Note: today’s post is riding theory geekery. You may wish to come back tomorrow if that is not your cup of tea.
Revelation
Horses are narrow side-to-side, but long front-to-back. People are wide side-to-side, but narrow front-to-back.
What This Means: Rider’s Weight
A rider’s lateral balance has a small tolerance. When I am sitting on a horse, there is not much horse extending to the left or to the right. To stay aboard, I cannot lean too far in either direction. If I bend down on one side to adjust my stirrup, I have to shift the weight in my seat to compensate. Riders learn early to keep their lateral balance.
A rider’s longitudinal balance has a wide tolerance. When I am sitting on a horse, there is a lot of horse extending in front and in back. I can lean forward to touch the horse’s ears. There is horse underneath me. I can – given the right horse – lean back to rest on the horse’s butt. There is horse underneath me. My weight can roam back and forth as on a large, fuzzy sofa. Since riders are not required by gravity to be exact in their longitudinal balance, they get sloppy. We are not forced to, so we don’t.
If one is farting around in the back pasture with one’s semi-retired, 25-year-old gelding, one’s longitudinal balance can be approximate. To execute a sparkling show trot, or a tight roll-back, the horse needs to be in a particular frame. To ride the horse through these maneuvers, the rider needs to be balanced laterally and longitudinally over the center of that frame.
This is why my transitions are sloppy. Since I am out of position longitudinally, I either have make a big move to get back into position or make a big gesture so that the horse hears me. This is also why trainers can sit like the aforementioned sack of potatoes [Dueling Disciplines] and yet ride brilliantly. They are where they need to be in relation to the horse’s balance.
What This Means: Position Control
A person’s lateral balance has a wide tolerance. When I am standing on my feet, I have to lean a fair bit before I fall over sideways.
A person’s longitudinal balance has a narrow tolerance. When I am standing on my feet, I have to lean only a few inches before I fall forward.
Therefore, it is easy for me to fall out of longitudinal balance with the horse. To correct, I need to make subtle adjustments. I don’t do subtle.
Bottom Line
Longitudinal balance. Easy to lose. Easy to overlook that it has been lost.
Virtual Brush Box 
In any sport, the most important part of your body to train is between your ears. It is good that you know this.
My serious response: This is really something to think about, thanks.
My less serious response: your revelation reminds me of Anne Elk’s Theory on Brontosauruses (a Monty Python sketch): “All brontosauruses are thin at one end, much much thicker in the middle and then thin again at the far end.” While the lateral balance issues of bront-y riding would be less of a challenge I feel other factors might come into play to offset the benefits.
Ooooo, I wish I was artistic. I have a sudden desire to see a brontosaurus being ridden.
Try thinking about lungeing one 😀