You are halfway through a run when a sharp pain suddenly appears just below your ribs.
It may feel like a cramp, a stabbing sensation, or as though someone has poked a finger deep into your side.
You slow down, press your hand against the painful spot, and wonder what exactly inside your body has gone wrong.
This familiar problem has a medical name: exercise-related transient abdominal pain, or ETAP.
Most people simply call it a side stitch.
It is extremely common, especially among runners—but surprisingly, scientists still do not know its exact cause.
A Side Stitch Is Not Necessarily a Muscle Cramp
Calling it a “stitch” or “side cramp” makes it sound as though an abdominal muscle has suddenly contracted.
That may not be what is happening.
The pain can appear in different parts of the abdomen, although it is especially common beneath the ribs and often occurs on the right side.
Some people describe a sharp stabbing sensation, while others experience aching, pulling, or cramping.
It generally disappears relatively quickly once exercise slows or stops.
Running Is Particularly Good at Triggering It
Side stitches can occur during many forms of exercise, including swimming, horse riding, basketball, and cycling.
But running is one of the activities most strongly associated with them.
Research has found that around 70 percent of runners experience a stitch at least once during a typical year.
That may be because running repeatedly moves the torso up and down while the body remains relatively upright.
Every stride produces another small jolt through the abdomen.
The Old Diaphragm Explanation May Not Tell the Whole Story
One traditional explanation blamed the diaphragm—the large muscle beneath the lungs that plays a major role in breathing.
The idea was that intense exercise redirected blood toward working leg muscles, temporarily reducing blood flow to the diaphragm and causing it to cramp.
It sounds plausible, especially because stitches often occur when people are breathing heavily.
But the theory has problems.
Side stitches can occur during activities that do not place especially high demands on breathing, and the location and characteristics of the pain do not always match what would be expected from an oxygen-starved diaphragm.
As a result, researchers no longer consider diaphragm ischemia a complete explanation.
The Abdominal Lining May Be Involved
One of the leading modern theories involves a membrane called the peritoneum.
The peritoneum consists of thin layers that line the abdominal cavity and cover many of the organs inside it.
Normally, these surfaces can move smoothly against one another.
Researchers have proposed that repetitive movement during exercise may irritate part of this lining, particularly the outer layer known as the parietal peritoneum.
That theory helps explain several characteristics of a stitch, including its sharp and localized nature and its association with activities involving repeated torso movement.
However, it remains a theory rather than a conclusively proven mechanism.
Eating Before Running Can Increase the Risk
Many runners discover this through experience: a large meal followed immediately by a run is not an ideal combination.
Studies of side stitches have found that consuming large amounts of food or fluid shortly before exercise can make ETAP more likely.
A full stomach may place additional pressure inside the abdominal cavity or alter how internal tissues move during exercise.
The problem seems particularly associated with large volumes consumed relatively close to the start of activity.
This does not mean everyone needs to exercise on an empty stomach. Individual tolerance varies considerably.
Very Sugary Drinks May Be a Particular Trigger
Research has also found an association between side stitches and certain concentrated drinks.
Highly sugary beverages may be more likely to provoke ETAP than smaller quantities of water or more dilute fluids.
That does not mean drinking water causes stitches.
Staying hydrated during exercise remains important.
The practical distinction is between normal hydration and rapidly filling the stomach with a large amount of fluid immediately before or during intense movement.
Why Is the Pain Often on the Right Side?
Many people report stitches beneath the lower ribs on the right side of the abdomen.
This has inspired theories involving the liver, which sits largely on the right side of the body, and the ligaments supporting abdominal organs.
One older theory suggested that running caused heavy organs such as the liver to pull repeatedly on connective tissues attached near the diaphragm.
But side stitches can appear on either side and even in the lower abdomen, making a simple “the liver is bouncing” explanation unlikely to account for every case.
Posture May Play a Role
Some studies have found that people with certain postural characteristics—particularly increased curvature of the upper spine—report side stitches more frequently or more severely.
One possible explanation is that posture changes tension on nerves or tissues associated with the abdominal lining.
This is another reason the phenomenon is difficult to reduce to a single cause.
Food, movement, posture, individual anatomy, and exercise intensity may all influence whether a stitch develops.
Why Are Children and Younger People More Prone to Them?
Side stitches appear to be more common among younger athletes.
Susceptibility generally decreases with age, although adults can certainly still experience them.
This pattern may relate to changes in body structure, tissue sensitivity, experience with exercise, or several factors working together.
Interestingly, highly trained athletes are not completely immune.
Even experienced runners can occasionally develop a stitch.
What Should You Do When a Stitch Appears?
There is no universally proven instant cure.
Most strategies come from athlete experience rather than strong clinical trials.
Common approaches include:
- slowing your pace
- walking temporarily
- taking slow, deep breaths
- pressing gently on the painful area
- bending slightly forward
- stretching the affected side
The most reliable solution is usually reducing or stopping the activity that triggered it.
Because ETAP is transient, the pain typically fades as the mechanical stress of exercise decreases.
Can You Prevent Side Stitches?
There is no guaranteed method, but several habits appear reasonable based on the available research.
Avoiding a very large meal or large amount of fluid shortly before vigorous exercise may help.
Some researchers recommend allowing roughly two hours between consuming a large quantity and beginning exercise, particularly if you know you are susceptible.
Other commonly suggested approaches include:
- warming up gradually
- avoiding large amounts of concentrated sugary drinks immediately before running
- improving core strength
- maintaining comfortable upright posture
- increasing running intensity progressively rather than suddenly
Evidence for many of these strategies is still limited, so what works for one runner may not work for another.
Is Breathing Incorrectly the Cause?
Runners are sometimes told that a stitch occurs because they are “breathing wrong.”
There is no good evidence that one incorrect breathing pattern explains the condition.
However, slowing down and deliberately breathing more deeply can help some people when a stitch occurs.
That may reduce tension in the torso, change movement patterns, or simply accompany the reduction in exercise intensity.
Breathing rhythm can therefore be useful for managing a stitch without necessarily being its original cause.
Could It Be Something Other Than a Stitch?
A typical side stitch appears during exercise and disappears when the activity slows or stops.
Persistent abdominal or chest pain should not automatically be dismissed as a stitch.
Pain that occurs when you are not exercising, becomes increasingly severe, repeatedly appears in exactly the same location, or comes with symptoms such as fever, fainting, difficulty breathing, or vomiting may have another cause and deserves medical evaluation.
It Is Surprisingly Common for Something We Understand So Poorly
Side stitches are a good example of how an everyday bodily experience can remain scientifically mysterious.
Millions of people have experienced one. Athletes have described them for centuries.
Yet because the pain disappears quickly and is usually harmless, it historically attracted relatively little medical research.
That is changing, but the current answer remains more nuanced than the familiar “your diaphragm is cramping” explanation.
Final Thoughts
We get a stitch when running because vigorous movement somehow irritates structures inside or around the abdomen—but scientists have not yet identified one mechanism that explains every case.
The leading theory involves irritation of the abdominal lining, while factors such as repetitive torso movement, eating or drinking large amounts before exercise, posture, and individual anatomy may influence the risk.
The reassuring part is that a typical side stitch is temporary and harmless.
Slow down, breathe, give your body a moment to recover, and it will usually disappear almost as suddenly as it arrived.
So the next time a sharp pain interrupts a perfectly good run, you can at least appreciate one strange fact:
you are experiencing something extraordinarily common that science still has not completely explained.
Key Takeaways
- A side stitch is medically known as exercise-related transient abdominal pain, or ETAP.
- It is especially common in running and other activities involving repetitive torso movement.
- The exact cause remains uncertain.
- The once-popular theory that a stitch is simply a diaphragm cramp does not explain all of the evidence.
- Irritation of the abdominal lining is currently one of the leading explanations.
- Large amounts of food or fluid shortly before exercise may increase the likelihood of a stitch.
- Slowing down, deep breathing, gentle pressure, and stretching may help during an episode.
- Persistent or unusual pain should not automatically be assumed to be a harmless side stitch.
