Horses, unless they are in flight or play mode, naturally carry about 60% of their weight on their front end. This is fine when they are unridden but can become problematic when you give them a 100-200lbs backpack -- a rider. A horse carrying most of his weight on the forehand and having to contend with a rider is an unbalanced horse. A lot of training problems come from the horse being stiff and unbalanced and reacting to that. Watch people ride and the horses who are unbalanced and uncomfortable with the rider are the ones careening around the arena, leaning into corners, hollowing their backs, swishing their tails, and acting generally stressed. A horse, being a flight animal, does not like to feel out of balance. Also, if a horse is being asked to work while carrying most of your and his weight on his forehand, it puts him at risk of developing soundness issues since it's not an effective way of using his body. It's like if you carried a heavy backback long distances in an unbalanced way for years -- you'd develop physical issues as well. However, horses are not born with the knowledge of how to carry themselves and a rider in a safe and balanced way. We have to teach them that.
So what is the best way to carry the rider? That's where the answer to your question comes in. The hind end of the horse is far more powerful and the joints are bigger and better designed to propel the weight of the horse and rider than the front end. When people talk about "getting a horse to use his hind end," they mean teaching the horse to shift his weight back and push off the hindleg, rather than pull off the foreleg. With correct training and properly built up strength, this is less wearing on joints and tendons than leaning on the forehand. It also means that the horse and his 150lbs backpack will be better balanced and the result of that will be greater maneuverability, lightness, and the ride will be far more comfortable for both parties. There are gradients of being on the hind end. When you start a youngster or are retraining a horse who has never been taught this, you introduce them to the idea of just pushing through with their hind leg while maintaining fairly even weight distribution between haunches and forehand. This is what dressage people call impulsion -- thrusting off the hindquarters. As training and strength increase, you can begin to ask the horse to actually rock his weight back and to flex the joints of the hind end, the hips, stifles, and hocks, and lift the forehand. This is collection. Watch youtube videos of Grand Prix dressage and you'll notice, especially in movements of extreme collection like the piaffe, that the hindquarters of the horse are noticeably lower than the forehand. The pinnacle of collection is the levade, where the horse lifts his forehand and sits, perfectly balanced, with 100% of his weight on the hindquarters. The horse in a levade has the front legs tucked up, holding them out of the way, because he is so strong and well balanced on his hind end he does not need to use them for balance (a horse just rearing up holds the front legs out because he does need them for balance).
Those of us riding for pleasure who have no ambitions (or lets be honest, means) to achieve Grand Prix or High School levels still can and should think about impulsion and to an extent, collection. Any horse sound and athletic enough to carry a rider can at least learn how to push off his hindquarters and have weight distributed pretty evenly between his hindquarters and forehand. Even this is far healthier than lugging around with most of his weight on his forehand.
Last edited by thesilverspear; 11-11-2009 at 10:32 AM.