 Hey, you guys. So in the last lecture, we introduced skeletal muscle and we looked at the anatomical and structural characteristics of skeletal muscle tissue and then how that tissue is used to put together skeletal muscle organs. And then we looked at axial musculature, which is awesome. Today, we're going to look at some of the functional consequences of the structure of skeletal muscle tissue. And so we're going to kind of do this like in and out thing where we're going to explain stuff that we see in muscle function every day that we're really comfortable with. We're going to explain it on a cellular and subcellular level of organization. So this lecture is pretty interesting, but we have to make sure that we're starting with a really solid understanding of skeletal muscle structure. So I have this image from the OpenStacks textbook just as a reminder of where we are in our overview. And then we'll go into addressing some specific questions. First of all, remember that the skeletal muscle organ is the giant structure that you can grab like my oh so massive biceps brachii. Skeletal muscle organs are made of bundles of muscle cells and those bundles of muscle cells are called fascicles. So each one of these little bundles right here is a fascicle. I'm just going to label that because my brain, I was scolded for my horrible handwriting. What? One of my face-to-face students, wasn't even a YouTube student said, dude can you type your notes please? Sorry about that. No, I'll try to write a little neater. So a fascicle is a bundle of muscle cells and we know that our muscle cells are called myofibers. So check this out. This is my myofiber. All a myofiber is a very long, like crazy unique cell. It has multiple nuclei and it's filled with all these really bizarre protein bundles. And this right here is a protein bundle and it's called a myofibril, myofibril. And myofibrils are made up of myofilaments. So let's just say that myofilament is the little, the little lines that I have right here. We know that we have thick filaments and thin filaments. It looks like my thin filaments are in light blue and it looks like my thick filaments, my thick filaments are in red and you can see that they overlap each other and that is the phenomenon known as the sarcomere. Here are my thick filaments, do you see that? Remember we had the z-lines, this is the z-line. So from z-line to z-line, that was one sarcomere. And remember that a sarcomere is nothing more than overlapping thick and thin myofilaments. Remember that the thick filaments had those little contractile heads that grabbed a hold of the thin filaments and you slide across and you actually get the shortening of the muscle. So if we were to look at this, if we were to contract this single myofibril right here, it would shorten in this direction. That my massive biceps brachii right here, when I flex my forearm, you can barely see, you can't even see it, how's that? Look, I'm flexing my massive biceps, flexing my forearm and I can feel the muscle shortening. It's shortening because my sarcomeres are going like that. Okay, we're going to rely on our understanding of sarcomeres and myofibrils when we're talking about some other characteristics in the rest of this entire lecture. So make sure you're comfortable with this little image right here so that you have a good solid understanding going into the next set of questions. The first and most important question that we're going to answer is, why am I so unbelievably strong?