Extreme athletes banded together to design Reebok Spartan Races' intense courses orchestrated over standardized distances, each strewn with natural and man-made obstacles to test mind-body fitness, resilience, stamina, and strength, designed to leave participants exhausted and exhilarated. In waves of about 200, runners collect smudges and stains as they perform box jumps, haul heavy sandbags, and juke feral linebackers. Depending on where in the world they're participating, the course may be as short as 3 miles or, for extremely practiced athletes, as long as a full marathon.