Human runners broke the four- minute mile in 1954 and have been getting faster and faster ever since.

But the winning times for horses in classic races like the Kentucky Derby have remained surprisingly static. Secretariat's world-record-breaking Derby time of 1:5925, for example, set in 1973, remains unsurpassed more than a quarter of a century later. Monarchos, this year's Derby winner, clocked the second-fastest winning time in history, running the race in 1:59.97, but still did not break Secretariat's record.

The reasons, scientists say, may have to do both with the unique physiology of the horse and the nature of the sport.

Horses are designed to run, their fleetness of foot having evolved over millions of years as a strategy of escape from predators. At one hour old, a foal is on its feet; at two hours, it is ready to go. And the horse's natural running ability has been nurtured and enhanced in thoroughbreds, which have been selectively bred for racing since the 17th century.

So while people require years of training and daily practice, thoroughbred racehorses enter the world much closer to their performance limits, said Dr. James Rooney, an emeritus professor at the University of Kentucky and an expert on equine biomechanics.

"The human is not, from the point of view of construction, a particularly good thing to start with and be an athlete," Dr. Rooney said. "But the horse is born to be an athlete. And the more they learn about horse physiology, the more people begin to realize that this animal has evolved to a certain point and you can't change it very much."

It is the best racehorses, those that compete in top stakes races like the Derby, that appear to be the closest to hitting a physiological speed barrier. In a study of racing performance between 1952 and 1977, for example, Dr. Patrick Cunningham and Dr. Barry Gaffney of Trinity College in Dublin found that though winning times for elite races had remained stagnant, horses as a whole were improving, at the rate of about 1 percent a year as calculated by a formula based on handicapping.

"Even though the winners are not getting better, the losers are getting better," Dr. Cunningham said.

Dr. Cunningham and Dr. Gaffney speculated that the elite horses may have been limited by the rate at which lactic acid, a waste product produced in strenuous exercise, can be cleared from the muscles or oxygen circulated in the bloodstream.

Yet an equally significant reason that records are not being broken may have to do with the goals of racing itself: racehorse owners do not necessarily aim to beat the clock, or to improve the speed of the breed as a whole. They simply want their horses to beat other horses.