The thing has a formal description in the research literature, which translates roughly as short-lived abdominal pain related to exercise. The word stitch is the one everybody actually uses.
The numbers are remarkable for something so poorly understood. Surveys of people who run regularly find that a clear majority have had one within the previous year. Across the general population the proportion who have experienced it at some point approaches everybody. It turns up in running, swimming, riding, cycling, team sports and aerobics, and it turns up in people of every level of fitness, including people in extremely good condition.
It is worth being precise about what is being discussed, because the word covers only one specific thing: the familiar sharp pain that arrives during exertion, sits in one place at the side of the abdomen, and goes away entirely when the exertion stops, leaving nothing behind. That is the subject of this piece and the only thing any of it refers to. Abdominal pain in general has a very large number of causes and nothing here is about any of them.
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The Explanation Everybody Repeats Is Folklore
Ask around and the answer that comes back most often involves the spleen. The story is that the spleen holds a reserve of blood, that hard exercise makes it contract to release that reserve, and that the contraction is what you feel.
There are several problems. The first is anatomical: the pain is at least as common on the right side as the left, and the spleen is on the left. The second is that the squeeze described does not happen in humans the way the story requires – it is a real phenomenon in some other mammals, in dogs and horses and notably in some diving mammals, which is almost certainly where the idea came from. The third is that the story predicts the pain should be tied closely to how hard you are working, and it is not reliably.
The spleen explanation has nonetheless been passed down for generations, and it is a good example of why a tidy mechanism is so much more memorable than an unresolved one. It names an organ, it gives it a job, and it feels like an answer. None of which makes it true.
The Oxygen-Starved Diaphragm Theory, and Why It Fell Apart
The first serious scientific proposal was that the great sheet of muscle across the base of the chest is simply running short of blood. On this account, hard exertion diverts circulation to the working limbs, the muscle gets less than it needs, and the pain is the pain of a muscle working without adequate supply – the same category of thing as cramp.
It is a sensible proposal and it dominated for decades. It has been largely abandoned for three reasons.
First, measurements of blood flow to that muscle during hard exercise do not show the shortfall the theory needs. It is a muscle that has been working continuously since birth and is unusually well supplied.
Second, the pain occurs in activities that place almost no demand on breathing at all. People get it while riding horses, and while being driven over rough ground, where nobody is short of breath and nothing is being diverted anywhere.
Third, it does not match the character of the pain. Muscle starved of blood produces a deep, spreading ache. A stitch is sharp, and it is in one findable spot, which is a different kind of sensation entirely and points at a different kind of tissue.
The Tugged-Ligament Theory, and Why It Fell Apart Too
The second proposal was mechanical. There are ligaments running from the underside of the diaphragm down to the organs below it, the liver on one side and the stomach on the other. Running involves repeated vertical jolting. A heavy organ bouncing on the end of a ligament will pull on whatever the ligament is attached to, and the theory was that this repeated tug produces the pain.
This had a great deal going for it. It explains why the pain is so strongly associated with running and jumping rather than with steady effort. It explains why a recent large meal makes it worse, since the organ doing the pulling is heavier. It even explains the two commonest locations, one on each side, which correspond neatly to the two attachments.
Then it ran into swimming. Swimmers get stitches at much the same rate as runners, and in swimming there is no vertical jolting, no bouncing and nothing being dropped against a ligament. Nor does the theory account for the pain arising while someone is seated.
It also struggles with a detail that turns out to matter: the pain is sometimes felt not in the abdomen at all, but in the tip of the shoulder.
The Explanation Currently in Front: Two Surfaces Rubbing
The leading current hypothesis is about neither muscle nor ligament. It concerns the lining of the abdominal cavity.
The inside of the abdomen is lined with a thin membrane, and the organs within it are wrapped in a continuation of the same membrane. The two layers face each other across a very small amount of fluid, and they slide against one another constantly – every breath, every bend, every step. The fluid is what makes the sliding frictionless.
The proposal is that under certain conditions the sliding stops being frictionless. Anything that reduces the lubrication, or that presses the two layers together harder than usual, produces friction between two surfaces that are not built to tolerate it, and the result is pain.
That immediately accounts for the awkward observations. It does not require jolting, so swimming is fine. It does not require hard breathing, so horse riding is fine. It does not require a particular side, because the lining is everywhere. And it predicts that anything distending the stomach – pressing the inner layer outward against the outer one – should make it dramatically worse, which is the single most reliable finding in the whole field.
The Evidence That Fits, Including the Clue in Your Shoulder
Two features of the pain support the membrane account in a way that is quite specific.
The first is the character. The body has two broadly different pain systems in this region. Pain arising from the organs themselves is vague, hard to locate, and tends to be felt somewhere other than where it comes from – which is why people with problems in a particular organ so often cannot point at it. Pain arising from the lining of the cavity wall is served by ordinary sensory nerves and behaves like pain from skin: sharp, immediate and precisely locatable. A stitch is firmly the second kind. You can put a finger on it. That alone rules out a great deal.
The second is the shoulder. The nerve supplying the upper part of that lining enters the spinal cord at the level of the neck, because of where the diaphragm happens to have developed from in the embryo. The brain interprets signals arriving at that level as coming from the shoulder, since that is what normally arrives there.
So irritation of the membrane under the diaphragm produces pain felt at the tip of the shoulder – a region with nothing wrong with it at all. This referral pattern is well established from entirely unrelated circumstances in which that membrane is irritated, and it is exactly the pattern that a proportion of people report with a stitch. A theory about a sheet of muscle or a stretched ligament has no reason to predict that. A theory about that specific membrane predicts it precisely.
The One Thing That Does Reliably Make It Worse
Amid all the uncertainty about mechanism, one relationship has come out of the research consistently and strongly, and it concerns what is in the stomach.
Large volumes of fluid taken shortly before exercise are associated with far more of these pains than small volumes. More striking, the composition matters independently of the volume: concentrated, sugary drinks are associated with substantially more than plain water, and the more concentrated the worse. Fruit juice performs particularly badly in these comparisons.
The explanation offered fits the membrane account well. A concentrated solution leaves the stomach more slowly than a dilute one, so it sits there distending it for longer, pressing outward on precisely the surfaces in question.
This finding holds up well and it is reported independently in different populations and different sports, which is more than can be said for most of the field. It is also the only part of the subject where the research produces a clear, repeatable pattern rather than a set of competing explanations – though what anybody does with that is not the business of an article about mechanism.
Why Something This Common Is Still Unsettled
It seems extraordinary that a sensation nearly every human has felt remains an open question. The reasons are mundane, and they are the same reasons a lot of ordinary things go unexplained.
It cannot be produced on demand. A phenomenon has to be reliably inducible to be studied properly, and this one is not – researchers trying to bring it on in volunteers succeed only erratically.
It vanishes the instant the activity stops, which means it cannot be examined while it is happening. By the time anybody could look, there is nothing to look at, and there is no trace afterwards: no mark, no change, nothing measurable.
And it is regarded in the literature as self-limiting and not a sign of damage, which means it attracts almost no research funding. Serious conditions get investigated. A pain that stops when you stop and leaves nothing behind does not.
The result is a subject where almost everybody has first-hand experience, the folk explanation is about the wrong organ on the wrong side, two scientific explanations have been tried and found wanting, and the current best account is a sensible, well-supported, still unproven idea about two slippery surfaces that have stopped being slippery.
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