Longe on a deep surface, for the horse.

I would like to refute this point... I don't think that lunging has as much to do with any increase in Navicular (do you have sources that state this is true?) - rather, I would look at better diagnosis (i.e. More cases possibly getting diagnosed due to better equipment?), poorer breeding selection (breeding for a short performance career rather than a long-term healthy horse), and starting horses too young and too hard.

I've never seen a horse breakdown because of lunging on grass, or in deep footing. Honestly, I think way to many people are to freaky out ish about 'oh there's strain on the horses _____ when they do ______!!!' I'm not pointing any finger or trying to insult anyone. But horses are fine to go in circles, go up and down hills, they're tough. IMHO a horse isn't going to ever have a problem going in big circles, or anything like that. If they were that delicate try would all be exctinct, or would only be able to be ridden for like, 2 years.
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What hurts horses is consistency in a poor environment. The fact is, that too deep footing DOES cause suspensory strain on the horse. You can lunge on most any surface once or twice without much harm if any, but you lunge a horse in poor footing consistently and you are putting strain on their legs and/or hooves. Bottom line, improper care is improper care, and even though horses are rather hardy they will break down earlier if we don't step up and do things right. A horse on improper footing is going to break down earlier than the exact same horse on good footing, there is no avoiding that.

To really get the depth right for your longe path, you'd need to see if the hooves are still striking hard surface under the cushion, & add more if needed. So, start at a few inches. A banked longe pen is best.

I agree with roro.

After my TB started to show signs of wear and tear on his hind right hock, I've become more aware to what situations aids in breaking down our horses joints overtime - and lunging is one of them, especially in deep footing.

After talking with an Upper Level Eventer in my area, about Lunging Nelsone while using the Pessoa Training System, she encouraged against it due to the strain given to our horses joints by lunging.

Lunging our horses in a circle smaller than 20 meters, is rough on them.

The fact of the matter is, there are many situations that wear and tear on our horses joints. It is up to us to protect our horses through proper care and daily living.

It's all about calculated risk versus reward. ANYthing we do with our horses will cause wear and tear to their joints as compared to them sitting in a perfectly manacured pasture. We have to assess the risk versus reward, and decide what's best for the horse on different levels. In order for our horses to maintain a healthy fitness level, we must exercise them - unless the horse is suspended, this is going to cause wear on the horse's joints. When managed properly, the wear is minimal; it's when the risk is mismanaged that the risk outweighs the benefit that we encounter problems.
Any living being's body is going to degrade and wear down eventually, it's just a matter of time. When managed properly, we can extend the horse's career and healthy days as long as possible; however we shouldn't wrap them in bubble wrap.
There's being cautious, then there's "ohmigod I don't want to walk out the door this morning because I'm sure I might sprain my ankle!" Many horses, like humans, like to have a "job" - they like the interaction, they like to be challenged (slightly personifying here, but you know what I mean).
ETA -
It's just as much a "mental" thing as physical. I'm injured right now and can't do as much exercise as I would like, or need to keep my mental state where it should be.

I also want to add that this is a completely individual thing as well. While lunging might be detrimental to some horses (MIE's Nelson with hock problems is an example), it is beneficial to others.

Ridergirl, I don't mean to say that grass isn't a good working surface for a horse, as long as it provides cushioning (isn't dead on hardpack). An interesting fact found while studying: Arabians didn't troll through the sand of the interior Arabian desert; they skirted it, where the terrain is hard & rocky!