What is the biological explanation of the feeling often called “a stitch in your side” while exercising?
The pain referred to as a "stitch" is caused by movement of blood away from the diaphragm to the limbs during exercise [1]. The diaphragm is the muscle that separates the stomach and abdomen from the heart and lungs, and it is one of the main muscles involved in breathing [1]. Some studies suggest that the pain is caused by a reduction in blood supply to the diaphragm, which cramps due to the movement of blood away from it [1, 2]. Additionally, some research shows that an irritation of the lining of the abdominal or pelvic cavity may be the cause of the pain [2]. When there is a lot of movement and friction in the torso, this irritation can occur and lead to localized pain in different areas, including the tip of the shoulder [2]. Experts currently refer to this pain as "exercise-related transient abdominal pain" or ETAP [3].
The biological explanation for the feeling often called “a stitch in your side” while exercising is not known [1]. Some studies have shown that a movement of blood to the diaphragm or muscles during physical activity can lead to a side stitch [1]. It is also possible that an irritation of the lining of the abdominal and pelvic cavity may be the cause [1], or that jolting movements during exercise put stress on the ligaments in the abdomen that hold the organs in place [4]. Posture or problems with gait or form could also be a factor [3]. One thing we do know is that a side stitch is not a muscle cramp [3]. It is common for side stitches to affect younger people more often than older people [2].