I've always felt instinctively that the horses here benefit from the variety of surfaces on our tracks. There are gradients (we don't really have ANY flat land at Rockley!) and they have to climb up and down hillocks, step over tree stumps, weave in and out of the woods and cross bridges just to get from one yard to the next.
Underfoot they will encounter rounded pea gravel, rippled concrete, rubber, chipped wood, shillet, earth in the woods, timber and sand - and often they can choose which of those surfaces they want to walk over.
This provides a great variety of stimulus for their hooves, and this is particularly important for horses just out of shoes. Research shows that in humans, proprioceptive input is restricted by shoes, and it seems just the same for horses. Not only that, but where horses have been or are still lame, proprioceptive impulses allow them to adapt their movement to minimise pain.
Here is another great quote, this time from a book by Jean Marie Denoix and Jean Pierre Pailloux: "Physical therapy and massage for the horse", which explains why you need to rehabilitate not just the feet but the rest of the horse's body following lameness:
"A horse which has regained soundness of movement will continue to retain for some weeks a vivid memory of its distress. It will move gingerly, out of apprehension that its movements will still be painful."
The books goes on to explain that re-establishing and re-educating the horse's proprioception
is an important part of restoring correct movement. and then they explain how to achieve this, which you do by letting the horse move over different types of terrain :-) The different surfaces, stimulation and sensations boost proprioception and "eventually lead to soundness of movement" :-)