Some years back a HUGE number of articles came out from...various authors....LOL...that discussed the role of the abdominal muscles. PRETTY SOON, all you HEARD about were 'abdominal muscles'. And you could engage them by tickling your horse with a pencil, and that humping up of his back that he did when you poked him with a pen in the belly, that was GOOD. EVEN WORSE...we were told, 'Oh! That's his abdominal muscles that do that!' Yeah-um-no. Now...I will admit, that during a particularly enlightening dressage lesson, a top trainer praised my horse's abdominal muscles, and I - I told the truth. "Well, he's numping the mare he's turned out all day with, all day, so..." The clinician, Carol Grant, said brightly, 'Well it's working!" So...good trainers do look at abdominal muscles, and abdominals DO have A role. They do seem to do SOMETHING. They aren't the be all and end all of muscles, though. The hysterically prominent role given them, as if they're the only muscles that are involved is - it's not accurate. And there are two very important things going on when horses move. For one, especially at the gallop, their body moves both up and down, and from side to side. Their spine has a regular undulation from side to side(best shown in Jackson's 'Effective Horsemanship'). Their guts slosh back and forth(backwards, forwards) as their body rises and falls with each stride, and changes angle with each stride(forehand goes down, then up, hind quarters go up, then down). It's kind of a pump action. It's much more in the gallop, too. But there is this regular sloshing type motion that goes on. And that actually contributes to the horse's motion and momentum. And muscles both allow and limit that action. And yes, their back moves. NOT solely or even chiefly, due to abdominal muscles. There's an entire complex of muscles that wrap AROUND the spine and stabilize it, for one thing. Then there's another layer (layers) that move that spine. There's the effect on the back of the neck, shoulder and hind quarter as well. There's the effect of the weight moving through space, which causes other muscles to contract and give rhythmically, to keep that movement controlled. SOME of the muscles involved in all these different motions are incredibly complex, like muscles of the neck that are divided into many compartments, where one end of the muscle has one job, and the other end has another job, and both ends have completely different characteristics. Some are fast response, and others slow response, low energy use muscle. It's like an orchestra, and it all works together. No one part is indispensable, and no one part is more prominent than the other. What the back does with a large, heavy, fast moving horizontal animal, is not, in the end, much at all, like what it does for a little two legged mammal, us. Being bigger and heavier and faster DOES make a huge difference. Being horizontal makes a huge difference. The back of the larger animal has a much larger weight to carry(suspend), and because it moves at much greater speeds, a great deal of stabilization to do as well. And no. Your abdominal muscles don't flex(hollow) your spine. But they sure as heck do a lot to stabilize you as you flex your spine, and that's a lot of what riding is about, and even, what the horse's motion is about. Muscles A and B may contract to make Motion Y, but in fact, there's a whole ORCHESTRA of muscles working WITH THEM at the time, to stabilize that body while it's in motion. Think about a giant Sherman the Dog balloon at a parade. Two ropes essentially haul him down the street, but ALL the other ropes are still involved in keeping him parallel to the street.