Running is great exercise. A common side effect of running is known as “stitches” in the side. This can be a very sharp, painful condition that feels like a knife stuck in your side that can stop you in your tracks.
Current literature and testing have provided results to confirm that a combination of muscle spasms and visceral ligament stretching contributes to this intense pain.
Both the diaphragm and the intercostals between each rib are the primary muscles involved in this pain. Another cause of the pain is due to the altered blood supply to visceral ligaments associated with the side lung area.
When you run at a faster pace than your body is conditioned to handle, you require an intense amount of oxygen. The breathing becomes labored and results in the body’s own autonomic response of hyperventilation.
This process of fast paced breathing often results in a spasm in the muscles whose primary function is to breathe. Sedentary breathing does not require the intense contractions required for deeper breathing, which is required during intense activity.
Resource: OnFitness Journal