i think it's also improtant to remember that a cavalry charge was not, for most of its approach, moving at top speed, but in a steady, controlled manner. the charge in Braveheart, where the english knights set off at a walk and move to a trot and canter as they get closer, was the norm. later drill manuals only told the riders to gallop at around 50-100 yards form teh target, to prevent the horses being "blown" and to keep formation