The Two-Thousand-Year-Old Pain Beneath the Ribs
Why nearly every runner gets a stitch, and why science still cannot fully explain it.
Three kilometers into a steady run, something sharp twists below the lower ribs. The pain is precise enough to be alarming, vague enough to defy description. Most runners simply call it a stitch. They slow, bend forward, push a thumb into the rib margin, and wait for the sensation to dissolve as mysteriously as it arrived. Within a few minutes, it usually does.
It is one of the most common complaints in athletic medicine. Surveys of recreational and competitive runners place the annual incidence near 70 percent, and the lifetime incidence approaches universality among anyone who has tried to jog more than a mile 1. Swimmers report it. Cyclists report it. So do equestrians, basketball players, and the occasional camel rider in Saharan endurance events. Yet despite a presence in athletic life so persistent that Pliny the Elder mentioned it in the first century, and despite the discomfort being trivially easy to reproduce in any willing test subject, the medical literature still does not fully agree on what a stitch actually is.
This is unusual. Modern physiology has mapped the molecular cascade behind muscle fatigue, the precise neurochemistry of runner’s high, the cellular response to lactate accumulation. The stitch, by comparison, remains a stubborn anomaly. It has a clinical name (exercise-induced transient abdominal pain, abbreviated ETAP), a body of research stretching back decades, and a list of competing mechanistic theories, none of which fully satisfies the evidence.
What follows is an attempt to trace that mystery, from ancient Roman observation through twentieth-century assumptions to the modest, careful revisions of a researcher in rural Australia who built much of what is currently known.
A pain older than the marathon
Pliny the Elder, writing in the Natural History around 77 AD, described a sharp pain in the side that came upon runners and could sometimes be relieved by holding the breath or pressing the affected area 2. The Greek physicians had noticed it too. It was familiar enough to be unremarkable, which is part of why it never attracted the kind of medical attention that more dramatic afflictions received. A stitch killed no one. It merely halted them, briefly, and then released them back to their activity.
For most of recorded medical history, the explanation defaulted to the diaphragm. The reasoning was intuitive. The diaphragm is a dome of muscle directly below the lungs, responsible for the mechanical work of breathing. Hard exercise demands hard breathing. Therefore, the logic went, the diaphragm must occasionally fatigue or cramp under load, and the cramp manifests as that sharp, localized pain beneath the ribs.
The theory had a respectable lineage. Variants of it appeared in early twentieth-century sports medicine textbooks, where the stitch was sometimes described as a diaphragmatic ischemia: a temporary shortage of oxygen reaching the muscle as blood was redirected to working limbs. Coaches passed the explanation to athletes. Athletes passed it to one another. The stitch became, in the popular imagination, a problem of breathing.
It was a clean story. It also turned out to be largely wrong.
The man who asked the obvious question
Darren Morton, an Australian sports physiologist based at Avondale College in New South Wales, became professionally curious about stitches in the late 1990s. He was not the first researcher to investigate them, but he was the first to investigate them at scale. Over several years, Morton and his collaborator Robin Callister surveyed nearly a thousand athletes across multiple sports, collected data on when stitches struck, what activities preceded them, what helped, and what did not 3. The resulting body of work, published across roughly a dozen papers between 2000 and 2015, remains the most systematic investigation of ETAP in the medical literature.
The diaphragm theory was the first casualty. If diaphragmatic ischemia were the mechanism, one would expect a clear correlation between breathing intensity and stitch onset. Morton found none. Runners breathing at maximum capacity were no more likely to develop a stitch than runners cruising at a moderate pace. Athletes with respiratory pathologies showed no elevated incidence. Most damaging of all, swimmers, who breathe in controlled, rhythmic patterns very different from running, reported stitches at comparable rates. Horseback riders, whose diaphragms are not particularly taxed, got them too. The single common feature across these activities was not heavy breathing. It was movement of the torso.
Morton turned his attention to something less obvious. The peritoneum is a thin, two-layered membrane that lines the abdominal cavity and drapes over its organs. The outer layer (the parietal peritoneum) adheres to the inner wall of the abdomen. The inner layer (the visceral peritoneum) wraps the organs themselves. Between them sits a thin film of serous fluid, perhaps fifty milliliters in a healthy adult, just enough to allow the two surfaces to slide against each other as the diaphragm rises and falls and the gut shifts with every step.
Morton’s hypothesis was simple. If something disturbed that lubricated glide, if the two layers began to rub rather than slip, the result would be friction-generated irritation along a membrane densely populated with pain-sensing nerves. The parietal peritoneum in particular is innervated by the same somatic nerves that supply the abdominal wall and, critically, share spinal segments with the shoulder. A pain originating there would be sharp, well-localized, and prone to radiate. Which is exactly what athletes describe.
Why a glass of orange juice ruins a run
Once friction became the working theory, several otherwise puzzling observations clicked into place. Runners had long known, anecdotally, that eating too close to a workout invited stitches. Coaches advised against drinking sugary fluids before exercise. The conventional explanation involved digestion competing with muscles for blood supply, which sounded plausible but had little experimental support.
Morton tested the question directly. In a study published in 2004, he and Callister had volunteers run on a treadmill after consuming different pre-exercise drinks: plain water, a hypotonic sports beverage, a more concentrated sports drink, and undiluted fruit juice 4. The pattern was striking. The denser and more sugary the drink, the worse and more frequent the stitches. Reconstituted fruit juice, the most concentrated of the trial beverages, produced symptoms in nearly every participant. Plain water produced almost none.
The mechanism, Morton argued, was osmotic and mechanical. A high-sugar liquid in the stomach draws water into the gut and empties slowly, leaving the stomach distended for longer. That distension pulls on the visceral ligaments that anchor the gut to the upper abdominal wall. The dragging force, transmitted through those ligaments to the diaphragm and the surrounding peritoneal tissue, increases friction precisely where the inner and outer membranes ought to slide freely.
Which predicts something the data confirms. Stitches strike more often on the right side, beneath the lower ribs, where the liver hangs heavily from its peritoneal attachments. The right-sided dominance is one of the more reliable features of ETAP, present in roughly two-thirds of reported episodes. The diaphragm theory could never quite explain it. The friction theory does: the liver is the largest abdominal organ, anchored by ligaments that transmit its weight directly into the peritoneal sheets when the torso bounces or tilts.
It also predicts why running, with its repetitive vertical impact, is the worst offender, and why swimming, with its horizontal posture and rhythmic core engagement, sits in second place. The activities that most aggressively jostle the gut produce the most stitches.
The posture problem
With Mitchell Plunkett, another researcher in his orbit, Morton noticed a further pattern that opened a second dimension of the puzzle. When athletes who reported frequent stitches were measured for spinal alignment, a striking proportion (Plunkett and Morton reported roughly two-thirds) showed measurable thoracic curvature or postural asymmetry 5. Slouching, in other words, predicted susceptibility.
The link is mechanical. A forward-curved upper spine compresses the upper abdomen, narrows the space between the ribcage and the iliac crest, and shifts the lie of the abdominal organs against the peritoneal membrane. The same gut motion that would slide smoothly in a tall, extended torso instead grinds against compressed tissue in a slumped one. Some sports physiotherapists have since incorporated postural assessment into their protocols for runners with chronic stitches, and the anecdotal evidence is encouraging. Many runners who learn to run tall, with extended thoracic spines and lifted sternums, find their stitches fade.
This is not the kind of finding that will revolutionize medicine. It does, however, suggest that the stitch is less a glitch of physiology than a small mechanical fault in how a moving body is organized. A slightly different posture, a slightly different gut content, a slightly different stride, and the friction stays below the threshold of awareness.
The complication from the spinal column
The peritoneal friction model was, by the mid-2000s, the leading theory of ETAP. It explained more of the data than any of its predecessors and was consistent with the relief produced by stretching, exhalation, and direct pressure. But theories in physiology rarely stay tidy for long. A 2014 review and several subsequent studies have suggested that the picture is more complicated, and that at least some component of stitch pain may originate not in the abdomen at all but in the spinal nerves that serve it 6.
The argument hinges on referred pain. The phrenic nerve, which supplies the diaphragm, originates in the cervical spine at levels C3 through C5. It shares those spinal segments with the nerves that innervate the tip of the shoulder. When the diaphragm or the parietal peritoneum near it is irritated, the brain often receives the signal as if it originated from the shoulder, because the cortex cannot reliably distinguish between two inputs that arrive on the same spinal highway. This is why gallbladder pain often radiates to the right shoulder, and why pneumonia in the lower lung can present as upper-body discomfort.
In the case of a stitch, the same logic applies in reverse. Mechanical irritation in the peritoneum sends signals up the spinal cord, and the brain, processing those signals through the shared neural circuitry of the upper torso, may locate the pain ambiguously. Some patients feel it precisely under the ribs. Others feel it radiate toward the shoulder tip. A few feel it primarily in the shoulder and only secondarily in the abdomen.
The more recent research suggests that this referred pain pattern is not just a curiosity but a hint about etiology. If the spinal pathway itself is involved, then conditions that alter spinal sensitivity (postural strain, prior injury, even visceral inflammation) might modulate how readily a stitch develops. It would explain why some runners are chronically prone to stitches while others, with apparently identical training and nutrition, almost never experience them.
For now, the friction model and the neural-referral model coexist uneasily. They may turn out to be two views of the same underlying process. Or one may eventually subsume the other. The honest answer, more than two millennia after Pliny first wrote the words, is that we still do not fully know.
What actually helps
The lack of mechanistic certainty has not prevented sports medicine from converging on a fairly consistent set of practical recommendations. Most of them follow logically from the friction theory, and most of them have at least modest empirical support.
Avoiding high-sugar fluids in the hour or two before running is the most reliable intervention. Plain water in moderate volumes is better tolerated than concentrated juices, sodas, or some sports drinks, and the effect is dose-dependent. Smaller volumes of liquid empty faster and produce less abdominal drag. Pre-run meals matter similarly. A heavy meal eaten ninety minutes before a run leaves a stomach still substantially loaded by mile three. Lighter, lower-fat foods empty faster.
Posture during the run matters more than most runners realize. Lifting the sternum, drawing the shoulders back, and extending the thoracic spine reduces the compressive force on the upper abdomen. Some runners find that consciously elongating the torso, almost as if being pulled upward from the crown of the head, makes the difference between a clean run and a painful one.
Breathing technique has its own modest evidence base. A long-standing observation in running coaching is that stitches correlate weakly with footstrike pattern. Many runners habitually exhale on the same foot every stride, which means the diaphragm contracts and relaxes in synchrony with the impact load on one side of the body. Switching the exhale to coincide with the opposite footstrike (typically the left, for right-sided stitches) distributes the mechanical stress more evenly across the abdomen. Whether this works because of friction redistribution or for some other reason is unclear, but many runners swear by it.
Once a stitch has started, the time-honored remedies still apply, and they have plausible mechanisms behind them. Slowing the pace reduces the mechanical jostling of the gut. Deep, slow breathing extends the diaphragm fully and stretches the peritoneal layers in opposing directions, which may briefly reset their alignment. Pressing firmly into the painful spot, an instinctive gesture that almost every runner adopts unbidden, applies external support to the abdominal wall and may reduce friction enough to allow recovery. Bending forward at the waist while breathing slowly accelerates the process. Stretching the torso upward and to the opposite side, once the pain has begun to fade, helps prevent its return.
None of these are dramatic. They form, collectively, a kind of folk pharmacopoeia handed down through generations of runners, much of it later validated by sports physiology. Pliny would have recognized most of them.
A small mystery, honestly held
It is striking how much of human physiology remains genuinely open. The big questions, the metabolic pathways and the architectures of the immune system, have been mapped in extraordinary detail. The small questions, the ones that shape ordinary experience, often have not. Why we yawn, why we hiccup, why dreams take the forms they do, why a sharp pain reliably appears beneath the lower ribs of a running body. These are the puzzles that sit closest to lived experience and yet remain partially unresolved.
The stitch is a useful reminder of that condition. Two thousand years of observation, a century of clinical interest, and twenty years of careful investigation by Darren Morton and his colleagues have produced a working theory, several robust correlations, and a set of practical recommendations that genuinely help. They have not produced a complete answer. The pain originates somewhere in the moving interface of organs, membranes, and nerves that constitutes the abdominal interior, and the precise mechanism remains under negotiation.
What the research has clarified is the nature of the conversation. A stitch is not a sign of weakness. It is not a failure of fitness or a deficit of breathing. It is a small mechanical complaint from a body asked to carry food, lubricated tissue, and habitual posture through a motion the species spent most of its history performing on near-empty stomachs and over level ground. The peritoneum complains. The spinal nerves relay the complaint. The runner slows, bends, breathes, and continues.
The next time it arrives, somewhere around the third kilometer, it is worth remembering that the sensation has a lineage. Pliny noted it. Morton studied it. A thousand laboratories still cannot fully explain it. And it will, in a few minutes, almost certainly fade.
Sources
- Morton, D. P. and Callister, R., ‘Characteristics and etiology of exercise-related transient abdominal pain,’ Medicine and Science in Sports and Exercise, 2000. — https://pubmed.ncbi.nlm.nih.gov/10694105/
- Pliny the Elder, Natural History, Book XX (c. 77 AD), Loeb Classical Library edition. — https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Pliny_the_Elder/home.html
- Morton, D. P. and Callister, R., ‘Exercise-related transient abdominal pain (ETAP),’ Sports Medicine, 2015. — https://pubmed.ncbi.nlm.nih.gov/25281762/
- Morton, D. P. and Callister, R., ‘Influence of posture and body type on the experience of exercise-related transient abdominal pain,’ Journal of Science and Medicine in Sport, 2004. — https://pubmed.ncbi.nlm.nih.gov/15518293/
- Plunkett, B. T. and Hopkins, W. G., ‘Investigation of the side pain ‘stitch’ induced by running after fluid ingestion,’ Medicine and Science in Sports and Exercise, 1999. — https://pubmed.ncbi.nlm.nih.gov/10449014/
- Morton, D. P., ‘Exercise related transient abdominal pain,’ British Journal of Sports Medicine, 2003. — https://bjsm.bmj.com/content/37/4/287
- Muir, B., ‘Exercise related transient abdominal pain: a case report and review of the literature,’ Journal of the Canadian Chiropractic Association, 2009. — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2729472/
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