When we talk about Collection, where I usually see a lot of people's minds go is to the position of the horse's poll, maybe the hind quarters, and sometimes they understand the impulsion or forward motion component. But they rarely realize one of the most important pieces of the horse's body that makes it all work, or not, and that is the ribcage.
Nine times (or nine and a half times to be exact) out of ten, when I get on a horse that I've never ridden before and we start to engage in a conversation about collection, I find the horse doesn't know anything about two important body parts that are imperative to actually having true collection (as opposed to the "false frame" concept), one part is their shoulders, and the other part is their ribcage.
I am a firm believer of working the horse through it's full range of motion, and developing obedience and trust when doing this. By going at the subject of collection like this, the results are astounding. It is a gymnastic approach, and so the horses loosen up through their neck, shoulders, and back, and the outcome is that everything starts working together across the topline. If a horse has any lateral stiffness or crookedness, collection will only be mildly successful, and it will only access a limited part of the horse's true potential through any maneuver we are asking him to perform.
So why is it so important to remember the ribs when we working on collection? Because if the ribs are supple and moveable the horse can be truly straight when going in a straight line, and/or it can truly be straight while traveling on a circle or an arc, and this "straightness" quite simply produces power. If the ribs are out of the way, the inside hind leg can come up and under to take the weight of the horse, as is captured in the image in this article. You can see the hind leg coming under to step where the front foot is leaving, and the top muscles of this horse's neck are working, I like to call them the "carrying" muscles. The opposite of those are the "pulling" muscles when the lower neck is being used. This is when a horse is called "ewe-necked" or "bull-necked" and usually associated with horses that over use their front end to pull themselves along, and these muscles are far smaller and weaker that their counter parts in the hip, back and topside of the neck. When a horse moves upside down like this, it is because his ribs are stuck to one side, or they are flexed to the inside of the circle or arc. With the ribs to the inside, the horse's weight to the inside forequarter, you can forget about ever achieving thorough collection, or what dressage riders call "through," where the energy and power is generated from the hindquarter, and then travels up and through the horse's entire topline with no leaks or interruptions, and the front end stays light and soft.
Mechanically this is a very simple concept. Where it can be challenging is adjusting the mental part and motivation of the horse. The level of challenge depends on how long the horse has been traveling incorrectly, and it also is effected by how much easier it is for horses to load their front end and move their hind end...there's a lot more weight to move around in the front end vs. the hind end. The mental difficulty is similar to us humans getting past having been told repeatedly to squat down, keep a straight back, tight core, and to use our legs when lifting a heavy object, when it is so much easier to just lean over to pick up that same object, and we happily run the risk of straining or throwing our back out because we didn't want to go through the effort to do the movement correctly. Developing and encouraging a good work ethic in a horse is so important, and could be a whole other blog post, so for now we'll stick to the mechanics of the ribs and how they effect collection.
Without the ribs/shoulders being supple during collection, in my mind's eye the horse has an "accordion" effect that happens to his body. For example, the nose is in so the poll is flexed, neck is high, shoulders are down, back is hollow or at best flat, and the hindquarters are being told to shove up under, but they can't because the ribs are in the way...it's a bit of a mess. I see it like the cartoon where the Roadrunner tricks the Coyote into running into a wall or off the cliff. When the Coyote hits the wall or ground, he turns into a zig-zag or accordion shape; his body length is shortened, but all the angles are sharp and disunited, instead of the frame being shortened but staying together, flexed in a longitudinal arc, and loaded with kinetic energy like a bow...two very different mental images, and two very different outcomes in the way a horse travels and uses himself.
Sometimes just having a good clean mental image to give the horse can change everything, after all, a picture is worth a thousand words. When working on collection, our mental picture is all the horse has to go off of, and the better our mental picture is, the better the result we'll get from our horse, and the sooner those good results will appear. So next time you go out to work on your horse's collection, along with the other components, you should be thinking about his ribcage, where it should be positioned for correctness, and what that should look and feel like...I think you'll be pleasantly surprised with what starts happening to your collection, and how much better your horse starts moving and operating ;-)