I know someone who does timing for Cowdray Park's polo matches and is desperately horsey-countified, but even she doesn't like the mechanics of the sport, viz. the issue raised quite reasonably in the letter.
The ponies are raced at great speed, with the accent on sharp and sudden turns - great bursts at the gallop for short distances, with manoeuvres such as 'riding off', where one rider deliberately leans his horse in on the other to intimidate it and put the other rider off his stroke. Of course there are great stresses on the horses' bodies, especially spines, pelvises, hips, and all the leg joints. They are then tied down with martingales (and yanked around by Pelham bits) so that they can't throw their heads up too far when given very strong aids to accomplish all the twisting and turning necessary. They're also driven by spurs. So you have an animal which has to go from a stand to a full-out, 'spike' activity, turn on its hindquarters at the gallop, lean left and right on command, and come to a virtual full-stop when needed, from the gallop, many times during a chukka.
Of course it all takes its toll, although some ponies are still playing in their teens, and one pony won't play through all the chukkas.
I don't have a problem with anyone, horsey or non-horsey, raising what are quite reasonable questions about the stressors on sport animals, whether they're muscular, skeletal, or mental. I think that these inquirers should be treated with respect and commonsense replies, and not just reactionary responses which are defensive and subjective. The downfall of much of the hunting fraternity was that it kept avoiding the true response it should've given - that it just LIKED hunting and wanted to continue it. Obfuscations about heritage and tradition work about as well as saying that's why we should still have bear-baiting or public executions.
There is a vast amount of information now available, because of the Internet, on the various ailments besetting sporting equines, so it's not as if the subject isn't constantly being examined, evaluated, and researched. There are many more products to assist equine health, and there are considerable advances made in surgical procedures, too, but I do think that it's worth asking whether animals are being asked to produce too much for our pleasure at times.
Polo ponies have mouths of iron, due to the hauling and yanking that goes on. It's not a sport of subtle riding techniques. The natural reaction to having someone wrenching your mouth around is to throw your head up to avoid the bit. That's why they're all fitted with standing martingales, so that the head can only go up SO far, and no further. To me, this is a brutal way to treat a horse, and it's the reason the Cowdray Park time-taker, although she loves the excitement of the game, hates the way the ponies are ridden.