Obstacle course racing (OCR) is a sport in which a competitor, traveling on foot, must overcome various physical challenges in the form of obstacles. Races vary in length from courses with obstacles close together to events of several kilometers which incorporate elements of track, road and/or cross country/trail running. Courses may include climbing over walls or up ropes, monkey bars, carrying heavy objects, traversing bodies of water or mud, crawling under barbed wire, and jumping through fire. Since the beginning of modern OCR in 1987, the sport has grown in popularity such that more than 2500 events are held annually across the world and several run organizing companies are commercially successful.