lower long track world records considerably; from 1971 to 2009, the average speed on the men's 1500 metres has been raised from 45 to 52 km/h. Similar speed increases are shown in the other distances. Professionalism After the 1972 season, European long track skaters founded a professional league, International Speedskating League, which included Ard Schenk, three-time Olympic gold medallist in 1972, as well as five Norwegians, four other Dutchmen, three Swedes, and a few other skaters. Jonny Nilsson, 1963 world champion and Olympic gold medallist, was the driving force behind the league, which folded in 1974 for economic reasons, and the ISU also excluded tracks hosting professional races from future international championships. The ISU later organised its own World Cup circuit with monetary prizes, and full-time professional teams developed in the Netherlands during the 1990s, which led them to a dominance on the men's side only challenged by Japanese 500 m racers and American inline skaters who changed to long tracks to win Olympic gold. North American professionals During the 20th century, roller skating also developed as a competitive sport. Roller-skating races were professional from an early stage. Professional World Championships were arranged in North America between the competitors on that circuit. Later, roller derby leagues appeared, a professional contact sport that originally was a form of racing. FIRS World Championships of inline speed skating go back to the 1980s, but many world champions, such as Derek Parra and Chad Hedrick, have switched to ice in order to win Olympic medals. Like roller skating, ice speed skating was also professional in North America. Oscar Mathisen, five-time ISU world champion and three-time European champion, renounced his amateur status in 1916 and travelled to America, where he won many races but was beaten by Bobby McLean of Chicago, four-time American champion, in one of the races. Chicago was a centre of ice speed skating in America; the Chicago Tribune sponsored a competition called the Silver Skates from 1912 to 2014. Short track enters the Olympics In 1992, short track speed skating was accepted as an Olympic sport. Short track speed skating had little following in the long track speed skating countries of Europe, such as Norway, the Netherlands and the former Soviet Union, with none of these nations having won official medals (though the Netherlands won two gold medals when the sport was a demonstration event in 1988). The Norwegian publication Sportsboken