Here, we're going to talk about levers. Levers are in your textbook in the muscle section, and there's a decent chance there will be a question on levers in your test. But to me, really understanding how levers work really helps you understand how the body moves. Typically, when we think of levers, we're thinking about moving something heavy, and a lever can make it easier to do that. Now that happens in the body sometimes, but another thing that levers do is that if you build a lever differently, you can make things move really fast. And that's often how the body is using levers. The example I like to use is that there are human beings that can throw a baseball 100 miles per hour. That means their fingertips have to be moving at least that fast. There are no muscles in the body that can contract at 100 miles per hour. So how do they do it? They use a system of levers. Alright. So let's see how this works. We're going to say that muscles generate force using lever systems, and a lever system is going to be a rigid rod, and in the body, that's just a bone, and there's a pivot point in the body that's a joint. And there are going to be 3 aspects of levers we need to learn here. There's going to be the load, and the load is going to be the weight that has to be moved. And usually, this is the body weight, but in the example of the baseball, it might be moving the baseball as well. There's going to be a fulcrum. That fulcrum is the pivot point, and we already said in the body that's going to be a joint. And then finally, there's going to be the effort, and this is the force that's going to be applied to that rigid rod. And we are going to say that this force is going to be applied at the insertion of a muscle. So we're going to look at different joints here and think about what types of levers these are. When you're thinking about where the effort is applied, don't always look where the muscle is, look where the muscle inserts. Alright. There are 3 types of levers here. Our first-class, second-class, and third-class levers. And I'm going to introduce a little mnemonic device that we'll build out, and you'll understand where it comes from in a second. Our mnemonic device for first, second, and third-class levers is that the Eiffel Tower had an elf, but he fell. It's kind of sad, actually. Alright. A first-class lever. This is our Eiffel Tower. And this is the type of lever that you probably think of when you think of a lever. When we distinguish these different levers, we're going to be talking about what order that load, fulcrum, and effort are in. So here we have an image of a person sort of pulling down on the rod, then comes the fulcrum, and the load is on the other side. So we are going to say that we have the effort, the fulcrum, and the load in that order. That's where our Eiffel Tower comes from. Alright. Again, this is the lever that you're probably most familiar with. The second-class lever, we're going to change the order here, and you can see that we have the person way out on the end, applying the effort to the end of this rod. In the middle is the load, and the fulcrum's on the other end. So we are going to put this one in the order of ELF. Effort load fulcrum. That's our l. Now you might not recognize this as a lever or think when does this happen, but this is how a wheelbarrow works. Right? If we just sort of draw a wheel on here and a bucket, you can imagine that's a wheelbarrow. And I don't know about you, sometimes I put a lot of rocks in a wheelbarrow, and it's pretty easy to lift the wheelbarrow on my end because I have the lever to help me. Our third-class lever, we're going to change the order again. We're going to put the fulcrum on one end. We're going to put the force in the middle. And we're going to put the load on the end. This is the cell, the l cell. So we're going to put here the FEL fulcrum effort load. And you might look at that and say, well, that looks like it's going to be hard to lift the thing. Right? If this load is way out at the end, that looks like it's going to be hard. And you're right. It is going to be harder. So why would you do something like this? Well, this is how a catapult works. Right? You put a ton of force near the fulcrum, and you can get this load out at the end moving really fast. Alright. So let's look at where these happen in the body, so that when you're learning the muscles you can sort of think what type of lever system is happening here, and how does that affect movement. As we do this, we're going to talk about mechanical advantage and disadvantage. An advantage means that it's going to be easier to do the movement based on the structure of the lever, and a disadvantage means that it's going to be harder to do the movement based on the construction of the lever. So a first-class lever. A classic example of this is your head. We have the muscle here, the splenius capitis, and that has an insertion on the back of the head. When it contracts, that effort is going to sort of pull down on the head like that with the fulcrum in the middle, and the load is the head. So the load is pulling down here, and it's going to move this way. How easy it is to do a movement with a first-class lever is going to depend on the location of the fulcrum. You can see in this example up here, the fulcrum is very close to the load. So you can imagine it's going to be relatively easy for this person to pull down and move that rock in this image. But if we drew a different fulcrum, if we drew the fulcrum way up here, right, now the fulcrum is very close to the effort. You can imagine that's going to be very hard. That's going to create a mechanical disadvantage for the person. Okay. A second-class lever. A classic example of a second-class lever in the body is your calf muscle. The calf muscle is going to pull up on your heel. The load is sort of in the middle, your body weight sort of centered over your foot, and the fulcrum is out here over the toe. This creates a mechanical advantage. And remember I said that's like the wheelbarrow. I can put a lot of rocks in my wheelbarrow, and it's still easy for me to get them off the ground and wheel them around because I'm working at that mechanical advantage. Our third-class lever system, this is how a lot of the muscles in the arms and the legs work. A third-class lever system, we talked about sort of like that catapult, is going to work at a mechanical disadvantage. And we can see that here with the biceps brachii. The insertion is there on the forearm. The fulcrum is at the elbow. So that insertion is really close to that fulcrum, and the weight that you're trying to move, maybe throwing a baseball, is way out there at the end. That means that this muscle has to work really hard to move that weight. Alright. So just to sort of formalize or talk about advantage and disadvantage, we're going to say that advantage is determined by the location of the fulcrum. If the fulcrum is closer to the load, that is going to equal an advantage. If the fulcrum is closer to the effort, that's going to equal a disadvantage. Okay. So why would you want to work at a disadvantage? Well, a mechanical disadvantage, yes, it requires more effort to do the movement, but it creates a greater range of motion. You can throw a baseball a 100 miles per hour because your body is working as a series of levers in series, all working at a mechanical disadvantage. You're moving your shoulder. You're moving your upper arm. You're moving your forearm. You're moving your hand. You're moving your fingers. All at mechanical disadvantages. That means you need to put a ton of force into it, which is why pitchers often injure their arms. But in doing that, you can move that baseball, the tips of your fingers, very, very fast. Alright. Again, you may get some questions about levers on a test, but understanding how levers work will really help you understand how the body moves. For that, there's an example to follow and practice problems after that. Give them a try.