When Jet Run was racing at Charle Town, he concealed any evidence that he possessed ability.

Even by the standards of the infirm, ill-bred and untalented creatures who compete in West Virginia, he was a very bad racehorse. Jet Run started three times as a 2-year-old in 1970 and never finished better than sixth. He tried four times the next year and still never earned a cent. When he struggled home near the tail end of a field of rock-bottom $1,500 claimers, trainer Dale Jenkins gave up on him.

Horses with similar records of futility have wound up in the glue factory Instead, Jet Run wound up in Capital Centre. He is the principal equine celebrity in the Washington International Horse Show this week. And next year, he and his 28-year-old rider, Michael Matz, will give the United States its best hope for a gold medal in the Olympics.

If Jet Run's status in the horse world defies his origins, so does Matz's. Most patrons at Capital Centre probably assume that horse shows are a game strictly for Middleburg millionaires, but Matz doesn't fit that stereotype. He is the son of a Reading, Pa., plumber, and got his first exposure to horses when he was driving a tractor, cutting grass, on a farm near his home.

The owner of the farm bought a couple horses and invited Matz to go riding. Pretty soon the teen-ager was hooked. He weighed 98 pounds and, he remembered, "It was a pretty big challenge to take that big animal and do things with him."

From that time on, Matz's life was similar to that of any kid who wants to be a jockey. He dreamed the same sort of dreams. He rode every chance he could. He went to horse shows, observed the riders' techniques, then came home and practiced them. He slept in tack rooms. His parents badgered him about getting a job. He hoped for some miraculous break.

"I was waiting for an opportunity," he said, "but I had no idea what it was going to be."

Like would-be jockeys, aspiring horse show riders need a successful trainer to take them under wing. One day Matz was riding at a horse show in Devon, Pa., when he attracted the attention of trainer Jerry Baker. "If you come with me," Baker said, "you'll never regret it." Matz never regretted it.

While Matz was honing his skills, fate was beginning to bring him and Jet Run together. The horse had been sold at the end of his racing career to an owner of show horses, and quickly began to display more of a liking for this new game. He was no longer a $1,500 animal nobody wanted. Jet Run was sold again, for $25,000, then again for a reported $250,000 to a wealthy Mexican businessman, Fernando Senderos.

When Senderos rode Jet Run to victory in the 1975 Pan American Games. it was a bit like the inept Ron Franklin holding on for dear life when Spectacular Bid won the Kentucky Derby. It was a triumph of horse over rider. Two years later, Senderos sold Jet Run to F. Eugene Dixon, the multimillionaire owner of the Philadelphia 76ers. Dixon's trainer was Jerry Baker, and now Matz had the opportunity of a lifetime.

Matz already had risen to the upper stratum of his profession because he possessed the essential skill of a show rider: the ability to communicate with a horse.

"A lot of this sport," he said, "is psychological. You have to learn not to get excited. If you let yourself get worried you give that feeling to the horse. They can sense it."

Nowhere is this subtle form of communication more crucial than when a half-ton animal is preparing to leap over a five-foot wall.

"I try to think what the horse is feeling." Matz sand. "How is he warming up? What did he do the day before? I feel him out going to the first fence. I see how he reacts to my aids. If I tell him to shorten stride but he's not listening, I know I have to be more forceful. If he's slow, if he's creeping over the fence, I know I have to be stronger with my legs. Ideally, you go to a fence completely balanced, with the most delicate feel of the horse's mouth, and you just sit and follow him over the top."

All the top riders possess this almost mystical sort of rapport with their horses.

"When you're talking about the 20 or 25 best riders," Matz said, "90 percent of it comes down to who has the best horse."

A few years ago, it was Rodney Jenkins who dominated the show circuit riding the great Idle Dice. It might still be Jenkins today if his brother Dale had not sold an unpromising 3-year-old who couldn't win at Charles Town.

"But now," Matz said, "I have the best. Jet Run is a special horse. He's very intelligent. He's very careful.He has tremendous stamina."

Matz finished third abroad Jet Run in the 1978 World Championships, and won the 1979 Pan American Games, but he is obsessed by the 1980 Olympics, which will give him the chance to redress the greatest frustration of his career.

He was riding a horse named Grande for the U.S. equestrian team in Montreal, when even a decent performance in the final round would have clinched a bronze medal. When Matz's mount knocked over five rails, America's hopes went down with them.

"You know on Wide World of Sports, when they talk about 'the thrill of victory and the agony of defeat'? Well, I know about the agony of defeat. There were 60,000 people there and I felt like I didn't have a friend in the stands

"When I got Jet Run, I told myself I wouldn't let that happen again. I'll never have another opportunity like this. I may be no special rider, but with that horse I'm special."