Thrilling obstacle courses

Part of a team of runners take a tumble in the final muddy obstacle near the end of the 10K race course at the 20th annual Marine Corps' World Famous Mud Run at Camp Pendleton, June 1, 2013. (Don Bartletti/Los Angeles Times/MCT)

Obstacle races are huge in the United States, where mud and risk are par for the course.

WITH macho-sounding names like Tough Mudder, Warrior Dash and the Spartan Race, obstacle course races in the US have turned mud, sweat and tears into a flood of revenue.