Movement of the lumbar spine
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Looking at the anatomy of the lumbar spine, you might think that there isn't as much movement there as in the thoracic spine, and you'd be correct. But there still is more movement than you might think. And now that we have established that western saddles can and do put weight on the loin of the horse without detrimental effect, knowing how it moves is important in regards to saddle fit.
Types of motion
There are the three basic types of movement which applies to the entire spine: flexion/extension, lateral bending and axial rotation. Flexion/extension is the "rounding and hollowing" of the back, lateral bending is curving side to side and axial rotation is tilting or twisting of the vertebra compared to the upright position.
A caution regarding interpreting the research
There has been a fair amount of research done to figure out how much motion there is all three directions in different sections of the equine spine. But it is important to realize that the results of the research is reported in different ways. Sometimes the amount of movement is reported between two vertebrae. Sometimes it is reported as the movement compared to the vertical or horizontal. And while some of the research is done on cadavers where they can measure maximum movement with stress applied, other research looks as live horses and how they move naturally. So that is where knowing what the number of degrees or percentage is actually measuring makes a big difference in interpreting the data.
The hind limbs as drivers
Remember that the sacrum is attached to the pelvis, and the pelvis is solidly attached to the hind legs, and the hind legs move. If you watch a horse walk, you see that his pelvis tilts from side to side depending which leg is on the ground. Basically, all animals "wiggle while they walk". This "wiggle" is transmitted through to the sacrum and therefore to the lumbar vertebrae and on up the spine. In fact, the hind legs/pelvis/sacrum connection is really the driver of the motion of the spine itself. We said when we talked about movement in the thoracic spine that the spine doesn't flex and extend all as one unit, but that motion goes along the spine like a wave. The same is true for lateral bending and axial rotation. In general terms, the movement of the hind limbs transmitted through the pelvis and then the sacrum is what starts that "wave" and it moves forward along the spine.
Lateral bending and axial rotation
When they tested equine cadavers, they found very little movement between the vertebrae in terms of lateral bending and axial rotation. Thinking about it, the large transverse processes would hit each other if there was too much sideways movement, and there are joints between the transverse processes of the last couple lumbar vertebrae that basically preclude any movement in these directions. (I've read that some of those joints can actually fuse so there really can't be any movement between these vertebrae.)
However, when you measure these movements in a live horse, not between the vertebrae but against the vertical and horizontal, you do find both movements. In fact, the axial rotation in the lumbar vertebra is greater than in the thorax at the walk and the trot. Why? Because the tilt from the pelvis and sacrum is transmitted forward along the spine through the lumbar vertebrae to the thoracic vertebrae. The farther forward you go, the less the vertebrae are affected by the hind limbs (and the more they are affected by the front limbs and head) but that hind limb movement really does drive the motion of the spine.
When it comes to flexion and extension of the back, the winner by far in terms of magnitude of possible motion is the lumbosacral junction (between L6 and S1). Remember how there was that "hole" between the forward leaning dorsal spinous processes of the lumbar vertebrae and the backward leaning ones on the sacrum? That allows for a whole pile of extension (hollowing) not available in the joints ahead of it, and it also means that there is more room for the ligaments between them to give so there is the ability for more flexion (rounding) too. In the cadaver studies, they measured only one or two degrees of movement between adjacent lumbar vertebrae. But they measured over twenty degrees of motion between L6 and S1. Only some of the joints in the neck have more than that.
So even through there is lots of room for movement between S1 and L6, flexing the sacrum still affects the lumbar vertebrae. Remember back to that supraspinous ligament and how the head pulling on it affected the vertebrae, especially at the front of the thorax? Well, the same thing is true at the back end. The supraspinous ligament pulls forward from the sacrum onto the dorsal spinous processes of the lumbar vertebrae, and since they lean forward, it acts to straighten them up a little which rounds the back a bit. It isn't as much flexion and extension as is possible in the thoracic vertebrae, but when they measure it in living horses, it does happen. So when you see that sliding stop where (if done correctly) everything is at maximum flexion, his back isn't evenly rounded all the way down. Most of the reason the horse can "plant his butt in the ground" is because of the movement he has at the lumbosacral junction.
It isn't a one way/t'other way, but a flow...
The movement of the horse's back is very complicated. While all three types of motion can be differentiated in studies, in practice they all flow together with a motion that twists and turns and bends with every portion of every step. And how that flow occurs changes with the different gaits, just to make life interesting. It is this flowing, moving, changing shape we are working to fit with our saddles. And it is very possible!
Next up - the rib cage.