A side stitch is usually a pain signal, not a torn muscle
That sudden, sharp pain during a run is commonly called exercise-related transient abdominal pain, or a side stitch. It often appears just below the ribs, though it can occur elsewhere in the abdomen. Despite how intense it feels, it usually passes when you slow down or stop; it does not generally mean an organ has been injured.
The exact source is still unsettled
There is no single proven explanation for every stitch. One leading idea is irritation of the parietal peritoneum, the thin, sensitive lining along the abdominal wall and underside of the diaphragm. Running repeatedly jostles the torso. The tugging and movement may irritate this lining, producing a small, sharply localized pain—rather than the diffuse ache associated with many internal organs.
The diaphragm, the main breathing muscle beneath the lungs, has also been implicated. Its movement and the tissues connected to it may contribute, but the familiar explanation that a stitch is simply a cramp from breathing too hard is not established as the whole story.
Why pace and a full stomach can matter
A stitch is more likely to show up when the torso is bouncing and the body is working hard. Eating a large meal or drinking a lot shortly before exercise can also make symptoms more likely for some people. Slowing down, changing posture, or pausing often eases the repeated motion that may be aggravating the area.
Because stitches are usually brief and tied to exercise, they are different from pain that persists after stopping or comes with other concerning symptoms. The useful distinction is that a stitch can feel dramatic while still being a temporary response to movement—not proof that something inside has been damaged.
