There is a reason many trainers these days start their young horses in a round pen, and begin their under saddle work in a fenced-in riding arena. There are actually many reasons, and I find that most trainers inherently "know" these reasons, but most don't talk about it. It's not just so the horse doesn't run away with you! "Using" the walls of your arena to train your horse is a valuable tool, it helps clarify the lesson to your horse, makes you more effective, and saves you energy - a must for trainers working many horses in a day. In fact, it is such a valuable tool that you can achieve many of these benefits just by having ONE wall!
Ok, so your horse won't get away!
This pertains mainly to the young horse in the round pen. At any stage in the horse's training, it is always important to be able to drive them forward. This teaches the horse in their own language that you are the herd leader. The round pen walls are valuable in that the horse is never so far away from you that you cannot affect them with your energy or a lunge whip, but far enough away that you will not get kicked. It is important that the horse learns at this stage that you have control over which direction they are going, and at what speed they are going. The round pen walls help you with both. More about round penning in another post! (Marsha!)
The walls also help a novice RIDER to learn to push a green horse forward without worrying about direction or being run away with. If you are a novice rider with a green horse, it might be scary to ride your horse as forward as he needs without having the feeling of brakes or steering! If you are riding the horse in a round pen, it gives you the confidence that you can ride the horse at a forward trot or canter without losing control. You don't have to worry about steering, and it lets you relax. In fact, most of my novice riders are started out on the lunge line or in the round pen for this reason - they can relax and find their balance without having to worry about where the horse is going.
The wall serves as a visual barrier
Most horses will respect the walls and not try to climb over or jump out. If this is the case, the wall itself will serve as a visual barrier to the horse to "stop." So, when you have the horse in the round pen, you automatically have the horse between your driving aids (lunge whip) and your restraining aids (the wall). This nicely transfers over to the fact that under saddle we want the horse between our leg (driving aid) and hand (restraining aid). Everything we do with our young horses is to teach them something that transfers over to under saddle work.
We can also use this visual barrier as a sort of "half-halt" before the horse even knows what that is. For example; when training a leg yield - where the horse moves forward and sideways - we can position them so that their head is angled at the wall. You want the horse to achieve no more than a 45 degree angle to the wall, but this position allows us to teach the horse that the leg sometimes means sideways, not always forward. In order for the hind leg to step sideways under the weight, we have to slow the front end of the horse down. Angling the horse towards the wall allows us to do that without using our hands to slow the horse down, which helps us to achieve lightness and obedience to the leg and seat.
The wall keeps the horse straight
When beginning work in-hand, it helps to work the horse next to a wall. We traditionally lead the horse from the left side. We always work the horse from the back to the front, which means we drive the horse forward first, and then direct him. This means that when teaching the horse to lead, it is incorrect to teach him to follow by pulling him forward with the halter (Pet peeve). Pressure on the head of the horse guides the direction or tells him to stop, it does not tell him to "go." It is important to teach the horse to lead by driving him forward with a ground person or a dressage whip. The difficulty with working young horses in hand by yourself is that if you are leading the horse on the left side, your right hand is on the lead, and your left hand comes behind the horse with the whip (while you are looking forward in the direction you are going), and the horse can swing it's haunches to the right, thus avoiding the driving action of the whip. Working your horse in-hand next to a wall on the right side of the horse helps you to avoid this problem, and the horse learns to go forward from the whip. It is important, however, to work your horse equally from both sides even though it is tradition to lead from the left. Always do everything with your horse on both sides!
You can also employ this method when using ground support at any stage in the horse's training, but it is particularly useful in teaching the piaffe, where the rider aids the horse from the saddle, and the ground person teaches the horse to engage and activate the hind leg with a stick or whip. It is especially important at this stage in training that the horse does not swing the haunches out.
Many trainers find it useful to teach their horses to lunge on a lunge line in the round pen for the same reason. You can use the lunge whip aimed at the haunches of the horse to ask the horse to go forward, rather than having the horse swing it's haunches out and face you. The wall keeps the haunches from swinging out.
The wall also helps keep the horse straight when teaching lateral work on the rail. When introducing the inside driving leg for the shoulder-in, for example, some horses will tend to step out with their outside hind leg rather than step under with the inside hind leg. In cases like this, the wall helps to support your outside aids.
The wall helps the horse get the correct canter lead
If your horse is having trouble picking up the correct canter lead, you can use the arena wall to help him get the right lead. This method can be used either under saddle or on the lunge line when lunging in an arena. The canter is a series of little jumps, where one leg is leading. A horse will not usually "jump" into a wall, he will be inclined to jump away from it. The best time to ask for the canter is when the horse is on a circle and is just about to touch the wall. The wall discourages the horse picking up the wrong lead by inhibiting the outside front leg, while the inside front leg has room to pick up the correct lead. It is also advantageous to ask young horses to pick up the canter just before touching the wall, because they are often exuberant about the canter. Since the wall serves as a visual barrier, they will often rock back over their hindquarters to pick up the canter, and are less likely to get strong and run off. However, if you have a lazy horse who does not want to pick up the canter, you should ask after you are coming off the wall, so that the horse feels like he has all the room in the world to pick up the canter.
Be sure to ride "off" the wall!
As valuable as using the arena walls can be, it is important to focus on riding away from the wall as well. Many horses become "rail magnets" and start to drift over to the wall if the rider does not intentionally ride him on the second track, or in other patterns. Riding on the second track will make the rider honest about their outside aids and make sure the horse is between all their aids. Also, make sure to keep your horse fresh and asking "what are we doing today?" by riding him outside of the arena regularly. This tests his obedience to you and may help you discover some new areas that you need to work on.