For the most part jumps on advanced level courses are around 4'3", spreads can get much large, and drops are around 5'-6'. Obviously jumping of any kind is hard on a horse's joints. These upper level horses MUST have good, dense bone and very, very good leg conformation to even make it up to this level in the first place. On top of that, most of them will only ever do those huge down banks at competitions, so it's not like once a week they're going out to hop down 6' down banks. Not to mention it takes an incredible amount of upkeep and conditioning to keep these horses going. I believe Anebel mentioned on another topic that per international level competition horse it's near $8,000-$10,000 in upkeep alone every month, when you consider the near constant soundness exams, farrier bills, etc. etc. Some horses will have more wear and tear than others, of course, but for the most part the biggest job is making sure the horses stay sound. If there's even the slightest issue they'll be taken off work for at least a few days, even if there's only a little heat in one leg and they're not actually lame. The utmost care is taken with these animals to keep the fit for the level they're at, and if they don't come up to par then they won't be competed. At any international level event there are what we call jogs in the mornings before dressage (first phase) and before show jumping (last phase). There are multiple vets and others assessing every single horse that is still in the competition, and if there is any doubt whatsoever the horse will be retired and not be able to finish the competition.
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