about how the crunch will blow apart the abdomen and pelvic floor, about how spinal flexion is oh so bad for function, etc. Basically, I repeated received wisdom.
I vowed to ban the crunch from my life. Like most vows based on exclusion, I broke it. I totally crunch now. And, gasp, I like it. It hasn’t worsened my pelvic floor or abdominal separation. If anything, it has helped.
Therefore, I shall defend the maligned crunch. It does not deserve the wrath brought forth by well-meaning postpartum apostles. (Just as the TvA does not deserve the unequivocal adoration heaped at its feet.)
Ah, but don’t jump to the ground and do 1,000 crunches. I shall defend the crunch, but I shan’t give it carte blanche.
The Case Against Crunches
Before defending the crunch, let me explain the case against it, especially in regard to diastasis recti. I jumped on the “crunches are bad” bandwagon after reading some blogs and then devouring Stuart McGill’s textbook on low back pain (He is a well-respected Canadian professor of spine biomechanics). It’s a good book, and I learned a lot, but it also made me afraid of spinal flexion.
His argument against spinal flexion goes like this: During a sit-up (and crunch), the contents of the spine are compressed, which can squeeze the intervertebral nucleus and cause back pain, theoretically. According to his research on pig spines, a sit-up can generate 3,350 newtons of compressive force, which is over the 3,300 newton threshold established by the US National Institute for Occupational Safety and Health.
McGill believes the spine has a limited number of flexion cycles before herniation, and he doesn’t think wasting those flexion cycles on crunches makes much sense. As a result of his research, McGill advocates alternative core exercises, such as the plank, the bird dog, or the “McGill curl-up,” which is like a mini-crunch that keeps the spine in neutral and one leg extended.
The Crunch and Diastasis Recti
As if bulging discs aren’t scary enough, many postpartum fitness professionals also specifically warn women with diastasis recti and pelvic floor disorder against the crunch. In 2013 one fitness blogger even wrote that “the absolute WORST THING that you can do is ‘traditional ab work’ like crunches and planks. Those types of movements actually pull the muscles apart and only make [diastasis recti] WORSE.” Dear god, crunches and planks pull the muscles apart! This blogger then explained how to activate the transverse abdominis, which she said helps close the gap.
Other professionals, not unreasonably, warn about the affect of crunches on a weakened or broken pelvic floor. They say that too much outer abdominal work will cause intra-abdominal pressure overload, which the pelvic floor cannot stiffen against, thereby putting excess stress on support structures. Instead of crunches, women are encouraged to do single-leg extensions, wall push-ups, etc.
I took all of this to heart. I stopped all crunches. I even avoided planks. And, I diligently contracted my TvA instead. I was smug about it, like I knew something other women in fitness classes did not. But, as often happens when one learns a little, I knew enough to think I knew enough, but not enough to know I didn’t know enough.
I’m by no means an expert now, but I do know that the case against crunches isn’t so cut and dry. Therefore, let’s look at the other side—the case for crunches, or, at the very least, a defense of the crunch.
In Defense of the Crunch
The best defense of the crunch I’ve read is an essay by Bret Contreras and Brad Schoenfeld called “To Crunch or Not to Crunch: An Evidence-Based Examination of Spinal Flexion Exercises, Their Potential Risks, and Their Applicability to Program Design.” They make some amazing points that are simultaneously obvious and overlooked.
For example, they address McGill’s research with pig spines, pointing out that disc degeneration seen in the lab might not easily correlate to the actual backs of living and breathing people.
Contreras and Schoenfeld write,
It should also be noted that after an exercise bout, spinal tissues are allowed to recuperate until the next training session, thereby alleviating disc stress and affording the structures time to remodel. Exercise-induced disc damage results when fatigue failure outpaces the rate of adaptive remodeling, which depends on the intensity of load, the abruptness of its increase, and the age and health of the trainee. Provided that dynamic spinal exercise is performed in a manner that does not exceed individual disc-loading capacity, the evidence would seem to suggest a positive adaptation of the supporting tissues. In support of this contention, Videman et al. found that moderate physical loading resulted in the least disc pathology, with the greatest degeneration seen at extreme levels of activity and inactivity.
In case block quotes make your eyes glaze over, I’ll summarize: In real life, spinal tissues are allowed to remodel after spinal flexion. People don’t do crunches or sit-ups all day long. Damage happens when overtraining stresses the discs at a faster rate than adaptive remodeling. Adaptive remodeling is essential and even helpful. Therefore, crunches and other spinal flexion exercises might actually inhibit back pain, not cause it, by allowing this tissue adaptation. Whether or not the crunch itself is a problem depends on a particular person or situation.
They also note that flexion helps hypertrophy the rectus abdominis muscle, more so than other abdominal or general strength training exercises. Put simply, the crunch can target the rectus abdominis and doesn’t necessarily cause disc degeneration if loads are sufficient. Like all exercise ever, placing an adequate load on the body supports tissue and muscle regeneration, whereas a too great load encourages breakdown.
Does this mean McGill is wrong? Not necessarily. There is more to his research than I’m able to summarize right now; nonetheless, many suspect he is being overly cautious about the way in vitro models apply to the in vivo world.
McGill’s advice has seeped into popular magazines, resulting in countless articles railing against the crunch. On one hand, McGill’s campaign has caused some positive changes, like the military reconsidering its sit-up requirements (I’ve always thought those were a bit silly), but on the other hand, too much anti-crunch talk makes people think their bodies are fragile or that flexion in itself is dangerous. Sometimes it is, but sometimes it is helpful.
Where does this leave us? Contreras and Schoenfeld offer a middle ground. They suggest performing small bouts of crunch-like flexion, not exceeding 60 reps in a workout session at the higher end and 2 sets of 15 reps at the lower end, to provide the benefits of spinal flexion without a high risk of distress. They also suggest waiting about 5 minutes after prolonged sitting before engaging in spinal flexion exercises. I don’t know many people who jump up from a desk and start doing 100 crunches, but maybe it happens.
After listening to the debates and paying attention to my own body, my bias is that we shouldn’t be afraid of crunches. We don’t want to do 100 sit-ups for time or flex under high load (especially when using weights), but we probably shouldn’t worry about some spinal flexion in a normal exercise routine, especially if our rectus abdominis needs to be strengthened, which is often the case if we’ve grown babies.
Diastasis Recti and Pelvic Floor Disorder
Ah, but what about women who have diastasis recti and pelvic floor disorder? Crunches are still bad for them, right? It depends. I should tattoo that phrase across my forehead. IT ALWAYS DEPENDS (although, hopefully you can keep out of depends).
I did a lot of crunches very soon after pushing out my first child. I was given the go-ahead after the 6 week checkup, so I started doing like 100 crunches a day for a couple weeks. I didn’t know what I was doing, and it ended up being a really bad idea.
I stumbled across a popular DR technique, which told me to STOP crunching and to instead splint and pulse my transverse abdominis, which I did…but that was also a bad idea. Why? Because both the crunch and the TvA pulse were repetitive movements that didn’t take into account what was actually happening to my abdomen and pelvic floor during exercise.
Good Versus Evil
I don’t think a crunch is categorically awful, but personal experience and common sense suggest it can be counterproductive in the wrong circumstances. Therefore, what makes a crunch good versus bad?
A crunch is good if you can modulate the pressure increase, control this pressure with proper breathing, form, and abdominal and pelvic floor stiffening. If you do this, your rectus muscles will likely move closer together during flexion (or at the very least not move apart). A crunch is bad if you strain, hold your breath, dome your belly, or let your pelvic floor descend rather than ascend on an exhale, and if the crunch moves your rectus farther apart.
A TvA contraction can also be good or bad. Unlike a crunch, a TvA contraction will likely move your rectus muscles farther apart, but this isn’t necessarily bad since tensing the TvA will make an abdominal separation more shallow (even if slightly wider). It also won’t dome the belly. And, performing a Kegel usually contracts the TvA, which is a good thing. This tensing probably works the tissue in essential ways. However, if you pulse the TvA indefinitely, this widening is not being counteracted by any other dynamic movements.
The TvA works WITH the other abdominal muscles. It makes sense to isolate it when figuring out where it is and what a contraction feels like, but it certainly doesn’t make sense to contract it over and over and over ad infinitum. TvA contractions also compress the abdomen, which if not done in coordination with pelvic lifting, can cause too much pressure downward. Oh, and it’s not a corset. It compresses by flattening, not by cinching.
You might be surprised to learn that the TvA doesn’t immediately bring the rectus closer together. A ton of postnatal sources say the TvA will narrow the gap, but common sense, as well as the research, doesn’t support this assertion, at least in terms of immediate narrowing.
Let’s look at the TvA origins and insertions. The TvA originates at the lumbar fascia, at the bottom of the rib cage (costal margin), and at the front two-thirds of the iliac crest, more or less. It inserts at the aponeurosis of the rectus sheath (linea alba), again, more or less. This means it wraps around the torso, which is why it creates abdominal support, forced expiration, and increased intra-abdominal pressure (which in turn helps with that abdominal support). A muscle contracts by moving closer to its origin and by pulling on its insertion. A contracted TvA pulls on the linea alba. This stiffens it. Stiffening is good, but it does mean the rectus isn’t being pushed together.
Recent research confirms this observation. For example, a 2015 study looked at the inter-recti distance of postpartum women when they performed a crunch versus a drawing-in exercise (aka an isolated TvA contraction). I liked reading this study because it specifically looked at postpartum women, not just healthy college kids, and because it compared women who’d given birth vaginally with those who’d had c-sections. This comparison helped answer the long-standing question of whether c-sections increase the risk of lingering DR (they don’t). The results of the study were clear — during the crunch, the inter-recti distance decreased significantly, and during the drawing-in, the inter-recti distance increased. It stayed the same size when drawing-in was combined with a curl-up.
Similarly, a 2016 study compared the inter-recti distance of men and nonparous women to parous women (moms) when doing a curl-up. At rest, the inter-recti distance of women who’d had babies was larger than that of men and women who hadn’t been pregnant (shocker). The inter-recti distance of the moms decreased when performing a curl-up, whereas the inter-recti distance of the men and nulliparous women stayed the same (actually surprising).
But, maybe the gap was getting smaller while intra-abdominal pressure was off the charts? Maybe the crunch was still really bad for their pelvic floors? Actually, other studies have shown that the crunch on average doesn’t create nearly as much intra-abdominal pressure as many other activities (e.g. standing up from a sitting position, jogging, supermans, lifting stuff, etc…)  One study even showed that a ho-hum crunch generates less pressure than simply standing. Also, the amount of intra-abdominal pressure is highly variable among people, suggesting technique matters a lot. Therefore, banning the crunch for all women makes as much sense as uniformly banning lifting, jogging, standing, or getting up from the floor.
What to Take Away from These Studies
My takeaway is that a curl-up doesn’t increase inter-recti distance in most postpartum women. Admittedly, these studies didn’t look at diastasis recti specifically. The postpartum women had larger gaps than the controls, but this doesn’t mean they had DR (which is usually defined as a gap larger then 2 or 2.5 finger-widths). Nonetheless, diastasis recti is a difference in degree, not kind. Most postpartum women have larger gaps than nulliparous controls. Women with diastasis recti have larger than normal gaps, but not a completely different kind of abdomen. Therefore, if the inter-recti distance didn’t increase for the postpartum women with moderate gaps, it probably wouldn’t for many women with diastasis recti either.
I can think of a scenario in which a crunch might increase the gap. It’s possible that a larger gap correlates with weaker abdomens and that weaker abdomens use compensation strategies that overdo the intra-abdominal pressure, which in turn might increase rather than decrease IRD. I think this happened to me when I did those 100 crunches for a couple weeks. Nonetheless, the actual studies did not show crunches widening the gap, so we should turn down the de facto crunch hysteria.
Don’t misunderstand me; I still don’t think the crunch should be the very first exercise offered to postpartum women. Perhaps I’m biased by my own experience, but crunching didn’t help me when at my weakest.
HOWEVER, I don’t think we need to ban the crunch either. Indeed, incorporating it into postnatal routines will probably help women, as long as the belly doesn’t pooch or splay out, and as long as the pelvic floor can react against any intra-abdominal pressure. Pelvic tilts also offer a gentle shortening of the rectus and perhaps act as a good precursor to the crunch.
At what point the crunch becomes appropriate depends on the strength levels and techniques of individual women. This may be very soon for some women and a little later for others. Therefore, making a rule either way seems silly.
What about the transverse abdominis? Should we toss it out the window? After all, it INCREASED inter-recti distance in the 2015 study! I say, off with its head!
Not so fast, Meredith. Neither of those studies looked at inter-recti depth. I suspect tensing the TvA made the gap larger, but also shallower. Indeed, Lee and Hodges did a small observational study of women with DR and specifically noted that activating the TvA before a crunch made the gap shallower (what they call less linea alba “distortion”).
Furthermore, the studies asked participants to do a drawing-in maneuver, which is a pretty vigorous TvA contraction. Many practitioners, myself included, use varied cues that encourage a gentle stiffening, not a huge drawing-in. Normal TvA contractions done in concert with other abdominal activation likely do not permanently widen the gap.
The TvA helps with stability and often co-contracts with the pelvic floor. And, the pull on the linea alba might actually help the tissue remodel over time (I don’t know this, but it’s not completely implausible).
Thus, I still really like my TvA contractions, but they have to be put in context, a context that includes rectus and oblique shortening, a context that doesn’t vilify moves like the crunch. Put simply, the TvA isn’t magic and rectus concentration through flexion isn’t evil. We don’t need to go all medieval on any abdominal muscle.
This is good news because, honestly, I struggled with some cognitive dissonance over crunches. They were supposed to be bad and the TvA was supposed to be good, but I couldn’t figure out HOW an isolated TvA contraction would smoosh the rectus together. This just didn’t make anatomical sense to me.
Furthermore, my gap got a little smaller after I reincorporated the crunch into my routines. I don’t know if the crunch itself helped this narrowing, but I do know it didn’t pull anything apart or damage my pelvic floor any further.
I started to feel a little insane. What was wrong with me? Why weren’t my TvA contractions pushing my rectus closer together? Why was the crunch making me stronger when I was told it would overload my abdomen and pelvic floor? Turns out, the simplistic narratives were the problem, not me.
Where Does This Leave Us?
We are left pretty much where we started. My advice: Vary your contractions. Learn how to contract your TvA, your rectus abdominis, your obliques, your back muscles, your leg muscles, your butt muscles, your arm muscles, etc… And don’t be afraid of the crunch. Some people might insist we employ the precautionary principle and avoid spinal flexion “just in case.” This is probably true at high numbers and with load, but doesn’t make much sense otherwise. If we used the precautionary principle with everything, we wouldn’t do any movements.
Ultimately, pay attention to how a movement feels! A neutral spine is still good, and we don’t want to be flexed all the time. A crunch is a discrete action, not a permanent position. It also shouldn’t overdo the intra-abdominal pressure, and it should’t be splaying apart your abdomen.
As I always say, “Don’t be stupid, but don’t be afraid.” If the crunch makes things worse, stop doing it (don’t be stupid), but if a crunch narrows your gap, don’t give it up (don’t be afraid).
Therefore, I sayeth the crunch is not forboden. Go forth and use it wisely.
- Sancho, Pascoal, Mota, and Bo. “Abdominal Exercises Affect Inter-Rectus Distance in Postpartum Women: A Two-Dimensional Ultrasound Study,” Physiotherapy, Volume 101, Issue 3. ↩
- Chiarello, McAuley, Hartigan, “Immediate Effect of Active Abdominal Contraction on Inter-Recti Distance,” Journal of Orthopaedic and Sports Physical Therapy, Volume 46, Issue 3. ↩
- Shaw, Janet, et. al. “Intra-abdominal pressures during activity in women using an intra-vaginal pressure transducer,” Journal of Sports Science. 2014 Jun; 32(12): 1176–1185. ↩
- Lee, Hodges, “Behavior of the Linea Alba During a Curl-up Task in Diastasis Rectus Abdominis: An Observational Study,” Journal of Orthopedic Sports Physical Therapy, July 2016. ↩