Levers - Online Tutor, Practice Problems & Exam Prep

Lever Systems

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.

Levers Example 1

This tells me that the origins of the three hamstring muscles are on the ischial tuberosity. The insertions are on the proximal end of the tibia and the fibula. And it says, knowing this, what type of lever system is the hamstring a part of when it performs the movement of flexing the knee? Then it wants to know, would you expect the hamstring to work at a mechanical advantage or disadvantage when it performs this movement? And to guide us a little bit, it has an image of somebody flexing their knee, bending their knee backwards like this, and it shows one of those muscles of the hamstring, the biceps femoris. So, to figure out the lever system, I want to look at this muscle. The biceps femoris has its origin on the ischial tuberosity. It also has an origin on the back of the femur there, but this is not relevant to the problem. It has an insertion on the proximal end of the tibia and the fibula right there.

Now, to determine the lever system, I need to identify the fulcrum, the effort, and the load. First, the fulcrum is the pivot point, which for this movement is the knee. So let's represent the knee as the fulcrum. Next, the load is the weight being moved, which is largely the leg itself. For our diagram, we can assume the center of gravity of the leg represents the load. Finally, the effort comes from the spot where the muscle attaches and exerts force, which in this case is the insertion on the proximal end of the tibia and fibula.

Recalling our mnemonic device: "The Eiffel Tower had an elf, but he fell," it spells out 'left' when arranged in one way, but not relevant to our options. Reading it the other way, it spells out 'fell,' indicating a third-class lever system. In a third-class lever system, the effort is between the fulcrum and the load. This means that the hamstring works at a mechanical disadvantage because the effort is closer to the fulcrum, necessitating more effort to move the load. Despite this disadvantage, the third-class lever arrangement allows the end of the leg to move rapidly and with a large range of motion, which is advantageous for running. This is why many muscles in the arms and legs operate as third-class levers.

In what type of lever is the force applied between the fulcrum and the load?

• A. First-class.
• B. Third-class.
• C. Second-class.
• D. Force cannot be applied between the fulcrum and the load.

The majority of the lever systems in the arms and legs are third-class levers; there are few second-class levers. Knowing this which of the following statements is correct?

• A. The muscles of the arms and legs usually work at a mechanical disadvantage.
• B. These joints will have relatively small range of motion.
• C. Most movements of the arms and legs prioritize power over speed.
• D. The fulcrum will most often be placed between the effort and the load.

Here’s what students ask on this topic:

What are the three types of levers in the human body?

The three types of levers in the human body are first class, second class, and third class levers. A first class lever has the fulcrum positioned between the effort and the load, like a seesaw. An example in the body is the head, where the fulcrum is the neck joint. A second class lever has the load between the fulcrum and the effort, similar to a wheelbarrow. The calf muscle lifting the heel is an example. A third class lever has the effort between the fulcrum and the load, like a catapult. The biceps brachii lifting the forearm exemplifies this type.

How do levers provide mechanical advantage in the human body?

Levers provide mechanical advantage in the human body by positioning the fulcrum closer to the load. This arrangement makes it easier to move the load with less effort. For example, in a second class lever like the calf muscle lifting the heel, the fulcrum (toe joint) is far from the effort (calf muscle) and close to the load (body weight), making it easier to lift the body. Mechanical advantage allows for efficient movement and reduces the amount of force needed to perform tasks.

Why do third class levers in the body operate at a mechanical disadvantage?

Third class levers in the body operate at a mechanical disadvantage because the effort is applied between the fulcrum and the load. This arrangement requires more force to move the load. For instance, in the biceps brachii lifting the forearm, the fulcrum is at the elbow, the effort is applied at the forearm, and the load is at the hand. Although this setup requires more effort, it allows for a greater range of motion and faster movement, which is essential for activities like throwing a baseball.

What is an example of a first class lever in the human body?

An example of a first class lever in the human body is the head and neck. The fulcrum is the neck joint, the load is the weight of the head, and the effort is applied by the muscles at the back of the neck, such as the splenius capitis. When these muscles contract, they pull the head backward, demonstrating the typical first class lever arrangement where the fulcrum is between the effort and the load.

How does the calf muscle function as a second class lever?

The calf muscle functions as a second class lever when lifting the heel off the ground. In this lever system, the fulcrum is the toe joint, the load is the body weight centered over the foot, and the effort is applied by the calf muscle at the heel. This arrangement allows the calf muscle to lift the body with less effort, providing a mechanical advantage similar to how a wheelbarrow works.