Diaphragm spasm
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A diaphragm spasm, commonly referred to as getting the wind knocked out of you or getting winded, occurs when sudden force is applied to the abdomen which puts pressure on the solar plexus. It results in a temporary paralysis of the diaphragm that makes it difficult to breathe for a short period of time. It can also occur from a strong blow to the back.
When the abdomen is struck, a large difference in pressure occurs across the diaphragm. The diaphragm then stretches, which also stretches the diaphragm's nerves. The resulting mechanical force puts the diaphragm into a muscle spasm, comparable to having a charlie horse in the leg. It takes a few seconds for the diaphragm to relax again before breathing can resume normally. Martial artists are often taught to breathe out heavily when struck in the stomach, to minimize this effect.