AMAモトクロス
The new regulations resulted in competitors aboard four-strokes made by smaller European manufacturers, with Husqvarna, Husaberg, and KTM winning world championships on four-stroke machinery. In 1997, Yamaha unveiled a prototype 400cc four-stroke motorcycle, the YZM 400, which was debuted in the FIM Motocross World Championship. The motorcycle made its U.S. debut in 1997, where Yamaha Factory Racing rider Doug Henry led every lap of the main event at the 1997 AMA Supercross Finale and became the first person to win an AMA Supercross race on a four-stroke powered motorcycle. Following Yamaha’s release of the production model in 1998, Henry won the 250 AMA Motocross Championship and became the first person to win a major AMA Motocross title on a four-stroke powered motorcycle. This success motivated the remaining major manufacturers, Honda, Kawasaki, and Suzuki, to develop their own four-stroke motocross race bikes. By 2006, every manufacturer had begun competing with four-stroke machines in the AMA 125 (FIM MX2) and 250 (MX1) classes.