"In many dressage manuals the rider is told to "go with the movement of the
horse" with his midsection at the walk. To me going with the horse's
movement implies moving back and forth with my hips a bit. Aren't we
supposed to have a midsection that is quietly stable and strong so we can
literally suck the horse's back up into collection? Whenever I watch upper
level riders at the walk their midsection hardly moves; they appear to sit
totally still. How quiet should my seat be at the walk?"
That’s an excellent question. It really goes beyond the walk and applies to the trot and canter as well. One of the goals of good dressage training is to enable the horse to move as beautifully under the rider as he does at liberty. In order to do that, the rider must not only balance, straighten, and supple the horse. He must also allow the horse’s back to move and to swing with the same range of motion, as if he were not carrying any weight at all. In other words, the movement of the horse’s back must go through the rider’s body undiminished. If the rider blocks the horse’s back movement with stiff joints, the horse loses impulsion, suppleness, the purity of the gait, and turns into a legmover, instead of becoming or remaining a backmover. Some horses even stop, unable to continue forward, if the rider’s seat is very stiff.
So the task for the rider is clear, but how do you sit in a way that allows the horse to move with the greatest possible freedom in his back? If you try to relax your entire body and be very loose, you will most likely flop around like a rag doll, which is not only unaesthetic, it is also ineffective, because you can’t control the horse’s rhythm, tempo, stride length, direction, bend, etc. this way. On the other hand, if you try hard to sit still, you are likely to become locked up and stiff, which then actually leads to inadvertently kicking legs and hammering hands, and shuts the poor horse down completely. The solution lies in finding a healthy balance between stability and suppleness.
What makes matters more complicated is that you will need a different degree of muscle tone for different muscle groups in your body. You have to be able to separate and differentiate between the different muscles. That is one of the great challenges in learning how to ride. Most people can either relax their entire body or contract all of their muscles. However, in order to ride well, you have to be able to contract (or tone) some of your muscles, while letting go with other muscles at the same time.
Your main shock absorbers are your hips, knees, and ankles. If you stiffen or brace any one of these joints, you are blocking the horse. Unfortunately, virtually every novice rider starts out with tight hips, and many riders also have very stiff, inflexible ankles. It takes many years of serious studying to develop the strength, suppleness and body awareness to maintain supple joints at all times.
So you can say that the rider’s hips, knees and ankles have to allow the horse’s back to move. They have to let the movement come out. But you also have to control the gait somewhere in your body. The control center, the “conductor in the orchestra of the aids”, as the Old Masters called it, lies in the core muscles, the abdominals, obliques, and back muscles. They are the ones that have to be toned. They connect the rider’s leg to the seat, they connect the leg to the hand, they connect the seat to the hand, and they lend authority to the rein and leg aids by backing them up with the body weight. Increasing the muscle tone in the core muscles is a prerequisite for being able to relax the hips. A rider who has a weak tone in his core muscles will inevitably tip forward, lock up his hips and grip with his hands and legs, in search of stability and security. This rider will find it impossible to let go with his hips until he has increased the tone in his core muscles and centered himself, which leads to another prerequisite for being able to follow the horse’s movement properly: the vertical alignment of the rider’s shoulders, hips, and heels.
The picture that emerges after this lengthy description is that the rider needs to keep his hips, knees and ankles supple and relaxed while maintaining a sufficiently high muscle tone in the muscle ring that surrounds the lumbar back on all sides, as well as in the upper back muscles, between the shoulder blades. The upper arms have to be relaxed but connected to the rib cage, so that the contact the horse makes with the bit enters the rider’s body at his waist, not at his shoulders. If the rein is connected to the shoulders, the leverage of the rider’s torso will work in favor of the horse, if he decides to engage in a “divide and conquer” strategy, because it is very easy for the horse to pull the rider onto his neck this way. If the rein is connected to the rider’s waist or hips, on the other hand, the leverage of the torso works in favor of the rider, and if a horse tries to root or lean, he basically half halts himself against the rider’s core muscles, and the rider’s seat never loses its integrity.
Back to the question of how much do you need to move. This is something only your horse can answer, because every horse’s movement is a little different. It depends on how large the horse’s back movement is. To help you narrow it down, you can say that if you “follow” the motion so much that your seat starts to look noisy, then you have gone too far. There should be no visible pushing, shoving, grinding, gyrating, or “belly dancing” on the horse. Excessive, visible motion is usually counterproductive and hinders the horse, instead of helping him. To find the right degree of following, you may have to experiment with following more or less, until you find the degree at which the horse feels the best. Then check yourself in the mirror, or ask a ground person, whether there is any excessive movement visible in your body. Good riders look as if they were sitting perfectly still, because they are moving exactly as much as necessary to absorb the horse’s motion in their body, no more and no less. The stillness is an optical illusion. But the mobility lies beneath the surface.