One way to classify skeletal muscles is by their fascicle arrangements, but also just knowing the fascicle arrangements of a particular muscle is going to let you know something about how that muscle works. So let's recall that fascicles are groups of muscle fibers that are bound together by perimysium. Importantly, the muscle fibers run the entire length of a muscle fascicle. In a skeletal muscle, there's never a muscle fiber that's joined end to end to another muscle fiber. As we think about the fascicles and the muscle fibers, we're going to have these two relationships that are pretty important. And that's going to be that the length of the fibers, in a rough sense, is going to be proportional to the length of the contraction for that muscle. Now that makes intuitive sense. Right? If you have a muscle with really long fibers, they can shorten a lot. So a muscle with really long fibers can get a lot shorter. A muscle with short fibers, well, they can't shorten as much in a total sense, so that muscle can't shorten as much. Our other relationship is going to be that the total number of fibers in the muscle is going to be roughly proportional to the strength of the contraction. Now that also makes sense. Right? You got more muscle fibers, they can contract with more total force. You get a stronger contraction in your muscle. What we're going to look at here is how the fascicles are arranged in a muscle can be arranged in ways that either optimize the length of contraction or they optimize the strength of contraction.
Now to see how that works, we have an anatomical model here with some key muscles shown, and we have 6 different fascicle arrangements that we're going to go through and look at the muscles that have those arrangements. First up is going to be what's probably the simplest one, the parallel fascicle arrangement. In the parallel arrangement, the fibers run the length of the muscle all in parallel. And our example here is the sartorius, and you can see that the sartorius is this long, thin muscle, and it's actually the longest muscle in your body. You can see it here. It starts at the hip, it sort of wraps around the thigh, and inserts on the tibia. So because it's long and thin like that, well, the fibers run-in parallel. The fascicles run-in parallel. The fibers are really, really long. That means that this muscle can shorten a lot, but it doesn't have a lot of total fibers. So that means it can't contract with as much force as some other muscles. As sort of a subset of the parallel arrangement, we're going to have the fusiform arrangement. Fusiform means that it's going to be spindle-shaped. And by spindle, we mean that it is wider in the middle than it is on the end. So here we have the biceps brachii. You can see the biceps brachii is kind of narrow on each end, it gets nice and fat in the middle. So that's that spindle shape, and we pull that out, of course, from the upper arm there. By getting a little wider in the middle, that's a way that the muscle can pack in a little bit more force while keeping this parallel arrangement.
Our next arrangement is going to be the convergent fascicle arrangement. In a convergent arrangement, the origin of the muscle is going to be larger than the insertion. And our example for this is going to be the pectoralis major. So that's this sort of major muscle of the chest here. So we have it pulled out, and the right edge here, you can see it gets really wide. That's all the origin of this muscle, and all the fascicles come in in one direction, and they all basically run the whole length of the muscle, and they converge at this small insertion point, which for this muscle is on the humerus. So because you're getting so many fascicles running into a single point, that's a lot of muscle fibers, this can contract with a lot of force. But the fascicles are all still pretty long, so that also means it's got a pretty decent amount of contraction that it can do. So that seems like it gets the best of both worlds, but the drawback to a convergent muscle like this is that the origin has to be really big. It takes up a lot of space in the body to have a convergent muscle like that.
Our next fascicle arrangement is going to be the circular arrangement. Now this is also sometimes just called a sphincter. A circular arrangement or a sphincter is something that wraps an opening. And I'm sure that everybody's aware of the anal sphincter, but our example here is going to be the orbicularis oculi. So you can see here, this is a circular fascicle arrangement, and it surrounds your eye. So when you close your eye like that, what you're doing is you're squeezing down on the sphincter, and that circular muscle is getting smaller, and it closes that opening around your eye. Those first three fascicle arrangements, in all cases, the fibers are the fibers and the fascicles are still basically running the entire length of the muscle. That's going to be different for our next three. So our next three are all versions of the pennate fascicle arrangement. And that word pennate comes from the Latin meaning feather. And you can see here we have a feather here. That's also the root for the word pen. Right? People used to write with feathers as a pen. So pennate and pen come from the same Latin root, meaning feather. So what you see with the structure of a feather, though, it has this shaft running down the middle, and then the soft parts of the feather are actually kind of short, running off the sides.
So a pennate muscle does the same thing. So our first type of pennate muscle is just called a pennate muscle, or sometimes called a unipennate muscle. And these ones, the short fibers, run at an angle from one side of a tendon. So our example here is the extensor digitorum longus. That's this muscle down here in the calf, but you can see as we pull it out here, this tendon runs along the side of the muscle, and the muscle fibers are actually kind of short. They're running off at an angle. Now, why would you do that? Well, short muscle fibers mean that it can't contract much in sort of total length, but you're able to pack in a lot more fibers. So you're able to contract with more force, again, at the expense of the total length of contraction. That's pennate or unipennate, where it comes off of one side of the tendon. Well, our next one is going to be bipennate. So you might be able to see where this is going. Uni to bi. Fibers here extend from both sides of a central tendon. And our example here is the rectus femoris. You can see this is that major muscle of the quadricep running right down the middle of the thigh there, and you can see here there's a tendon running down the middle of the muscle and these short fibers, relatively short fibers, running off the angles off of both sides of that tendon. Again, short fibers mean that this muscle can't contract a lot in terms of total length, but we fit more fibers in so it's able to contract with more force. Our final one, multipennate. Again, you might be able to see where this is going with that prefix multi. This one, the fibers extend from a branched tendon. And our example here is going to be the deltoid muscle. So you can see this muscle here. It's sort of one of those major muscles of the shoulder. It runs over the shoulder there. And you can see here how there are multiple tendons running through this muscle and multiple tendons all having these short fibers coming off of it at an angle. Again, short fibers, we're sacrificing the length of the total contraction, but we're fitting in a lot more fibers. It's able to contract with a lot more strength. Okay. Again, remembering these for different muscles can, again, sometimes just feel like another thing on the list that you need to memorize. But if you understand how these work, it's going to tell you something about the muscle. Like always, we have examples to follow and practice problems after that. I'll see you there.
