Break down/broken legs in Thoroughbred horses. Studies are starting to point to surface as the major factor.

Thoroughbred weighs more than half a ton, and as he gallops, no more than two legs at a time absorb the shock of earthly contact at more than 35 miles per hour. Sue Stover, a professor at the University of California at Davis veterinary research laboratory, says that many runners, animal or human, wind up with microscopic bone damage because of the repetitive pounding of training and competition. Regularly, the body swaps such damaged tissue for new, but weak spots can develop if damage occurs faster than replacement.

That does not mean a bone will break. It means the risk of injury is higher. After examining sample tissue from racehorses that died or were euthanized after leg fractures, Stover's laboratory found that more than 90 percent had preexisting bone damage. Nor are fatalities the only threat, she said. Imagine a stable of 50 horses that race regularly. In three months, there might still be 50 horses, but a fifth will no longer be competing. Age or illness will have taken some, but muscular-skeletal injury will have sidelined the rest, at least temporarily. "It's huge, it's huge attrition," Stover said.

Nobody, certainly not a family on a Sunday outing, wants to see a mercy killing on the track. Nobody wants to lose a prized investment, either. Racing is a $10.7 billion-a-year business that employs 146,000 people, according to a 2005 study done for the American Horse Council. So there are reasons both humane and economic to reduce track fatalities, which average more than one a day in the United States and Canada.

One idea is to replace dirt track surfaces with more forgiving, man-made materials. The California Horse Racing Board, in fact, has ordered major Thoroughbred tracks in its state to switch to synthetic surfaces by January 1, 2008. But a dream solution involves finding an early-warning system. Researchers believe markers in the blood can telegraph whether a horse is experiencing risky bone damage, says McIlwraith of Colorado State. If so, the animal could be held out of training or competition until it heals. "We're not quite there yet," McIlwraith said, "but we're getting there."