When the horse walks or trots, its spine moves from side to side in a snake-like motion as each step pushes energy up the horses back. The diagonal pattern of the gait transfers the thrust from the push phase of the gait up one side of the horses back and then the other. The energy flows through the muscles like a pulse and is recycled to the other side as the opposite hind leg exits the support phase of the stride and thrusts the energy forward. This is how the horse moves forward.
There is a natural frequency to this motion that is individual to each horse. The rider must strive to preserve this motion and allow it to happen as naturally as possible. The key word here is “allow” because as we sit on their backs, we cannot dictate this natural frequency, we must match it. You must develop the muscle tonus (tonus: the constant low-level activity of a body tissue, especially muscle tone) such that your body is stable enough to balance and support the movement, while not interfering with the movement.
This is not a completely relaxed state. Some riders are told to just totally relax, and let their body be like a rubber doll on the horses back. While this may allow the body to follow some natural movement, eventually the rider will not be able to control his balance against gravity and this “dead weight” will begin to move unpredictably, forcing the horse to change his natural movement to accommodate the shifts in the rider’s weight, like a loose load of goods on a donkey’s back.
On the other hand, the tonus needed is simply enough to provide stability to your posture while allowing parts of your body to move in the rhythm of the horse. While you are standing in line at a grocery store, you have a certain amount of tonus in your body to keep you upright. While riding in a car or driving, you have enough activity in your muscles to keep you sitting upright when going around a curve or hitting a bump. On the horse, this low-level muscle activity is constant, and the goal is keeping your body aligned with the center of gravity of the horse so that you do not disrupt the horses balance and movement.
As you ride you can feel the spine move from side to side. Coarsely, it can be felt in the belly swinging side to side as it moves out of the way as the hind leg on that side completes the swing phase and reaches under the belly. A much finer feeling is to learn to feel the movement of the horse’s spine in your inner thigh and seat. This where top riders listen and can communicate aids. However, rudimentary methods like “pedaling”, swinging your hips side to side, or driving with one seat bone then the other are detrimental because you will not be able to match the natural rhythm perfectly with these methods. A rider trying to drive the horse in this way easily gets out of sync with the rhythm and blocks it. The horse responds by protecting the motion of the spine and holding it rigid against the rider’s faulty motion, resulting in a tense, resistant back instead of a graceful, flowing gait.
At the canter, the spine moves more like a wave, up and down. The very same practice applies at the canter. The tonus of the riders muscles must keep them upright and balanced, which takes stronger activity of the muscles in the core at this active gait. The natural motion of the horses back will push the rider’s seat back and forth, with a great canter feeling like a “rocking chair” motion. Driving with the seat or physically trying to push forward and back will block the motion, for the same reasons as in the trot. The upper body needs to stay as upright as possible without being rigid. Some slight motion backwards from the vertical, as if you were sitting on a see-saw actually allows your body to stay in line with the center of gravity as the horse undulates underneath of you. As the horse’s fitness and balance increases, as well as the rider’s, this motion becomes very minimal.