Actually, I don't have a thing to add from the physiology camp that
everyone else hasn't already pointed out---there are just too many
physical, environmental and mental factors involved that you can't
pigeonhole individual rides as either "hard" or "easy". The physical
stress placed on a horse is going to be somewhat different doing a flat,
fast gallop than it would be doing a 50 mile boulder-hop, but unless you
want to specifically get into numbers of how many newtons of strain were
placed on this joint and that bone from doing such-and-such type and
speed of exercise, you really just have to realize that ANY ride can be
either physiologically tough or easy on the horse depending on yours and
your horse's fitness, conformation, mental preparedness and talents.
Or, as Truman pointed out, whether or not you're just plain Having a Bad
Day and the vet won't cooperate by giving you a real reason to pull <g>.
So much for cutting edge statistics from the lab rat. Sometimes you
just have to fade back and punt.
Susan