Roger Pielke Jr. wonders why the speed of the fastest horses has plateaued while the fastest humans continue to break records:
One possibility, advanced by [Mark] Denny and others, is that thoroughbred race times may have leveled off because the narrow genetic diversity of racehorses limits the genetic diversity in the pool of potential thoroughbred champions. Modern thoroughbreds are descendants of a small number of horses (less than 30 in the 18th century), and 95 percent are thought to trace their ancestry to a single horse, The Darley Arabian. Today, there are fewer than 25,000 thoroughbreds born each year in the United States. Compare that with the more than 7 billion people worldwide. The size of the human population may simply lead to a greater number of potential athletes with extreme speed.
[C]onsider that Usain Bolt started by propelling himself out of blocks down a specially fabricated carpet designed to allow him to travel as fast as humanly possible. Jesse Owens, on the other hand, ran on cinders, the ash from burnt wood, and that soft surface stole far more energy from his legs as he ran. Rather than blocks, Jesse Owens had a gardening trowel that he had to use to dig holes in the cinders to start from.Biomechanical analysis of the speed of Owens’ joints shows that had been running on the same surface as Bolt, he wouldn’t have been 14 feet behind, he would have been within one stride.
Cowen highlights other interesting parts of the speech.