Series history
The series began in 1972 with the introduction of two classes based on 500 cc and 250 cc engine displacement formulas. [5] A 125 cc class was added in 1974. As motocross technology developed, 500 cc two-stroke motocross bikes became too powerful for the average rider and, faced with diminishing numbers of competitors, the AMA discontinued the 500 cc class after the 1993 season. A women's national championship series was introduced in 1996. [6]
Facing tightening federal emissions regulations in the United States, the A.M.A. increased the allowable displacement capacity for four-stroke engines in 1997, in an effort to encourage manufacturers to develop environmentally friendlier four-stroke machines. [7] Due to the low relative power output of four-stroke engines, compared to the then-dominating two stroke design, the AMA had increased the allowable displacement capacity for four-strokes. By 1994, the displacement limit of a four-stroke power motocross bike was up to 550 cc in the 250 class, to incentivize manufactures to further develop the design for use in motocross. [8]
In 2006, the 250 cc division was renamed the MX Class, with an engine formula allowing for 150–250 cc two-stroke or 250–450 cc four-stroke machines. [9] The 125 cc class was renamed the MX Lites Class, allowing 0–125 cc two-stroke or 150–250 cc four-stroke engines. In 2009, the MX class was renamed the 450MX Class and the MX Lites Class was renamed the 250MX Class, to reflect the fact that all the competing manufacturers had adopted four-stroke machinery.
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Brad Lackey (Kawasaki)
Pierre Karsmakers (Yamaha)
Jean-Michel Bayle (Honda)
Greg Albertyn (Suzuki)
Stefy Bau 
Tania Satchwell
Steffi Laier
Chad Reed (Suzuki)
Dean Wilson (Kawasaki)
