You are twenty minutes into a run, finally finding a rhythm, and then a sharp cramp digs in just under your ribs on one side. It forces you to slow down, press a hand into it, and wonder what you did wrong. Almost every runner has felt it, and most have no real idea what it is. The medical name is exercise-related transient abdominal pain, which researchers shorten to ETAP. It is one of the most common complaints in running, swimming, and horseback riding, yet it stays poorly understood by the people who deal with it most. The good news is that the leading explanations point to fixes you can actually control.
A side stitch is almost always felt on the side, usually the right, just below the rib cage. It can range from a dull ache to a stabbing cramp that stops you cold in the middle of a workout. It tends to show up more in newer runners and fades as fitness improves, though even trained athletes are not immune. It is not dangerous and leaves no lasting harm, but it can wreck a run or a race in seconds. Understanding it starts with knowing what it is not. It is not your appendix, not a heart problem, and not a sign that something inside you is torn.
The oldest explanation blames the diaphragm, the sheet of muscle under your lungs that drives every breath. The idea is that hard exercise fatigues the diaphragm or reduces its blood supply, and the resulting cramp radiates as that familiar pain. Shallow, rapid breathing during a run makes it worse by keeping the diaphragm from fully relaxing between breaths. This theory explains why deep, controlled breathing often relieves a stitch mid-run. It also explains why the pain eases when you slow down and let your breathing settle. For years this was the standard answer, and it still holds part of the truth.
Newer research points somewhere else, to the peritoneum. That is the double-layered membrane lining your abdominal cavity, with one layer wrapping your organs and the other lining the wall. The two layers normally glide against each other with almost no friction. When you run, jostling and a full stomach can increase that friction or irritate the lining, producing the sharp, localized pain of a stitch. This model fits the evidence better than the diaphragm theory in several studies. It also explains why eating or drinking too close to a run so reliably brings a stitch on.
That connection to food and drink is the most useful clue you have. Eating a large meal within two hours of exercise is one of the strongest triggers there is. Sugary drinks and fruit juices before a run make stitches more likely than plain water does. Posture matters too, since slouching compresses the abdomen and stresses the connective tissue involved. Weak core muscles show up again and again in runners who get frequent stitches. Put those factors together and a clear pattern emerges, one you can plan around rather than simply endure every time you lace up.
When a stitch hits mid-run, a few things reliably help. Slow your pace and breathe deeply from the belly rather than shallowly from the chest. Try exhaling hard as the foot opposite the pain strikes the ground, which shifts the mechanical stress off the sore area. Pressing gently on the tender spot while bending slightly forward can ease it within a minute. If it lingers, walking for thirty seconds almost always resets things. Stretching the same-side arm up overhead opens the torso and can release the cramp when nothing else does.
Preventing stitches is far more reliable than treating them once they start. Stop eating heavy meals at least two hours before you run, and skip sugary drinks right beforehand. Build core strength through basic planks and similar work, since a stable trunk resists the irritation that causes the pain. Warm up gradually instead of launching into a hard pace from a standstill. Pay attention to your posture and run tall rather than hunched over. None of this guarantees you will never feel another stitch, but together these habits make them rare enough that they stop shaping your runs.
