Even veterinarians don't fully understand how it could happen, how a physically sound thoroughbred can be racing fluidly at one moment and be hobbling on a shattered leg a split-second later.

But a half-ton animal moving at 40 miles an hour generates a lot of force, and if for some reason its shock-absorbing mechanism fails for an instant, the results can be explosive. Those are the terms with which veterinarians described what happened to the filly Anifa in the Washington, D.C. International: It was as if her leg exploded.

As the field reached the final turn of the International, Anifa was running just behind the leaders, poised to make her move and confirm that she was one of the best racehorses on either side of the Atlantic. Suddenly, as the crowd at Laurel let out a collective gasp, she came to an abrupt halt and could barely stay on her feet.

Even casual racing fans knew that what had happened to her was serious, probably fatal. They don't shoot horses any more, but even with the great advances made in veterinary medicine in recent years, the outlook for thoroughbreds who break their legs is still grim.

If Anifa had been an ordinary racehorse, her life would probably have ended that afternoon. Because of her immense potential value as a brood mare, efforts are being made to save her. The difficulties she has encountered suggest why less expensive horses are destroyed under similar circumstances.

Anifa was sent to the University of Pennsylvania's famed New Bolton Center, where veterinarians found that she had broken all four bones in her right ankle joint. The operation lasted for hours.

"The techniques for fixing fractures in horses are basically the same as those for any species," said Dr. Richard McFeely, associate dean at New Bolton."But horses have slender bones and long limbs that provide fairly small support. Anifa's case was analogous to you or I standing on our middle finger. That was the digit, and Anifa had fractured the bone into a number of pieces."

After such an operation, horses encounter a series of obstacles to their recovery, the first of which occurs when they come out from under the influence of an anesthetic. Like humans they are going to be woozy and disoriented, and some horses start thrashing wildly, reinjuring themselves. This is what happend to Ruffian after she broke her leg in 1973. New Bolton uses various techniques to help horses -- sometimes immersing them in a pool so they can't hurt themselves -- and Anifa got past the first hurdle.

The veterinarians had fitted Anifa with a cast, immobilizing the damaged area so the bones would knit. This, obviously, is not easy. "If you or I break a leg," McFeely said, "doctors can put us into a bed and tell us not to stand on it. But you can't put an animal between clean sheets."

Instead, vets put the 4-year-old filly into an intensive care stall, one that is specially padded and contains any emergency equipment the doctors might need. But there is a limit to what humans can do for a horse at this stage. The animal must have the temperament to adjust to its immobility and confinement.

"Some animals seem to understand and act sensibly," McFeely said. "Anifa has; she's acted very well. She's standing, moving, eating well. She has a lot going for her. She's quite a horse."

But even for such a model patient, McFeely acknowledged that the odds are unfavorable.

"Seriously injured animals have a tough time," he said. "Complications are very common. Time is the major factor, and I can't even give an approximation of how long it might take. We hope the healing will continue and the bones will come together without major complications. But based on our experience, the prognosis has to be extremely poor."