 The last thing we're going to talk about is a couple of ways to describe different kinds of big picture muscle movements. So there are two main categories of muscle contractions, and they are isotonic contractions, and this is big muscle. So now we're taking all of the strategies used in the previous section by the central nervous system to get a muscle contraction. We're going to get those and we're going to look at, okay, what kind of contractions can we end up with? We can end up with isotonic or isometric contractions. Metric, sure. Iso, what does iso mean? Same. Isometric, I have to do isometric first, because that's how my brain does it. Isometric, same length. In an isometric contraction, you have muscle contraction, but you have no movement. What? It happens all the time. I can push on a wall. I can try to lift the building. It doesn't matter how much I can track my muscle. Sorry, I'm not going to be able to move some things. That is considered an isometric contraction. Isometric contractions happen because of elastic qualities of the sarcomeres. There are actually these little elastic structures running through the sarcomeres, and we didn't even talk about those guys, but they allow for some extra stretching where we can end up attempting to contract and we can stretch on the edges so that ultimately there's no shortening of the muscle, because all we've done is actually just stretched out some ends of our sarcomere. There's also stretchy components in tendons, and so you literally are stretching your muscle organ. You're applying the force and trying to shorten it, but you're not successfully shortening it because of the stretchiness. Think about it. This only happens if the load is too heavy to lift. So you're applying a force, but what you're applying the force to is too heavy. You can't lift it. So no amount of force that your muscle generates is going to move the object, which means you're literally stretching your muscle to the maximum, and depending on how big of a thing you're lifting and how much force you're trying to lift it with, you can do damage to your muscles. You can actually shorten them and break them. That's a bummer. Try not to break your muscles. Isotonic. Okay, I can take a deep breath. I wonder how I said it in my notes. Okay, the same, we're moving, but we have the same force being applied. So in an isotonic contraction, you apply a force, and the force that you're applying actually does move the thing, and it's the same force all the way through. So you have a very smooth movement that says smooth. Are you ready for this concept? There are two flavors of isotonic contractions. There are concentric, concentric. Concentric contractions are the kind that you would expect from like flexion of the forearm. My bicep muscle contracts and flexes my forearm. Concentric contractions shorten the muscle. That's a C. Okay, this is opposed to something called an eccentric contraction, and go ahead and get ready for this concept. Eccentric contractions result when a muscle lengthens. Does anybody want to call bull hockey on that one? Probably, but no, it's true. Imagine, if you will, my massive bicep has just lifted this gigantic pen. And now I'm going to lower my arm, and I'm not just going to let the pen drop to the ground. I'm actually releasing, I'm letting the pen go, but feel your bicep when you let your pen go, if you slowly put it down on the ground. Your triceps are contracting. This is an antagonistic group to the biceps group, so we are getting triceps contraction in order to drop the pen down back onto the table. But I'm contracting my biceps even while I'm lengthening them. In order to control putting it down, think about the central nervous system and the fine messages that need to be delivered to these muscles in order to carry out what, if you start thinking about it, become wholly complicated actions. Holi complicated. I love it. All right, my friends, this was exciting and fun, and I'm sure we'll, like, play tug-of-war or something fun in class when I see you. Okay, I might not make you play tug-of-war, you better show up. Okay, bye.