The sport of cross-country skiing encompasses a variety of formats for cross-country skiing races over courses of varying lengths according to rules sanctioned by the International Ski Federation and by various national organizations, such as the U.S. Ski and Snowboard Association (USSA) and Cross Country Ski Canada. International competitions include the FIS Nordic World Ski Championships, the FIS Cross-Country World Cup, and at the Winter Olympic Games. Such races occur over homologated, groomed courses designed to support classic (in-track) and freestyle events, where the skiers may employ skate skiing. It also encompasses cross-country ski marathon events, sanctioned by the Worldloppet Ski Federation, and cross-country ski orienteering events, sanctioned by the International Orienteering Federation. Related forms of competition are biathlon, where competitors race on cross-country skis and stop to shoot at targets with rifles, and paralympic cross-country skiing that allows athletes with disabilities to compete at cross-country skiing with adaptive equipment. Norwegian army units were skiing for sport (and prizes) in the 18th century. Starting in the latter part of the 20th century, technique evolved from the striding in-track classic technique to include skate-skiing, which occurs on courses that have been groomed with wide lanes for those using the technique. At the same time, equipment evolved from skis and poles that were made of wood and other natural materials to comprising such man-made materials as fiberglass, carbon fiber, and polyethylene plastics. Athletes train to achieve endurance, strength, speed, skill and flexibility at different levels of intensity. Offseason training often occurs on dry land, sometimes on roller skis. The organization of cross-country ski competitions aims to make those events accessible both to spectators and television audiences. As with other sports that require endurance, strength and speed, some athletes have chosen to use banned performance-enhancing drugs.