A side stitch is a sharp, localized pain in the abdomen that strikes during physical activity, and despite being one of the most common complaints among runners and other athletes, its exact cause remains surprisingly unresolved. Formally known as exercise-related transient abdominal pain (ETAP), the stitch affects roughly 70% of runners in any given year, and about one in five runners can expect to feel one during a single race or training run.1PubMed Central. Exercise-related transient abdominal pain (ETAP) It is benign and self-limiting, but that does not mean it is trivial, especially mid-race when performance is on the line.
What a Side Stitch Actually Feels Like
Most people describe the pain as a sharp or stabbing sensation when it is at its worst, softening to a cramp, ache, or pulling feeling as it eases.2PubMed Central. Exercise-related transient abdominal pain (ETAP) It almost always shows up on the lateral edges of the mid-abdomen, right along the lower border of the ribcage, though it can appear anywhere across the abdomen. The right side seems to catch the worst of it more often than the left, though left-sided stitches are far from rare.
Severity varies enormously from episode to episode. Some stitches are mild enough that you can run through them with a grimace. Others will stop an experienced athlete dead. Research confirms that the condition can be “very detrimental to the performance of many athletes from novice to elite,” which is a polite way of saying it has ruined more than a few race times.3PubMed Central. Exercise related transient abdominal pain: a case report and review of the literature
Three Theories and No Clear Winner
Researchers have proposed at least three competing explanations for what actually produces that pain, and decades of study have not settled the debate definitively.4PubMed. Stitch in the side: causes, workup, and solutions
The oldest explanation is diaphragmatic ischemia, the idea that during hard exercise the diaphragm does not get enough blood flow because blood is being redirected to the working muscles. This was the standard textbook answer for years, and it is probably the one your gym teacher told you. The problem is that the diaphragm is a respiratory muscle that is working hard during exercise, and the body is generally good at keeping essential respiratory muscles fed with oxygen. Studies that directly measured diaphragm function during exercise have largely failed to find the kind of blood-flow deficit this theory requires, which has pushed researchers toward other ideas.
The second theory involves the peritoneal ligaments, bands of connective tissue that help suspend the abdominal organs. When you run with a stomach full of food or fluid, the extra weight bouncing around may stress these ligaments, especially the ones connecting the gut to the diaphragm. A study that had runners exercise after drinking various fluids found results consistent with this notion: when the gut was engorged with fluid, stitch developed regardless of what the fluid was, supporting the idea that mechanical tugging rather than any chemical property of the drink was the culprit.5PubMed. Investigation of the side pain “stitch” induced by running after fluid ingestion
The third and currently most favored explanation centers on irritation of the parietal peritoneum, the membrane that lines the inside of the abdominal wall. This membrane is richly supplied with nerves and highly sensitive to friction or stretching. During exercise, especially activities that involve a lot of torso twisting and bouncing, the layers of peritoneal tissue can rub against each other or become irritated by changes in fluid between them. This explanation accounts for several features of stitch that the other theories struggle with: why the pain is so well-localized, why it can occur anywhere across the abdomen (not just where the diaphragm is), and why it sometimes radiates to the shoulder.
The Shoulder Pain Connection
One of the stranger features of a side stitch is that it sometimes comes with a sharp pain in the tip of the shoulder, usually on the same side as the abdominal pain. This is a textbook example of referred pain, where irritation in one part of the body is felt somewhere else entirely because the two areas share a nerve pathway.
The mechanism involves the phrenic nerve, which supplies the underside of the diaphragm and the peritoneum just below it. When those tissues become irritated during exercise, the signal travels along the phrenic nerve, and the brain interprets part of that signal as coming from the shoulder, because the phrenic nerve also serves the skin over that area.6PubMed Central. Exercise-related transient abdominal pain (ETAP) Athletes who experience shoulder tip pain alongside their stitch may actually be providing researchers with a clue about the mechanism: the peritoneal irritation theory explains both symptoms neatly, while the diaphragmatic ischemia theory does not account for referred shoulder pain as cleanly.7PubMed Central. Exercise related transient abdominal pain: a case report and review of the literature
What Makes a Stitch More Likely
Side stitches are particularly common in activities that involve repetitive, vertical jolting of the torso combined with an upright or extended posture. Running is the classic trigger, but horse riding, swimming, and aerobics are also frequently reported.8PubMed Central. Exercise-related transient abdominal pain (ETAP) Cycling, by contrast, produces stitches far less often, probably because the torso is relatively stable and slightly flexed forward.
What you eat and drink before exercise matters. That fluid-ingestion study mentioned earlier tested water, a sports drink, decarbonated cola, and a thick sugar solution that the gut absorbs slowly. Stitch developed to a similar intensity with all four fluids during the early bouts of running. The sports drink was the only one that led to a decline in stitch intensity over repeated bouts, eventually falling to the level seen when subjects drank nothing at all.9PubMed. Investigation of the side pain “stitch” induced by running after fluid ingestion The likely explanation is that the sports drink emptied from the stomach faster, reducing the time the gut spent heavy and sloshing. Highly concentrated or sugary drinks that sit in the stomach longer are more likely to keep you in stitch territory.
Solid food follows a similar logic. A large meal eaten within an hour or two of exercise is a well-known trigger, and most experienced runners learn this the hard way. The fuller the stomach, the more weight the peritoneal ligaments and abdominal contents have to manage during the repetitive impact of running.
Who Gets Stitches Most
Age is the strongest demographic predictor. Younger people get stitches more often and more severely than older people, with both the frequency and intensity declining steadily as people age.10Medicine & Science in Sports & Exercise. Factors influencing exercise-related transient abdominal pain This is a bit counterintuitive, because you might expect less fit or older bodies to suffer more, but the data consistently show the opposite. One possible explanation is that the peritoneal membranes become less sensitive with age, or that the ligamentous structures in the abdomen become less elastic and therefore less prone to the kind of repetitive stretching that triggers irritation.
Gender does not appear to matter. Neither does body mass index. Training status is similarly neutral as a blanket category, but here the data gets more interesting: people who train more frequently report stitches less often.11Medicine & Science in Sports & Exercise. Factors influencing exercise-related transient abdominal pain That is not the same thing as saying fit people are immune. It suggests instead that regular exposure to the stimulus gradually desensitizes whatever tissue is producing the pain, which again fits the peritoneal irritation model better than a simple blood-flow story.
Beginners starting a running program are therefore the highest-risk group, which lines up with what most running coaches observe. The reassuring flip side is that stitches tend to become less frequent and less intense the more consistently you train.
How to Stop a Stitch When It Hits
No controlled trial has formally tested the various folk remedies for stopping a stitch mid-exercise, so what follows comes from clinical observation and athlete experience rather than randomized evidence. That said, certain strategies are reported often enough to be worth trying.
- Slow down: Reducing intensity is the single most reliable way to ease a stitch. If you are racing and do not want to stop entirely, even dropping your pace for 30 to 60 seconds often takes the edge off.
- Bend forward slightly: Flexing the torso forward at the waist compresses the abdomen gently and may reduce the tension on whatever tissue is being irritated. Many runners do this instinctively.
- Press into the pain: Pushing your fingers firmly into the painful spot while exhaling deeply is another widely reported fix. The pressure may help stabilize the abdominal contents or provide a counter-stimulus that overrides the pain signal.
- Exhale on the opposite foot strike: If the stitch is on your right side, try timing a hard exhale to the moment your left foot hits the ground. The theory is that this changes the impact forces transmitted through the torso and may reduce the repetitive stress on one side. There is no firm evidence for why this works, but enough runners swear by it that it is worth a try.
- Raise one arm and lean away: Stretching the arm on the affected side overhead and leaning toward the opposite side opens up the torso and may reduce compression on the irritated tissue.
If nothing helps, walking for a minute or two almost always resolves the pain completely. The stitch is transient by definition, so once the triggering mechanical stimulus stops, the pain follows.
Preventing Stitches Before They Start
Because the strongest modifiable risk factor is what and when you eat and drink, the most practical prevention strategy is meal timing. Leaving at least two hours between a substantial meal and hard exercise reduces stitch risk considerably. If you need to drink close to a workout, small sips of water or a well-formulated sports drink are less problematic than gulping a large volume of fluid at once, especially concentrated or carbonated drinks.
A proper warm-up that gradually increases intensity also helps. Jumping straight into a hard effort is a reliable way to bring on a stitch, whereas easing in over five to ten minutes gives the abdominal tissues time to adjust to the rhythmic stress. Core strengthening may also play a role, at least anecdotally. Runners with stronger trunk musculature report fewer stitches, though this could reflect the same frequency-of-training effect seen in the survey data rather than a direct benefit of core work.
Breathing technique is another area where runners often experiment. Shallow, rapid chest breathing seems to be associated with more stitches, while deeper, diaphragmatic breathing may help. Again, this has not been tested rigorously, but the logic makes sense: deeper breathing engages the diaphragm more rhythmically and may reduce the erratic pressure changes within the abdomen that could irritate the peritoneum.
When It Might Not Be a Stitch
Because a stitch is a diagnosis of exclusion in medicine, meaning there is no blood test or scan that confirms it, any persistent or unusual abdominal pain during exercise deserves a second look. A true stitch goes away within minutes of stopping exercise and does not leave lasting tenderness. Pain that lingers for hours, returns at rest, or comes with other symptoms like fever, nausea, or blood in the urine is not a stitch, even if it started during a run.
Conditions that can mimic a side stitch include a muscle strain of the abdominal wall, a stress fracture of the lower ribs (rare but possible in high-mileage runners), gallbladder issues that flare with exertion, and even cardiac referred pain in some cases. The distinguishing features are usually duration and context. If the pain is reproducible only during exercise, resolves quickly on rest, fits the classic location along the lower rib margin, and has no accompanying symptoms, it is almost certainly a stitch. If any of those features are missing, it is worth checking with a doctor before assuming you just need to work on your breathing.
Why the Science Remains Thin
For something so common, side stitches have attracted remarkably little formal research. Part of the problem is that stitches are impossible to study in a traditional clinical framework: you cannot biopsy the peritoneum of someone mid-run to see what is happening, and imaging during vigorous exercise is impractical. The condition is also self-limiting and benign, which means it does not attract the kind of funding that serious pathologies do.
Most of what we know comes from survey studies and a handful of controlled experiments like the fluid-ingestion trial. The peritoneal irritation hypothesis has the most circumstantial support, but “most supported” in this context means “least contradicted.” No one has directly visualized the peritoneum being irritated during exercise in a living human. Researchers have acknowledged that the condition remains “under analyzed and under reported in the medical literature,” which is a diplomatic way of saying that something affecting millions of athletes every year has been given far less scientific attention than it deserves.12PubMed Central. Exercise related transient abdominal pain: a case report and review of the literature
The practical consequence of this research gap is that most advice about side stitches remains rooted in experience and plausible physiology rather than proven interventions. That is frustrating for the evidence-minded runner, but the good news is that the experiential advice, particularly around meal timing, warm-ups, and gradual exposure, is consistent enough across the athletic community that it clearly works for most people. Whether the underlying explanation is peritoneal irritation, ligament stress, or something no one has thought of yet, the stitch itself remains mercifully temporary.