Yes, actually. But I don't think that means that 21 1/2% is ALWAYS going to cripple a horse instantly, far from it.
The 20% I think is too high a percentage for speed work and top sport type work. Jockeys, for example, often weigh closer to 10% of the horse's weight.
I think it's in the ballpark for the kind of riding most people do, though, as I noted.
Does the horse's weight have anything to do with what it can carry? Yes, I think it does. The idea that an animal should only carry a certain percentage of its own weight is not such a bad rule of thumb and is based on some good science.
Weight definitely can have an effect in harder sport- some eventers studied jumping and proved that.
But it is of course, just a start, as I noted; then commonsense really has to rule.
of course the slim or poorly built animal is up to less weight. That only makes sense. In Brittain, where they have hunter classes, they have light, medium and heavy weight hunters. The heavy weight hunters have more bone, more weight.
As far as being what I experience, yes, I think that in my experience, I have seen heavy riders on less suitable horses cause those horses physical problems....that is....IF they are doing more demanding sport.
My vet thinks so too. A few months ago, he discussed a case with me in which a 300 lb rider was riding a rather small fine limbed horse that weighed about 800 or 900 lbs, that would be about 30%. I won't mention the type of riding, but it was a difficult type of horse sport and at a level that demanded a lot of hard work of the horse. He felt the horse's soundness problems were specifically due to the weight of the rider. So yes. I do think it's possible.
I think a 150 lb rider on a 1000 lb horse is 15% of the horse's body weight. And I think for sport and harder faster use that is not a bad matchup. I don't see a lot of heavy eventers, just one or two now and again. Most appear to be very slim and fit, but of course, they're jumping and galloping at speed and doing a very demanding sport.
20% would be a 200 lb rider on a 1000 lb horse. From what I've seen that's not unusual, but as I said, most people pleasure ride, they are not galloping and jumping an intermediate course, so I don't see 20% as a problem.
Some is tradition. In some riding sports it's common to see big heavy men on rather small horses. The horses are stocky, and that helps I'm sure, but it's more that it is a tradition. Since it's a tradition those in the sport are sure it's OK.
I'm not sure a concern with weight is all bad. We have a problem with overweight in our country and it does affect people's health.
But....I think an obsession with anorexic, model-thin unhealthy figures is very unhealthy too!!! We need to bury that somewhere deep.
Fact is these days it's tough to stay slim. But riding and caring for a horse can be a good exercise. Brushing, tacking up and stable work can be a part of an active lifestyle. The important thing is to do something one enjoys and be healthy and happy.