ANSWER:
The Body-Mass-Index is probably a good starting point for assessing how healthy a person's height/weight ratio is. There are several things to consider in this context. First of all, the old masters considered the rider's body weight to be the most powerful aid. If the weight is used and distributed well, the rider can help the horse tremendously and make himself feel lighter than he really is by supporting his own weight and moving in perfect harmony with the horse's back - which actually requires quite a bit of core muscle strength and can quickly become a somewhat aerobic activity. If the rider uses his weight poorly, he can interfere with the horse's motion so much that he actually destroys the gait and damages the back, and he feels heavier to the horse than he really is. Obviously, the heavier the rider and the smaller the horse, the more profound the effect of even the smallest weights shift in the saddle will be. Conversely, the smaller and lighter the rider and the larger the horse, the less the rider's movements will influence the horse. The same thing applies to tall riders on short or narrow horses. The leverage that a tall torso entails can be a tremendous asset or it can have tremendously destructive force, if it isn't wielded skillfully.
Some horses benefit from having a heavy rider, because the weight can help to flex the haunches and to break through certain resistances. I'm thinking of very large, bulky horses with straight hind legs, short backs, perhaps croup high conformation, especially if they have learned to lean and tug on the reins. The more efficiently you can use your weight, the less hands and legs you have to use.
Other horses have very sensitive, weak backs, e.g. horses with narrow backs, long backs, sway backs. If that conformation is compounded by a long, high set neck and/or hind legs whose joint angles bring the feet naturally out behind the horse's body, then these horses are more or less unsuitable as riding horses and should be used as driving horses instead. If they are ridden at all, these weak backed horses need lightweight riders with a good seat who can spread their weight out over the largest possible surface area, so that the number of pounds per square inch is as low as possible.
Most horses appreciate it very much when the rider sits lightly, i.e. when the weight is not sitting like a rock in the middle of the horse's back, but if it's channeled around the barrel, through the horse's legs, and into the ground, so that it's like water that runs off the horse's back and around his rib cage to the ground. The rider's joints also have to absorb the motion of the body elastically. That allows the horse to move with the greatest possible freedom. It's true that a heavier, but balanced and supple rider is much more comfortable for the horse to carry than a lighter, but stiff and unbalanced rider.
However, each horse has a maximum "payload" that still permits him to move with ease and pure gaits. If you exceed this maximum load, the back can no longer support the rider's weight and drops, no matter how balanced and supple the rider's seat is. The consequence is that the horse inverts, starts to suck back, and in some cases becomes quite belligerent, because the rider's weight is too uncomfortable or painful to carry. Therefore, heavier riders cannot ride certain horses well, if the horse is too small, or not strong enough, or if the conformation is not suitable for supporting heavier loads. This is true regardless of whether the rider is tall and heavy or "too short for his/her weight".
Conversely, petite, lightweight riders sometimes don't weigh enough to ride certain large horses well, because these horses can simply lift the rider up with their croup, so that the half halts can't go through, because the rider's weight is too small to back up the rein aids effectively. These horses need to be tuned up by a heavier rider first, to teach them respect for the seat, weight, and rein aids, before the lightweight rider can ride them easily as well.
The 19th century military authors occasionally mention that certain soldiers in their regiments made every horse swaybacked, because they were simply too heavy, and the cavalry horses had to carry additional baggage on top of the rider's weight, too.
Young horses should always be started by lightweight, balanced, supple, athletic riders, since they have not developed enough musculature yet to handle a heavier rider.
- Thomas Ritter
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Last Update: February 25, 2007