Off-road duathlon is a form of duathlon, where the competitors have to go through a trail-running stage and a mountain-biking stage.
Off-road duathlons are distinguished from conventional duathlons in that the terrain for the cycling 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.
References
Off-road duathlon Wikipedia(Text) CC BY-SA