While his teammates were working at Redskin Park yesterday, Mark Moseley was performing at Robert F. Kennedy Stadium. He was wearing the familiar No. 3 on his jersey, but the only thing he was kicking was the starter of a 125cc motorcycle.

"He promised not to do that," said Moseley's agent, Carolyn Wells, as she watched her million-dollar property fly up and over a series of dirt ramps that have been built on the playing field for today's Superbowl of Motocross. "He said he would ride but not race."

Minutes later, when some of the 60 professional riders in today's competition took the track for a few practice spins, Moseley's speed was made tame by comparison. Between autographs and interviews, the NFL's most valuable player and a former amateur motocross competitor watched the riders long enough for his jaw to drop.

"It would scare me to death if I was to take one of those jumps like you're supposed to," said Moseley, who was once ranked third in the southwest in the late 1960s. "I've got more bumps and bruises on my body right now at 35 from riding a motorcycle than I do from football."

In the promotional blitz that has preceded the debut of Supercross in Washington, the machine-crunching, bone-breaking aspect of the 10-year-old sport has been emphasized. In this case, the hype does not seem to exceed the reality. The medical histories of some of the riders reads like an emergency room training manual. And there are a few ailments, like "dislocated kidney" which seem to break new medical ground.

Jeff Hicks is a 22-year-old rider from Ohio who knows what even a relatively minor injury can mean in such a competitive sport. Last year he was one of fewer than 20 riders sponsored by a Japanese motorcycle manufacturer. Then he sprained his ankle and had to miss a few important events. When his contract was up, another rider had taken his coveted spot.

"I'm getting by and making a little bit of money," said Hicks, one of the "privateers" who follow the tour in a caravan of cars and trucks, hoping to earn enough points and attention to win salaried jobs racing for one of the big four bike manufacturers--Honda, Yamaha, Suzuki or Kawasaki.

The gates to RFK open today at 1:30 p.m. The first of five rounds of racing begins at 3 p.m. and the main event of 20 laps is expected to be concluded by 6. Tickets start at $10.