A word (or a few) about muscles
We are no longer building saddle trees. We have two saddle fit videos available on our westernsaddlefit.com website. Western Saddle Fit - The Basics, aimed at riders, is available either on DVD or streaming on Vimeo while the six hour series Well Beyond the Basics, aimed more for professionals but understandable by anyone, is available by streaming on Vimeo. (We left this website up because we have had many requests to keep the information available.)
In the series on anatomy, we are moving on from the bones to the muscles that are important in regard to saddles. I want to start with a few basic facts about muscles.
#1 Muscles only pull. They can't push.
When being taught about muscle physiology in vet school, I remember the prof saying "You can't push on a rope." as an analogy of how muscles don't work. When a muscle contracts, it shortens and tries to pull whatever the ends are attached to closer together. When it quits contracting, unless something else pulls those ends apart again, they will stay where they are because the muscle cannot push them apart. The lengthening of muscles is a totally passive process.
#2 Muscles work antagonistically.
So how can the ends move apart? Because either gravity works or another muscle or muscles pull on them in the other direction. Since gravity always pulls down, regardless of body position relative to the earth, it can't be relied on to always do the same thing. Thus bodies are designed with muscles that work antagonistically - one pulls this way while another pulls that way. For example - your biceps pulls your forearm closer to your upper arm (flexes your elbow), while the triceps straightens your arm out again (extends your elbow).
#3 It is more complicated than Muscle A against Muscle B.
If joints only moved in one plane, then two muscles per joint would probably work. But joints twist and turn all over the place, and so it isn't just one muscle bending the elbow and another one straightening it. There are more muscles than that involved. Some muscles also have more than one head and attach at more than one place, so they have multiple actions which depend on which part of the muscle is active and how active it is. Some muscles also cross more than one joint, so they can flex one joint while extending another. They also don't just work in one plane, so while they are flexing a joint front to back, they can also be pulling the leg closer to the body side to side - again depending on which fibres are active and how active they are.
#4) They are active when we don't think they should be, and vice versa.
As people have looked at anatomy over time, they surmised when the muscles should be active based on how they looked like they should work. But with more sophisticated testing equipment available, researchers are finding that muscles are active when they never figured they would be. For example, the your biceps actively flexes your arm, but it is also active at some stages when your arm is extending so the triceps doesn't hyperextend your elbow. Regarding the extension and flexion of horses' backs, we would think that the abdominal muscles being active would cause the back to flex (round) and the muscles above the spine would cause the back to extend (hollow). But when they look at when the muscles that are active at the trot (at least), that isn't the case. The abdominal muscles are active when the back is extending - limiting the extension - and the longissimus dorsi of the back is active when the back is flexing - limiting the flexion. From what they know now, back movements appear to be caused by leg movements, weight of the abdomen pulling down, etc. and the abdominal and spinal muscles act to stabilize the movement of the back rather than causing it to move. Who'da thunk it?
#5) Muscles generally thicken when they are active.
As a muscle contracts, it gets shorter (if it can) because all the micro fibrils slide against each other and in between each other. This also means the muscle gets thicker. End result - it changes shape as it contracts. Pretty basic stuff, but what it means practically is that the shape of the horse's back is constantly changing not just due to the skeletal movements, but due to the muscle changing shape as well.
#6) We don't know how or when this happens during the stride.
I had the opportunity a few years back to listen to some lectures from a top researcher in equine biomechanics. I asked her specifically "What muscles are active at what phase of the stride?" and her answer was "We don't know." From what I have been able to read since then, we still don't know. While there are some solid studies that give us a good idea on how the spine changes shape during different gaits, there isn't anything that tries to describe the back shape overall during movement. All we do know is that we don't know, and while we can make predictions, what researchers have learned so far is that their predictions often are not true.
#7) Muscles can attach in different ways.
Muscles can attach to bone by discrete tendons. These can be very long, such as the flexor tendons down the back of the legs or, more commonly, be much shorter. But they can also attach via an aponeurosis, which is a thick, wide sheet of fascia - that tough white, glistening material you have to cut off your meat because it is too tough to chew. Aponeuroses (plural of aponeurosis) attach to bone(s) over a wide area, not at a single discrete point like a tendon. This is important because pressure on an aponeurosis (or tendon) will affect muscle movement, and a number of important muscles are attached via aponeuroses over the back. Muscles can also attach to each other along the midline, like the abdominal muscles do down the center of the abdomen.
So if we don't know exactly when muscles are active during movement, and we can't tell how their contraction will affect the shape of the horse's back, why look at the muscles at all? Because while right now we don't know the specifics (that is all conjecture still), knowing where they are and what they (probably) do can give us a good idea how pressure, especially excess pressure, might affect them, and therefore how it might affect the horse and how he moves. At the very least we will know what we are looking at under the skin of the horse!
Now, on to specific muscles.