Are we reaching locomotory limits?
I came across another interesting paper on locomotory limits in humans and other species, including the horse that I thought was timely given the questions of why the current crop of three year olds in North America seem to be quite slow when compared to horses of the past.
The study looked at maximal speeds in horses, dogs and humans using datasets on the 10 best performers covering more than a century of races. The study firstly found that running speed in horses was heritable (h = 0.438, P = 0.01), suggesting the involvement of genetic factors in determining maximal speed. This is similar to other such studies on horses. They also noted that the maximal speed progression in humans is more likely due to an enlarged population of runners, associated with improved training practices. I would probably like to add the ability for a human to self-regulate training and respond to pain (horses can't exactly tell us when they are sore sometimes!) as a factor also.
The study showed that the maximum running speed over short to middle distances increased over the last century in dogs, horses and humans, but is currently reaching its asymptotic (peak) value. The authors noted that although speed progression in dogs and horses on one side and humans on the other have not been affected by the same genetic / environmental balance of forces, it is likely that further progress will be extremely limited.
My take on this is that they are probably right, but that we haven't as an industry done a stellar job in terms of selection. We still use some very arcane tools to select for matings and also for individual horses and have yet to really embrace genomics for what it can deliver. It is quite possible that with the addition of proper breeding values for thoroughbreds, along with genomic data, we could improve the breed still.