Distance runner Mary Decker is not one of the most stoic losers in American sports. "She's fiercely competitive," says one friend, "even when she cleans the house." So it's surprising that Decker, 21, was satisfied with finishing third last week in the 800 meters at the Athletics Congress National Finals. It is the distance at which she set a U.S. indoor record last February. "I was happy with my time and my race," she explained. "With the injury, I needed a place to start." The "injury," which had sidelined her for 10 weeks, was a strained tendon in her left foot. A more crucial test of how well it's healed is the Olympic Trials this week in her hometown of Eugene, Oreg., where she'll run the 1,500 meters.

Decker is a specialist at medical comebacks. After a three-month layoff in 1979 because of a pinched sciatic nerve, she returned last winter to set two world indoor records—800 meters (1:58.9) and 1,500 meters (4:00.8)—and run the fastest outdoor mile ever by a woman (4:21.7).

Decker began her career at 11 and by 14 was a world class competitor. However, she broke a foot bone that year while bicycling. At 15, after setting an earlier world indoor record at 800 meters, she suffered stress fractures in her right leg which kept her out of the 1976 Olympics. In 1977 she developed them in both legs and spent 12 weeks in casts.

Then Decker met Dick Quax, a brash New Zealander who won a silver medal in the 5,000 meters in the Montreal Games. When she described her medical problems to him, he pointed to scars on his own leg and suggested she might have compartment syndrome—the failure of the tissue sheath around the calf muscle to expand when the muscle grows. "At one point I was taking 12 aspirins a day for the pain and it only helped a little," she recalls. "Finally I couldn't walk." After two operations to relieve the constriction, Mary was able to resume running without pain. Later she was hindered by tendinitis and the sciatic nerve. In 1979 she managed just six races, winning three of them.

The New Jersey-born daughter of a tool-and-die maker, she was accident-prone even as a child. At 10, Mary moved with her family to California, finally settling in Garden Grove, where she fractured her skull in a motorcycle crash. On one weekend in 1970, when she was 12, Decker competed in seven events, including a marathon and one-and two-mile races. A few hours later she was taken to the hospital for an appendectomy. Twice in 1977 she escaped auto accidents unhurt; in one, her car rolled over four times.

A dropout from the University of Colorado, Decker moved to Eugene last July to become the only woman in Athletics West, a training program modeled on those in Eastern European countries. In addition to running, she is monitored for strength, body fat and stress.

Quax, who is separated from his wife and daughter, has just been appointed coach of Athletics West. Mary makes no secret of their close friendship, though until now Quax, 32, has spent much of the year in New Zealand. "It's been hard," she says, "when he was so far away. I date other people. People probably think I'm a flirt. I flirt with them and they flirt with me. I like to be looked at as a woman and not just as a runner."

Quax himself might have made it to Moscow as a marathoner, but like most New Zealand athletes he's boycotting the Games. His marathon debut last year, 2:11:13, was a record for first-timers. "The Olympics need track and field," Quax proclaims. "But we don't need the Games. If they were never held again, I wouldn't shed a tear."

Though Decker is sometimes volatile—she once threw a baton at a Russian who jostled her in a relay—she's surprisingly unemotional about the boycott. "I have at least 10 more years of competitive running in me," Mary says. "If I can't win an Olympic medal, maybe I can prove I'm best in other ways."