Why does my leg hurt the day after a long run?

There are many reasons why your leg can hurt after a long run, however delayed onset muscle soreness (DOMS) is the most common cause.  When you run, you create microtears in your muscles.  While these are not generally harmful, they can cause soreness in the muscles one to two days later.  Eccentric exercise, where the muscles are lengthening, is the most common cause of DOMS.  Gradually increasing your exercise routine can help to avoid some DOMS, however it is a common symptom of exercise.  It may actually help to be more active the day after a run to help alleviate some of the soreness, as increased blood flow helps repair the microtears in the muscles.  If you experience abnormally sore muscles that persist longer than normal after a workout, it is best to be evaluated by a doctor. 
After running for a long period of time, the body becomes sore; most soreness comes from eccentric movement. An example that stresses this eccentric movement is running down a hill. This soreness can last for hours or even days. It can be classified as delayed-onset muscle soreness (DOMS) and usually lasts 24-72 hours after exercise. (This answer provided for NATA by the Eastern University Athletic Training Education Program)

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