The Science Behind the Stitch

What a Stitch Actually Feels Like

ETAP most commonly shows up along the sides of the mid-body, near the lower ribs, though it isn't confined to one spot and can occur elsewhere too. When it's mild, it tends to feel like a cramp, ache, or dragging sensation. When it's more severe, it turns sharp and stabbing. 

Some people also feel it as a referred pain at the tip of the shoulder, indicating it’s linked to the diaphragm since the phrenic nerve carries sensation from both the diaphragm's lining and that spot on the shoulder. When one of them is irritated, your brain can read the signal as coming from the other.

It's also not just a beginner's problem. Well-trained and elite athletes still get stitches, just less often than less-conditioned people.

The Leading Theory: Friction Inside Your Abdomen

For most of the last century, doctors assumed a stitch was caused by your diaphragm running out of blood or oxygen, or by heavy organs tugging on the ligaments that suspend them from the diaphragm. Both ideas have mostly been ruled out. 

In one study, researchers had people perform breathing tests during an actual stitch episode and found lung function was completely unaffected, arguing against the diaphragm running low on oxygen. In a separate study, researchers attached sensors to the pain site to measure electrical activity in the muscle (the same kind of signal that shows up during a true muscle cramp), and found no such activity during a stitch, ruling out a simple muscle cramp too.

The theory that's held up best is friction. Your abdominal organs are wrapped in a membrane called the peritoneum, with one layer coating the organs themselves, and another layer called the parietal peritoneum lining the inside of your abdominal wall and the underside of your diaphragm. During exercise, especially anything involving repetitive torso movement, these two layers can rub against each other. Irritation of this parietal layer explains more of the actual features of a stitch than any competing theory: the varied location, why it worsens after eating or drinking since a fuller stomach presses the layers together more firmly, and why it's sometimes felt at the shoulder.

The visceral ligament theory, where the weight of organs like the liver tug on the ligaments that suspend them from the diaphragm, hasn't been ruled out entirely. It does a good job of explaining why low-intensity but jolting activities like horse riding cause stitches despite barely raising your breathing rate. But, it struggles to explain why stitches also happen low in the abdomen, or why swimmers get them at a meaningful rate despite there being little "jolting" involved. 

Right now, peritoneal friction is considered the better explanation of the two, though researchers are honest that the full picture isn't settled.

What Actually Triggers a Stitch

A few factors come up again and again across the research:

Eating or drinking too close to exercise. A large stomach volume presses the two peritoneal layers together more firmly, giving them more surface contact to rub against as you move.

What you drink matters as much as how much. Hypertonic drinks, meaning those heavily concentrated in sugar or salt, tend to increase both the likelihood and severity of a stitch compared with isotonic drinks or plain water. The likely reason is that the fluid inside your peritoneal cavity is sensitive to the same osmotic gradients as the rest of your body, so a strongly hypertonic drink may be doing something to that fluid layer directly, not just adding stomach volume.

Poor posture, especially through the upper back. People with kyphosis, an excessive rounding of the upper back, report more frequent and more severe stitches, and this connection was actually first noted almost a century ago, well before the peritoneal friction theory existed.

Age and experience. Younger and less experienced athletes are more susceptible, regardless of body type or sex.

The activity itself. Activities like running and horseback riding, which involve repetitive torso movement in an extended posture, tend to trigger stitches considerably more often than something like cycling, which flexes the torso and involves comparatively little movement.

Core strength. Runners with weaker trunk muscles including a thinner transversus abdominis, a deep core muscle, report more stitch symptoms than those with stronger cores. This suggests better core stability may limit how much your organs and abdominal wall can shift against each other.

Preventing and Managing a Stitch

Aside from avoiding the activities that trigger stitches, there are a few things you can do to reduce the likelihood:

Give yourself real time between eating and training. Aim for at least 2 hours before exercise, and closer to 3–4 hours if you're someone who's more prone to stitches. 

Avoid heavily salted, sugary drinks. If you do need to drink close to or during a session, favour drinks that lean more isotonic, and take smaller, regular sips rather than large volumes at once. Blonyx Hydra+ is formulated to be isotonic when mixed with 8oz of water, or hypotonic when mixed with 16oz, so you get carbohydrates and electrolytes into your system without leaning on a concentrated, hypertonic load that's been linked to worse stitch symptoms.

Work on posture and core strength. Improving thoracic posture and strengthening the transversus abdominis specifically have both been linked to fewer stitch symptoms in the studies above. Staying consistent with training also seems to help as well-conditioned athletes report stitches less frequently, even if the severity is about the same when it happens.

If a stitch does hit mid-session, the most commonly reported relief techniques are deep breathing, pushing on the affected area, bending forward, and stretching the site. What actually helps seems to vary from person to person, so treat them as tactics worth experimenting with, especially as none have been rigorously proven to work. The one strategy that will stop a stitch is slowing down or stopping, but unfortunately that isn't always practical mid-race.

Key Takeaways

While the exact cause of a stitch is still considered unresolved, peritoneal friction explains the most evidence. Here's what we do know so far:

A stitch is medically called ETAP, and it's extremely common with about 70% of runners experiencing one every year, and roughly 1 in 5 people getting one during any single running event.

The leading explanation is friction between the layers of tissue lining your abdomen and organs, not a lack of blood flow to your diaphragm or a simple muscle cramp. Both of those older theories have been directly tested and don't hold up.

Big meals, big drinks, and hypertonic beverages close to exercise are the clearest triggers. Poor upper-back posture and weaker core strength are also linked to more frequent and severe stitches.

Prevention beats treatment. Eating earlier, sipping smaller volumes of a lighter drink during exercise, and building core and postural strength all help reduce stitches while in-the-moment relief techniques like pushing on the area that hurts are more hit-or-miss.