 So we have an intuitive understanding and acceptance that muscle size is related to muscle strength. And so we look at these men here and we probably, most of us except for maybe me, would be pretty convinced that any one of these fellows would probably beat me in an arm wrestling match. I am pretty competitive and scrappy, and it might be a little bit of a battle, but unfortunately my muscles are not that big. And we know that the size of a muscle is generally associated with its strength. So the question that we would ask is, why do muscles get big? And like you can change how big your muscles are. And if I were a chilling human who didn't know anything, I'd probably go, dude, you just make more cells. Make more myofibers and that will bulk up your muscles so that someday I could look like that. So guess what? That's not true. When you are born out of your awesome mama's belly, you have all the myofibers that you're ever going to have. Now I had a conversation with actually two different classes this week. Why? I guess we're all talking about muscles. Regarding my small 10-year-old son, and I made the assertion that my small 10-year-old son will never be a buff stud. Like it just isn't going to happen. And my students were like, dude, you're the meanest mom on the planet. How can you say stuff like that? That's terrible. And I was like, I'm not being mean. Like it just, it's the way it is. And when we look at muscle anatomy, we realize that yeah, there are some people who are born with more muscle fibers than other people. So if one human cannot grow more muscle cells in their muscle, but obviously, holy crud, we can grow our muscles to be bigger. And even my little slime who can't ever look like that without some chemical assistance, which we all know we'll try to avoid, even he can make his little muscles bigger if he works them out. So how do they get bigger? Well, let's go figure that out. Muscle fibers get bigger in diameter and they add myofibrils. Look, let's write that down. Her to increase muscle size. And we associate muscle size with muscle strength and we'll talk about why that is in a second. To increase muscle size, we increase myofibrils. Myofibrils. Do I do that every time I write the word myofibril? This is a myofibril. Here's another one. Here's another one. And those are the protein bundles that are found inside a cell. Remember my little analogy here of spaghetti noodles from the last lab, lecture, whatever, whatever I'm doing right now, nobody knows. Remember how the organization of my thick and thin filaments gives me this stripy look. Each one of these pieces of spaghetti that I have colored on, that was such a fun activity. Each one of them is a bundle of myofilaments. So this is one myofibril. And do you agree that if I take this myofibril and I add another myofibril? Uh-oh, look, I got another one. Look what just happened to my diameter of my muscle. It got huge. How'd that happen? Wow, that's because protein synthesis, eat your protein after you work out, your muscles will respond to, holy crap, you're lifting all this massive weight and we need to get bigger. We need to respond to that and get bigger. So we're going to build more myofibrils. Now if you think about it, myofibrils are just proteins, myofilaments. So we're adding more myofilaments as well. But are we adding more muscle cells? No. The next question that I want you to think about is, okay, awesome, we added more myofibrils. It made our muscle organ bigger. But why is that associated with more strength? How is it possible that more myofibrils mean more strength? Well, to get there, we need to look, we need to remind ourselves about how the myofilaments are overlapping each other and how they're actually set up inside the myofibril. And remember that our thick filaments here, these purple things, have all those little heads and the heads grab a hold of the thin filaments and pull the z-lines in my sarcomere, pull the z-lines closer together and shorten the muscle. Now think about this a second. If you had more myosin heads, if you had more contractile proteins to grab actin and pull, grab the thin filaments and shorten the sarcomere, the more myosin that you have doing that, the more tension you can generate by that muscle, the heavier the muscle is going to be, the heavier things you can pick up. If you think about adding in more and more and more, it's almost like adding in more people in a tug-of-war battle. If you have more hands, like each one of these little things is somebody's hands reaching out and grabbing the rope in a tug-of-war battle, war of the worlds, the more people you get on that rope, the more tension you can generate, the more likely you are to win, win. That's something that you should probably be comfortable explaining. That would be an awesome short answer question. What is muscle strength? How does your body change to make you stronger? This is actually all under hormonal control. And also there's some mechanical stuff in the muscle itself that feeds back. The muscle tissue can be damaged and that feeds back and says, whoa, we need more bigger, stronger myofibrils. So let's just build more of those things. And you need lots of protein so that you can go through protein synthesis to build more myofilaments. How are you? I think I should show you a picture of the muscles. They went, that my son will build up. Never mind, they're cubing muscles. Did you know that those exist? My small child, I guarantee he could beat the crap out of anyone out there except maybe Felix Zemdex in a Rubik's Cube battle because he's built up a lot of Rubik's cubing muscles in his thumb. And somewhere I had a picture of thumb muscle anatomy to indicate that, yes, we have other muscles that we can work out and make very strong. Okay, my children are very cool. Don't worry, I'm not a bad mom. No, I'm not. Okay, I'll be right back and we'll talk a little bit more about what happens here in the sarcomere.