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The knee joint is injured often. Probably not as often as the shoulder. It can happen while running from overuse or too much too soon . More commonly the cause is accidental, like a fall, a collision or tripping. According to one orthopedist running is one of the most bio-mechanically efficient activities for people of normal weight. If you have knee pain check with your doctor, therapist or orthopedist. Once he determines your knee is ready to increase activity. You can start a running/walk program with a conservative gradual increase of not more than ten percent increase in any week, month or year, according Jeff Galloway a long time running coach and expert. One suggestion I would make is the couch potato to 5k which has been popular in recent years.