Here is an honest thing to admit at the start of an article: one of the most common pains in all of sport, the sharp jab under the ribs that most runners and swimmers know by feel, does not have a settled explanation. Not a controversial one. Not a complicated one. Nobody actually knows what causes a stitch. We have named it, counted it, timed it and argued about it for decades, and the mechanism is still open. That is not a failure of this article. That is the article.
01 · The thing itselfA pain so common it has a proper name
The stitch is real, it is specific, and it is astonishingly widespread. In the research literature it goes by the deliberately dull name exercise-related transient abdominal pain, or ETAP, a label pushed into use largely by two Australian researchers, Darren Morton and Robin Callister, whose surveys and experiments still make up most of what we know. When they asked 965 athletes across six sports, about 61% had felt a stitch in the previous year. It is sharp, it is usually well localised (most people can point to the spot), it tends to sit in the mid-to-lower abdomen and lean to the right, and it has one very cooperative feature: it stops almost the instant you stop moving. That last detail matters, because whatever causes it is clearly tied to the act of exercising, not to any lasting damage.
02 · The old answerWhy "your diaphragm is starved of blood" fell apart
For a long time the textbook explanation was that the diaphragm, the big breathing muscle under your lungs, was being starved of blood during hard exercise, cramping like an overworked calf. It is a tidy story. It is also, on the evidence, probably wrong. The diaphragm has a genuinely generous blood supply, more than enough to keep working hard, which makes a blood-starvation cramp hard to justify. And when researchers actually measured people’s breathing during a stitch, they found no impairment: lung function looked normal even mid-pain. A cramping, oxygen-starved diaphragm should have shown up. It did not. So the old favourite is not so much disproven as quietly abandoned by most people who study this.
03 · The tugging ideaWhere the "bouncing gut" theory runs into swimmers
The next candidate is more mechanical. The idea is that your internal organs hang from the body wall by ligaments and folds of tissue, and that the up-and-down pounding of running jolts them, tugging on those supports and on the nerves around them. It has real appeal, because it fits why running and horse riding, both full of vertical jolting, are such reliable stitch factories, and why a heavy meal (a heavier gut to yank about) makes it worse.
Then you remember swimmers. Swimming has no pounding at all, no vertical jolt to bounce anything, and yet swimmers report stitches more than any other athletes studied. A theory built on jolting has a hard time with a sport that does not jolt. The tugging idea is not dead, but it plainly cannot be the whole story, and the swimmers are the reason.
The most honest sentence in the science of the stitch is also the least satisfying: the cause is unknown. Every explanation you have ever been told is a hypothesis, and the leading one is a membrane you have probably never heard of.
04 · The leading ideaTwo slippery layers, rubbing
The current front-runner is subtler. Your abdominal cavity is lined by a thin, slippery membrane called the peritoneum, and it comes in two layers: one coating the organs, one lining the inside of the body wall, normally gliding over each other on a film of fluid. Morton and Callister’s proposal is that friction between these two layers, or irritation of the layer stuck to the abdominal wall, produces the pain. It is a good fit for several stubborn facts at once. It explains the sharp, precisely localised quality (that outer layer is richly supplied with nerves that can pinpoint pain). It explains the shoulder-tip ache some sufferers get, because part of that lining is served by the phrenic nerve, which refers pain to the shoulder. And it explains why a big meal or a concentrated sugary drink makes things worse, by changing the fluid and volume in the cavity so the layers no longer slide cleanly. It even survives the swimmers, whose torso rotation and extension could irritate the same membrane without any jolting. Notice the word doing the work in all of that: it “could.” This is the best idea we have. It is still an idea.
05 · What we do knowThe risk factors that are actually solid
The cause may be murky, but the risk factors are surprisingly clear, and some of them are counter-intuitive. Age is the big one: the stitch is a young person’s affliction. It is most common in teenagers and steadily rarer with each decade, dropping from roughly three-quarters of under-20s to around 40% of the over-40s in the surveys. Sex and body type barely matter. And here is the one that stings the dedicated: fitness does not make you immune. Being well trained might mean you get a stitch a little less often, but elite athletes still get them, and being fit does nothing to make the pain milder when it lands. The thing you can actually control is what is in your gut. Eating a large meal in the one-to-two hours before you run, and drinking concentrated, sugary drinks rather than water, both reliably raise your odds.
06 · Making it stopThe fixes, and how much to trust them
Because nobody has nailed the cause, nobody has a proven cure, and it is worth being honest about that when you pass on the folklore. What we have is the collection of things sufferers report helping: slow down or stop (this one always works, being the least popular), breathe deeply, press a hand firmly on the sore spot or bend forward over it, and try forcing the air out through pursed lips as the opposite foot strikes the ground. The pursed-lip and opposite-footfall tricks have a plausible logic about evening out the strain on the two sides of the torso, but the evidence behind them is thin, so treat them as reasonable, harmless bets rather than a formula. The prevention advice is on firmer ground: leave about two hours after a big meal, skip the sugary sports drink for water, build intensity up gradually, and, since some evidence links a stronger core to fewer stitches, it may be worth the sit-ups.
07 · The payoffWhy "we don't know" is the right answer
So, why do you get a stitch when you run? The most truthful answer is that a common, harmless, self-cancelling pain has quietly resisted explanation for decades, that the tidy version you were taught (the gasping diaphragm) is probably wrong, and that the best current guess involves two slippery membranes losing their glide. It is a small and useful reminder that “science has an answer for everything” is not quite true, and that the everyday body still keeps a few secrets in plain sight. The next time one jabs you at mile three, you can at least take comfort in this: you are experiencing a genuine open question in human physiology, and it will be gone before you can finish thinking about it.
Quick questions
What is a side stitch actually called?
Its formal name is exercise-related transient abdominal pain, usually shortened to ETAP. Older names include 'a stitch' and 'side ache.' The term was pushed into the literature largely by the Australian researchers Darren Morton and Robin Callister, whose surveys and experiments are still the main body of evidence on it.
What causes a stitch when running?
The honest answer is that it is not settled. The best-supported current idea is that the two layers of the peritoneum, the membrane lining your abdominal cavity, rub against each other and become irritated, producing sharp, well-localised pain. That fits the symptoms, but it has not been proven, and other mechanisms have not been ruled out entirely.
Is a stitch caused by the diaphragm not getting enough blood?
This was the classic textbook explanation, but it is poorly supported. The diaphragm has a generous blood supply, and when researchers measured breathing function during a stitch they found no impairment. Most experts now consider diaphragm ischaemia unlikely to be the main cause.
Why do swimmers get stitches if there is no jolting?
This is one of the strongest clues that the old 'tugging on the gut' idea is incomplete. Swimming has no vertical pounding, yet swimmers report ETAP more than any other athletes studied. A friction-of-the-peritoneum mechanism can explain it, because swimming involves repeated torso rotation and extension, which the theory says could irritate the same membrane.
Does being fit stop you getting a stitch?
No. Well-trained athletes get ETAP too. Fitness may reduce how often you get one, but it does not make you immune, and it does not reduce how much it hurts when you do. Elite runners still report stitches.
Why does eating before a run cause a stitch?
A large meal close to exercise is one of the most consistent risk factors. In Morton and colleagues' work, runners who ate a big meal one to two hours before an event were more likely to get a stitch. The reason is debated: a full or heavy gut may load the supporting membranes, or the contents may change the fluid around the peritoneum.
Do sports drinks make a stitch worse?
They can. Concentrated, sugary (hypertonic) drinks appear to be more provocative than water or dilute drinks. In one experiment a large majority of people developed a stitch after a hypertonic drink, a somewhat higher share than with an isotonic one. If you are prone to stitches, plain water and smaller volumes are the safer bet.
Why does a stitch sometimes hurt in your shoulder?
Some people with a stitch feel a referred ache at the tip of the shoulder on the same side. That pattern points to the phrenic nerve, which carries sensation from part of the abdominal lining and, when irritated, is felt at the shoulder tip. It is one of the observations the peritoneal-irritation idea leans on.
Where in the body do you feel a stitch?
Most often it is a sharp, well-localised pain in the mid-to-lower abdomen, more commonly on the right side. In surveys most sufferers describe it as sharp and stabbing when severe and more of a cramp or ache when mild, and the majority can point to one spot.
How do you get rid of a stitch fast?
There is no proven cure, but the common self-help measures are: slow down or stop, breathe deeply, press on the sore spot or bend forward, and try exhaling hard through pursed lips as the opposite foot lands. These come from what sufferers report helping rather than from clean trials, so treat them as reasonable bets, not guarantees.
Does breathing out on the opposite foot really help?
It is a popular tip with a plausible logic, that timing your exhale to the opposite footfall reduces strain on one side, but the evidence is thin. Many runners swear by it, and it is harmless to try, so it is worth a go even though it has not been rigorously proven.
Are stitches ever a sign of something serious?
A classic exercise stitch is harmless and disappears once you stop. But pain that is severe, does not settle with rest, or comes with other symptoms is not a stitch and should be checked. If abdominal pain during exercise is new, one-sided and persistent, or very intense, see a doctor rather than assuming it is a stitch.
Do children get stitches more than adults?
Yes. ETAP is strongly age-related: it is most common in the young and becomes less frequent as people get older. Surveys have found prevalence far higher in teenagers than in the over-forties. Why age protects you is, like most of this topic, not fully understood.
Can you prevent a stitch entirely?
Not reliably, but you can shorten the odds: avoid large meals and sugary drinks in the couple of hours before exercise, build up intensity gradually, and strengthen your core, which some evidence links to fewer stitches. None of this is a guarantee, because the underlying cause is still unknown.
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