Enormous amounts of money are spent to acquire the best breeding stock for thoroughbred horse racing. The average time of the winning horse in the Kentucky Derby dropped steadily up through middle of the 20th Century (as shown in the red line above), yet there has been virtually no overall improvement in average time since 1950 (as shown in the blue line). If the improvements seen from 1896-1949 had continued, the average winning time today would be about three seconds faster.
Instead, Secretariat's 1:59.4 back in 1973 remains the Kentucky Derby's record, and only one other winner broke two minutes.
In contrast, in most human races (e.g., running, swimming, etc.), where there is no organized artificial selection, times have improved significantly since 1973.
This lack of improvement in thoroughbred racing is sometimes attributed to too much inbreeding: all thoroughbreds are descended from three Arabian stallions and 20 English mares.
Genetic variations are still possible, however. According to the leading veterinary pathologist, Secretariat's heart was twice as big as the average thoroughbred's and 30% bigger than the next largest he had ever seen in thousands of autopsies. This may explain why Secretariat was something of a bust as a stud: he must have had a unique combination of genes providing both a freakishly large heart and the optimal infrastructure to support it.