When I was a kid I thought balance was only important while riding horses so that the rider didn't fall off the horse when the horse moves. It wasn't until I started learning the theory of riding and equine biomechanics that I really began to understand how the rider's balance actually affects the well-being of the horse. I hear a lot of people saying "You're not going to hurt that big ole horse," and figuring the horse is too big and strong for a 5'5" woman to hurt by poor balance. I realize that they mean no harm by that, but as I've been thinking more about the physics of balance and weight distribution and realizing that our balance on a horse's back, no matter how big they are or small we are, can be very important to their physical comfort and well-being.
I mentioned this in an earlier post that I have heard folks say (and this may be a more common belief about riding and balance than I realized) that if you stay fairly straight in the saddle, but let your weight fall to the opposite side of the horse's weak side, then it will "pull them back up straight" if they are falling in to one side. Like I said before, without really thinking too deeply about it you would think that would make sense. But then after thinking about what I know of equine biomechanics I realized that wouldn't work at all. So, I did a little experiment with my daughter to see what would happen if I tried to get her to hold her body in a specific way by using different weight distribution on her back. This is by no means a truly scientific experiment, we have no control group, the backback is not exactly in the same place it would be on the horse's back and yes, she's walking on two legs instead of four. But it shows the basic principle of weight distribution and movement in regards to an organic being in a state of self-propulsion.
My daughter weighs about 65 pounds. To mimic the ratio of rider to horse weight, I put six pounds of weight into a backpack, so roughly the same as a 100 pound rider on a 1200 pound Warmblood. In this experiment I decided that she would emulate a horse with a weak right side, so a horse who was falling in to the right and needed to bring his right side up to meet his left for balance. A horse who is falling in on the right side like that is moving in what feels like a natural way for him (or else he wouldn't be doing it). Bringing up his weak right side and moving in a balanced way feels unnatural to him (or will until he gets stronger). So, to keep that aspect of the experiment the same, I decided to have my daughter walk normally with the weight evenly distributed on her back, then try to see if I could get her to lean to the left while walking, by bringing more weight to the left side on her back. See what happens. Does she lean to the left like I intended by attempting to "pull her down" with the weight distribution?
You see what happened when I tried that - I got exactly the opposite reaction from what I was looking for. She leaned her weight to the right and when I asked her why she wasn't standing up straight she said she was trying to make up for being off-balance. Not to mention, if your horse is leaning to the right because of a weak right side, and you put more of your weight on the left side to try and pull him into balance, he is going to work his left side even harder and thus exacerbate and intensify the strength imbalance in his body.
So, stay in the middle of your saddle, keep your weight balanced and encourage your horse to engage his weak side. But don't try to pull him over or balance him by changing your balance or you will do a great disservice to a horse who - like my daughter - is just trying to move in the most efficient way possible for himself despite an unbalanced weight on his back.