The possible causes of leg pain when exercising include excessive exercise, muscle strain, and myofasciitis. 1. Excessive exercise: it may be the lack of exercise in general, suddenly exercise too much, resulting in an abnormal accumulation of lactic acid in the body, which can not be eliminated within a short period of time, thus appearing the phenomenon of leg pain when exercising. 2. Muscle strain: it may be in the process of exercise, due to excessive exercise or improper exercise, resulting in muscle strain in the legs, causing symptoms of leg pain. 3. Myofasciitis: it may be due to a sudden increase in the amount of exercise, resulting in fatigue of the myofascia, which produces aseptic inflammation, i.e. myofasciitis. There may be other reasons for leg pain when you exercise, so it is recommended that you go to the hospital in time to improve the examination, clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.