Off-road duathlon is a form of duathlon

Duathlon

Duathlon is an athletic event that consists of a running leg, followed by a cycling leg and then another running leg in a format bearing some resemblance to triathlons. The International Triathlon Union governs the sport internationally....

, where the competitors have to go through a trail-running

Cross country running

Cross country running is a sport in which people run a race on open-air courses over natural terrain. The course, typically long, may include surfaces of grass and earth, pass through woodlands and open country, and include hills, flat ground and sometimes gravel road...

 stage and a mountain-biking

Mountain bike

A mountain bike or mountain bicycle is a bicycle created for off-road cycling. This activity includes traversing of rocks and washouts, and steep declines,...

 stage.

Off-road duathlons are distinguished from conventional duathlons in that the terrain for the cycling

Cycling

Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are cyclists or bicyclists...

and running stages are generally unpaved, rough, and very steep and hilly. They require different techniques than conventional duathlon races, and the athletes employ mountain bikes rather than road bikes.