When running, a stitch is most commonly caused by a spasm of the diaphragm, the muscle that plays a key role in breathing by separating the chest from the abdominal cavity. This spasm can happen when the diaphragm fatigues or cramps due to the increased breathing demand and physical stress of running. Other contributing factors can include weak core muscles, eating too close to running (especially heavy or sugary meals), shallow or improper breathing, and the physical jarring motion of running that strains ligaments connected to the diaphragm and internal organs like the liver. This strain or irritation leads to the characteristic sharp pain known as a side stitch. Side stitches are often felt on the right side just below the ribs, where the liver’s ligaments pull on the diaphragm during the up-and-down motion of running. Breath patterns and muscle fatigue also play a role, as shallow breathing can stress the diaphragm and surrounding muscles. To summarize:

• Diaphragm muscle spasm due to fatigue and stress from fast or long runs
• Weak core muscles causing lack of stability
• Digestive factors like eating too soon before running
• Improper breathing techniques (shallow or uncoordinated)
• Jarring motion stretching ligaments connecting diaphragm and organs

These factors combine to cause the sharp, localized pain commonly called a stitch during running.