Two classic causes of a "side stitch" are running and prolonged laughter. Those activities have at least one thing in common: exertion of the diaphragm. "When you laugh really hard, you're sucking in a lot of air, which fills the lungs and pushes down on the diaphragm while the abdominal muscles are also contracting and pushing up on the diaphragm," explains Robert Gotlin, D.O., a sports physician at Beth Israel Medical Center and former director of orthopedic rehab with the New York Knicks.
All of which, of course, happens scores of times each minute when you're howling. The repeated compression can produce a muscle spasm that we all know as a stitch. "Sometimes when you laugh a lot, you get a pain in your right arm as well as the side stitch. That's because the nerve that supplies the diaphragm also goes to the right shoulder," he says. So, in addition to busting your gut, a hearty laugh can mistakenly make you think you're having a heart attack. Try breaking the rapid cycle of diaphragm punishment that we call laughter by slow, deep breathing between fits of hysteria. And avoid eating big meals, which draw blood to the stomach, before settling in for an evening of "30 Rock" reruns.
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