Hi Lisa,
I wished they had you comparing thorobreds with Morgans (my expertise) but I can make some assumptions here much the same as you have done. It does get back to the percentage of muscle fibers. I believe there have been many papers out there on muscle biopsies between sprinters and marathoners where the percentage of fast-twitch vs slow twitch muscle are different. I can only surmise that these studies can be applied to horses where one breed tends to be trained to run all out and the other breed spends most of their time in a walking gait. I know that my Morgans, which were carriage horses were very easy to train and happiest to pull a carriage at slow but my thorobreds were anything but carriage horses and were happiest in a canter or all-out gallop with plenty of bursting speed qualities.
My muscle physiology professor at Howard was a Ph.D/ DVM. His name is Dr. Keith Cauthan. I do remember him saying that many of these studies have been done in horses since there is lots of money to be make in breeding the fastest horse with the most efficient cardiac reserve and making predictions based on muscle fiber differences. There has to be a body of research out there under muscle physiology in horses.
Good luck. This class sounds like lots of fun.
Natalie