 So I want to talk about the relationship between the length of an entire muscle organ and the amount of tension that that muscle organ can generate. Now I want you to just do a little thought experiment here. If you fully extended your forearm at the elbow joint, so your forearm is fully straight, you're almost in standard anatomical position. And in this position, I want you to imagine that you have, like, keep your elbow fully extended, like, not any kind of, there's no flexion at all in the elbow joint. I want it to be fully extended straight out, almost hyper extended so that your biceps brachii muscle, which is this massive bad boy right here, is as long as it can possibly be. You can't make it longer without, like, applying forces and pulling it apart and stuff like that, which let's not do that. And I want you to imagine it, and you might want to pause and do this. I grab something heavy and then slowly lift the heavy thing. So do like a biceps curl. So you're slowly flexing your forearm at the elbow joint. And then go all the way up, carry that brick or whatever it is that's going to be the heavy thing that you are lifting from a fully extended position to a fully flexed position. And think about, see if you can figure out at what point can you apply the biggest force? At what point do you have, like, is it the weakest? And do it, stop, and go do it. And now you're back because you always do exactly what I say. And I'm going to show you what I'm talking about. When the muscle is short, you actually are generating a small amount of force. Whoa, hello, get out of there. And this is actually, like, I totally feel that is an intuitive truth. If I'm carrying something that my brick and I have to lift it, I'm actually going to flex my forearm at the elbow joint before I lift it. I'm not going to try to lift it from a fully extended position. And I don't even think about that. Like, it just, it feels like it would hurt to lift the brick from a fully extended position. And so I'm just like, watch, like, my brain goes, oh, I'm just going to, I'm going to flex, even though I'm not lifting it anywhere. And now I'm going to lift it. Now I'm going to finish the lift. But to go from that fully extended position, it doesn't feel comfortable. It doesn't feel comfortable because you really can't generate very much tension when you're, I'm on the wrong end. When the length of your muscle is really huge, you can't generate very much tension. But you get the sense that when you shorten that muscle just a tad, you can generate more and more and more tension. This is supposed to be a smooth curve. And there's a point at which you're like, dude, go ahead, throw somebody's face in front of that brick and I'm going to knock the Holy Living Daylights out of them. It's a little violent. I wouldn't actually do that. Maybe, well, okay, we'll stop saying things that are violent in nature. But there is a point where you're like, whoa, I can generate a lot of force. It would be kind of in the middle of the shortening of your muscle. And then there's a point where, you know what? You go back down and when your muscle is super short, I mean, even though you can't really move it very far because you don't have much distance to move it, it doesn't feel very strong. Like if I want to hit somebody in the face with a brick, I'm going to start from here and go like that. I'm not going to start from fully extended and I'm certainly not going to start from up here. It just doesn't feel very strong. So my question for you is, my first statement is, this is the reality. This is the truth. My question for you is why? Why is it like that? Well, to answer that question, let's go back to our friend that you are growing to know and love, the sarcomere. Keep in mind that our myofibrils got there. Our myofibrils are nothing but a whole bunch of sarcomeres in series. And so if, now imagine this for a second. If I could take these thin filaments, the green ones, that's what I was going for. If I could stretch them out. So I'm going to fully extend the muscle and stretch those out so that maybe the amount of overlap starts like right here. And I'm going to take it all the way over here. My z-line is going to be all the way over here. Did you see what I just did? All I did was just pull it apart. And so this space in the middle is going to be really huge and the overlap between the thick and thin filaments is going to be really small. Why is that not going to generate very much force? Is it totally intuitive? If the overlap between my thin filament and my thick filament is like two myosin heads, how can I possibly generate very much force if I'm only using two myosin heads? And it's totally true. If you contract a little bit, shorten the sarcomere just a little bit, there's an optimal zone where we actually have, look at this, this is probably close to my optimal zone, where, dude, every single myosin head has access to the thin filaments to generate a contraction and generate more force. There's a point at which the actins are completely overlapped with the myosin and you can't get any shorter. If you can't get any shorter, you're not going to be able to generate very much tension. You're not going to be able to generate or smash a brick somewhere. That's not very nice with your muscles fully contract. And again, contracted, that makes intuitive sense. Think about sprinters who start, if they're in a race, the 200 meters, they get down into starting position. They don't start the race standing straight up because they want to be as explosive as possible. And in order to be explosive, they need to be partially contracted. They can't be all extended in the case of standing straight up and starting running from that position. All right, that's super cool. Let's talk about flexibility. Okay, good idea.