Mikael Holstrom DVM did his Ph.D dissertation on joint angles and bone
lengths, etc that typify successful elite and international-level
jumping, eventing and dressage horses. I know three-day eventing is
different from endurance, but it's still pretty hard work, especially at
the very elite levels he was looking at. Anyway, he measured several
hundred horses and came to the conclusion that slight to moderate cow
hocks are not a conformational problem and don't seem to cause hock
problems. Sickle hocks, on the other hand, did not appear in the top
level horses, so somehow or another, they were a weakness that kept
horses from top-level performance. I know of several endurance riders
that prefer slight cow-hocks because the horses don't tend to forge at
an extended trot.
When Dr. Holmstrom came to L.A. for a clinic, I got a chance to grill
him on his study and asked him about how it would apply to endurance
horses. He thought that some of the parameters were unimportant---for
example, extreme collection is unnecessary in endurance, so the joint
angles that contribute to that are not as important---but for the most
part, the results could still apply to endurance horses.
Hope this helps.