Wednesday I have a Vertebrate Anatomy final (go, me!) and the subject that really stumps me is musculature. We’ve been given a review sheet with topics to think about.
Basically we’re supposed to think about how the ratio of non-contractile fascia in muscles, versus pure contractile muscle fibers, causes the muscles to act differently and be used in different situations.
For example, the tensor fasciae latae muscle has a large amount of fascia (which doesn’t contract) and a small amount of muscle tissue (which contracts.) The sartorius muscle is made entirely of muscular tissue (entirely contractile). I understand this, and understand that one will contract to a greater extent than another… but that’s where my understanding ends. What’s the point of having non-contractile material attached to a muscle? How does this change the use and function of a muscle?
I just don’t get why all muscles aren’t made out of, well, muscle. Why have other stuff in there?
Two things muscle needs to do. Contract a distance and contract a force. Need a lot of force, but not moving things a long distance? Thick muscle of short length, make the rest of it noncontractile and hard to tear. Lots of distance to contract but not needing a lot of force? Long thin muscle does the job and the sartorius is the best example.
The sartorius, according to the review sheet my prof. handed out, originates on the ilium and inserts on the tibia and patella. When it contracts it flexes the thigh and shank (so, in other words, causes the leg to swing backwards and lifts up the shank? I have to admit that the word “flex” confuses me in this context. I know it means to bend a joint, but I’m not sure how you’d “bend a thigh”… I can see how you’d bend a knee, but not a thigh.)
The tensor fascia latae has similar origin and insertion points. So if this muscle is there to create a force without swinging the leg over a wide arc, I can see how it might be used to press against the ground if someone is walking, but not be used to swing the leg back all the way. Or maybe it might also be used to hold a leg stationary over the ground? I hope I am on track here.
Humans, and I assume other vertebrates, have two muscle systems. The first is known as the global system, and it consists of large muscles–the ones your average bodybuilder would know about–that are used for moving body parts and whatever they might be holding. Those are the ones that contract a distance.
The other muscle system is the local muscle system, consisting of a bunch of little muscles that I’m glad I don’t have to memorize. Those are used for stabilization during movements, and they contract a little with force.
I think this is what DSeid is saying, only from a different angle.
DSeid is exactly right, put simply, the fascia is the bag we keep our muscles in to keep them from sloshing around, or lying around, jittering. Neatness counts.
For the action of the sartorius, think of its word roots, same as sartorial pleasures, it means the tailor’s muscle, and it helps simultaneously flex and externally rotate the leg at the hip, to put the foot resting on top of the opposite knee, a classic tailor’s position. Not a muscle of much strength but a long motion to execute.
The tensor fascia latae doesn’t contract far but exerts a fair amount of force against the strong thigh muscles to keep them in place, providing a strong lateral wall, so that they can exert much more power. It mainly comes into play when driving your leg forward.
Got to admit, I don’t totally understand the role of the fascia. I’ve got something called chronic or exertional cmpartment syndrome of the forearms. So when I’m exercising intensely, the muscles in my forearms swell up more than the fascia can accomodate which cuts off the blood flow to my forearms and is very painful. Runners will sometimes get this in their legs.
The solution is to do a fasciectomy or to slice the fascia so the muscles have more room to expand. From all the accounts I’ve read, it’s a very effective treatment with no side-effects. So if you can safely slice open the fascia, what purpose was it serving initially? The muscles will be less powerful after a fasciectomy?
