Of all the ways to keep a horse physically sound, I believe keeping a horse's mind sound to be one of the most important. In fact, I believe a horse's mind rates right up there with basic conformation and physical conditioning. But then, physical conditioning for your horse is part of how you keep a horse's mind sound-by riding a horse without a lot of stress and pressure.
I know from personal experience, and observation of other people, that we don't always have time to ride a horse enough. We have jobs and schedules that limit our time. I remember rushing home from a day of work and having just enough time to saddle up, get the cattle in, lope the horse twice around the arena and rope before it got dark.
The horse got edgy and started doing things incorrectly at speed. I think this sets the stage for self-inflicted trauma. If not a significant single-trauma event, perhaps micro-trauma that can be cumulative and results in a clinical condition.
An example of a cumulative-trauma condition is a bone chip in a joint. This condition can show up acutely as a sore, swollen joint, and the bone chip is seen on radiographs. But the chip was not the result of a one-time trauma incident. When bone fragments have been studied histologically (microscopic tissue sections) it's been shown to be a chronic inflammation of the bone that finally gave way and separated.
It's kind of like a section of a glacier that's been weakening over a period of time, and all of a sudden breaks off and falls into the ocean. Tendon and suspensory injuries have a similar evolution in most incidences.
The best way to protect these susceptible and critical structures is by developing good muscle tone, avoidance of fatigue, and balanced, controlled moves for the job description. To accomplish this set of circumstances takes time and repetition at a slow pace. In other words, the very thing that is most apt to keep a horse's mind "right." I guess I've just defined what would be a good horse trainer's approach with a little insight on how good training procedures help ensure soundness.
Oh, and by the way, it doesn't hurt the condition, peace of mind and soundness of the rider, either.