You can blame some of it on our selective breeding, and you can blame the rest of it on our insane desire to have horses use their front ends for slowing forward speed or stopping.
It's all the rage.
A horse's knee can be compared to a human's wrist. It's comprised of two rows of four bones each. Actually the horse's knee joint is three joints. The radius makes a joint with the upper row of knee bones; the upper row makes a joint with the lower row; and the lower row joins with the cannon bone.
The knee is a complex system, and therefore presents plenty of opportunity for trouble.
The horse's knee was meant to flex, and normally should be able to be bent easily so the bottom of the horse's hoof touches the horse's elbow. Such flexion should cause no distress.
The knee was designed to bend only one way. When it is bent the wrong way, inverted toward the rear of the horse, damage occurs. Unfortunately, such inversion takes place frequently among performance horses, especially when they are being asked to slow down by a rider pulling back on the reins.
When a rider pulls on the reins to stop a horse, the horse is virtually forced to use his front legs as brakes. The knees are bent backwards when the horse tries to stop his mass with his front legs instead of allowing that mass to move over the stationary leg. A western slide stop is an exaggeration of how a horse should use his hindquarters to stop while his front legs keep moving.
The cartilage between the bones of the knee, the joint fluid and the bones themselves were designed to withstand a certain amount of stress. However, the stresses of sprint racing, jumping, pole bending, calf roping and polo--to mention just a few--are a little too great when compounded by a rider balancing on the horse's mouth or pulling back on the reins.
The result of consistent excessive stress to both young and old knees is a squeezing and pinching and deterioration of the cartilage, a leaking of joint fluid, and a chipping, denting or scratching of bone. All of which leads to inflammation, then arthritis, and often joint deformities. Once injured, a knee takes a long time to heal because the knee has a poor blood supply.
And seldom does an injured knee get help as quickly as it should, because knee injuries are hard to detect in the early stages.
A horse with a knee problem will show a little stiffness in the injured leg when first brought from a stall, but if you are not looking for it, you might not see it. The horse, if viewed from the front, will tend to swing the affected leg outward, eventually putting most of his weight on the inside heel.
But during the early stages, the horse will "warm up" and lose all symptoms of lameness. Just about the time the rider thinks the horse is a "bit off," the horse begins to travel fine. The rider goes on with the exercise as planned, and the sound horse is becoming one with a major knee problem.
Time off from work (not stall rest) is the greatest healer of knee injuries. There are some drug therapies, but they too require major amounts of "time off." When drug therapies are used to keep the horse in competition, there is no healing.
Horses which are "back at the knee" are the most susceptible to knee problems.
And, of course, today's competitive pleasure horse--of nearly every breed--has to move slowly, with no reach by his front legs. What kind of front legs move slowly with no reach? Front legs, hanging from a straight shoulder, back at the knees. So we are breeding them that way.
To compound the problem, the desire for excessive slowness is moving into other areas of competition. Now we have more and more riders cranking the horse's head down to force the horse to go slow by braking with his front legs.
We are literally bashing and breaking the horse's knees.
Of course there is no way to help the horse who's rider will not learn. The more the rider hangs and pulls on the horse, the more battered the horse's knees.
It's the "in" thing, man!