People often talk about the negative repercussions of a horse that moves with a hollow back. It's fairly easy to see once you know what you're looking for: the swayed-looking back, the dropped underbelly, and sometimes, you can also see an awkwardly neck jutting up and out. Some horses show obvious signs of discomfort but often, horses get used to working in this posture and generally truck along, legs moving but back tight and tense.
So the first thing to do is to understand how it looks and then, why it happens. Because only when we know WHY, will we be able to begin to do something about it.
What makes a horse hollow his back?
1. Conformation
Some horses could have a sway-backed conformation. The only way you'd know it's truly conformation (rather than having been developed through riding) is if you were able to see the horse as a youngster, long before he was ever ridden. Was he born with a longish back? Are there any specific areas of weakness over the back? It is possible.
The good news for these horses is that with proper work, you can actually build enough top line and supporting muscling that will allow the horse to be ridden in health for the length of his ridden career. But it's good to know when there's a conformation factor, because then you can understand and address the root cause.
Other than conformation, there are many rider-influenced reasons that the horse might hollow his back. Let's look at a few ways that our riding can impact the horse's movement.
2. Restricted "Forward"
I would say that lack of impulsion is probably one of the most common reasons for the horse to have to maintain a hollow back. If he moves with little activity and thrust from the hind end, the horse has to support himself with the front end. He also has to initiate movement from the front end, which will result in the front end pulling the hind end along.
When there is little reach of the hind legs underneath the horse's body, there is little he can do to maintain his own balance, never mind the balance of his rider's weight too. And so the hollow back is the result.
3. Effectiveness of the Aids
The aids are the rider's way of helping communicating with the horse and helping him maintain his balance even while doing movements that require him to change balance all the time. However, if the aids aren't timed correctly, are too harsh (so they block or stun the horse), or too vague (so they do nothing), then once again, the horse's balance is not maintained.
Luckily, the horse has four legs and so won't literally fall to the ground (well, not usually, at least). Instead, he might hollow his back and brace.
4. Riding "The Front Of The Horse"
This one is common and takes a lot of time to unlearn if it's a habit. We always do things with our hands and so it's easy to get into the mindset that the horse needs the most help from your hand aids. But too often, what we do with our hands stop the horse, block the energy, interfere with balance, or just plain pull the horse's neck out of alignment with his body!
5. Rider's Seat (and Position and Technique)
Well, it goes without saying that what the rider does on the back makes a whole lot of difference to the horse. So much of dressage training is rooted in the concept of developing a "good" seat, one that allows the horse's back to rise and fall, allows energy "through" the horse's body, and actually encourages the horse to work more from the hind end.
I'm sure there's more reasons why a horse might hollow the back. As I'm not a vet, I haven't even mentioned all the possible physical problems that could contribute to a hollow back. I'm only looking at this from a riding/training perspective. And I know what you're probably thinking: these are fairly broad generalizations that take years for the average rider to improve and overcome. It's true! (*See the links below if you'd like to read articles I've written about how to solve some of these concerns.)
The problem is that there is no magic pill, or "buttons" you can put on a horse, that will give you any short cuts on your way toward resolving the hollow back issue. It really is all about time, effort, $$ for lessons, making mistakes and learning from them. And the horses are saints, because they usually give us the time we need to develop these skills, even to their own detriment.
But when it happens - when you feel that first swinging back, or that first "real" impulsion that helps to send the horse's energy forward (not fast!) - it's just the most amazing feeling!
And then you build on these feelings and go step by step into a direction that not only helps you improve your riding skills, but also helps your horse move freer, more healthily, and even more happily... then, you'll realize how valuable your learning path is! Because the next horse you ride will benefit from your knowledge.
Finally! The Ultimate Rider-Centered Program!
I've already written about the heels of the rider before. In that post, I explained why I think we should not be 
