My definition of a good ride is as follows : Your horse is in good condition, he's fit and isn't over- or under-weight, he vetted well before the ride. After the ride, his pulse, hydration, respiration and gut sounds were excellent and he trotted out sound. But the acid test is whether or not he is perky and looks as if he can and wants to do another leg - now THAT was a good ride. Don't worry about your time and your placement, you've done well by finishing "fit to continue".
InversionReferring to Part 4 where we discussed the dangers of Heat and Humidity and Tying-up, there is a third warning signal of a fatigued horse, and that is inversion.
A horse uses respiration - breathing - for two things : to get oxygen into his body and to cool himself down. When he's tired, he'll be taking long, gulping breaths with his neck stretched out, trying to replenish the oxygen debt. His pulse will remain high and his body temperature won't come down, often hovering above 40 deg C. His respiration rate - number of breaths - will remain far below his pulse rate. He'll look deflated and dull, indicating fatigue.
In contrast to the above, a horse who pants but whose pulse has dropped as usual and his body temperature remains below 40 deg C, is trying to cool himself down by taking short, cool breaths. Hot blood is rushed over the lungs, cooled with the new air and the old, hot air is expelled. The respiration is much higher than his pulse, often as high as 100 breaths per minute. This is known as inversion.
Inversion is commonly seen in high heat and humidity conditions, where the horse is using all available means to cool himself down. Heavily muscled horses tend to pant more as well, as the muscle mass inhibits heat dispersal through the skin - he now has to use his respiratory equipment to help cool him down. It's estimated that respiration contributes as much as 33% to cooling the horse down.
When out riding, pay attention to the quality of your horse's breaths at different gaits. If he's exerting himself beyond his capability, he'll be taking gulping breaths of air while trotting or cantering, he's in oxygen trouble and you'd better slow down to help him recover - and don't push so hard, your horse told you that he cannot handle that particular exertion level yet. He'll be breathing easily and lightly if the particular exertion level is within his capability and you can safely continue.