Levers and Lever Systems

Hi, everyone. My name is doctor Adam Brazil.

I'm a lecturer in sports biomechanics at the University

of Bath, specifically within the department for health.

Most of my research is based within the sports biomechanics realm,

so trying to understand how we can achieve higher performance

through biomechanical analysis and specifically the

application of biomechanics to the realm of of physical

preparation, so strength and conditioning training,

trying to understand how biomechanics can inform,

better training, regimes for for athletes.

And most of my teaching is within the realm of

biomechanics as well.

So I teach across first and second year biomechanics,

with our students here on the sport and exercise science

course and the health and exercise science course,

and I do some additional teaching within strength and

power development, and coordination studies as well.

So what I'm gonna talk to you today about,

being levers and lever systems, does,

is involved within some of our first and second year, cohort,

material at the university.

So I'm in a good position to talk to you about those today.

If we think about levers, what they are designed to do

generally within mechanics,

but more specifically within the human body is to help us to create movement.

Okay?

So when we contract our muscles, a linear force is

developed, and the effect of that linear force about a

fulcrum, a pivot point, which is typically our joints,

creates a turning force, which is known as a joint torque or a moment.

And it's that torque or that moment that is the the effect

that a turning effect of a force that's applied.

And levers help us to achieve a turning effect,

which then allows us to rotate segments about our joints that

help us to produce movements, within our body.

And then that allows us to move and perform tasks in a whole

host of sport, exercise, going about our daily lives

type of activities.

So when it comes to lever systems, there are four key

features of levers that we need to think about.

The first of which is a rigid structure. Okay?

So this could be a rigid object, for example, a seesaw,

which we'll probably come back to a few times within this

session to provide an example.

Within the human body, those rigid structures are our bones.

Okay? So we need something that we're trying to move.

And as I said, within the human body,

they're they're our bones.

Then the next key feature of a lever system is a fulcrum,

which is essentially a pivot point,

which allows that rigid structure to move around.

And, again, if we're thinking about the human body,

those fulcrums are our joints, okay,

which allow rigid structures such as, let's say,

the forearm forearm here to rotate about that joint.

So the fulcrum provides the axis of rotation,

for that turning effect to happen.

Then we have two forces, which essentially oppose one another.

One is known as the load or the resistance.

So that is some kind of weight that provides resistance that

we need to move.

And the other is the effort,

which is typically the force that we're applying to try and

move the rigid object about that fulcrum point.

Okay?

So the effort most typically within our human system would

be the force produced by our muscles that then pulls onto a

tendon, which attaches to a bone,

which tries to move the bones around our joints.

And if we have those four, components of our lever,

and we take the resistance or the load, the effort,

and the fulcrum, they're the three key,

key aspects of that lever system that we can manipulate

that helps us to define different classes of levers,

different types of levers,

and also to consider the mechanical advantage of

different lever systems.

Okay. So now we've introduced the features of a lever system.

What we'll go on to now is to talk through the different

types of levers, which are known as lever classes.

There are three classes of lever, and they,

dependent on the position of the effort, the load,

and the fulcrum in relation to one another.

So we'll start from the first and work our way through to the third.

The first class of lever system,

kind the classic seesaw example.

So we have a fulcrum point in the middle,

and we have a rigid structure which spans either side of that fulcrum.

And then on either side of the fulcrum,

we have the effort and the load or

the resistance, and the fulcrum is therefore in the middle of those two compro

those two components.

We have the effort on one side, fulcrum in the middle,

and load on the opposite side.

So as I've said, a classic seesaw example,

fulcrum in the middle,

and the seesaw will be moving up and down based on how much

weight, is provided on each side.

Within the human body, first class lever system would be,

for example, raising our neck.

In this scenario, we have the neck joint as the fulcrum,

which sits in the middle.

We have the weight of our head, which we're trying to move,

which sits in front of the fulcrum point.

And the muscles that are trying to provide the effort or are

providing the effort sit behind the joint on the rear side of the body here.

And when those muscles contract, they provide an effort force,

which pulls the head around the fulcrum to lift our neck.

Okay? So that's an example of a first class lever system.

And we'll talk about mechanical advantage as we go forwards in

the talk, but this, in terms of advantage,

sits somewhere in the middle and is dependent on the

positions, of that effort and that load.

A second class lever is where the load is placed in the

middle between the effort and the fulcrum.

So in this example, we can imagine a wheelbarrow.

So we have the fulcrum, which is the wheel. Okay?

And that's the pivot point that we're trying to lift the

wheelbarrow around.

We come back a little bit,

and we have the resistance or the load,

which is what we've put into the wheelbarrow itself,

which provides the weight of the wheelbarrow that we're trying to lift.

And then if we come back a bit further,

we have the handles of the wheelbarrow,

which is where we apply our effort to lift the wheelbarrow itself.

Okay.

So we have this system where the load sits between the and

the effort this time.

From a human body perspective,

we can consider standing on our tiptoes as an example of a

second class lever if we simplify things.

Imagine the fulcrum in this scenario is the ball of our foot,

and we are pivoting around the ball of our foot to raise our

heels, off the ground.

We then have the load or the resistance,

which is our body weight in this scenario.

That sits behind the fulcrum,

around about probably in line with the ankle joint,

maybe just slightly in front depending on where our center

of mass is placed.

And then behind the ankle joints,

so even further away from that fulcrum,

are going to be our plantar flexor muscles,

so our gastrocnemius and our soleus.

And when those muscles contract,

they're gonna provide an effort force that's going to,

aim to raise the heel,

and that's gonna allow us to raise the center of mass into

the air pivoting around the ball of the foot,

which is the fulcrum.

So in this scenario, we have the load in the middle.

The body weight sits between the effort provided by the

gastrocnemius on the posterior aspect of the calf and the

fulcrum which sits in front of that body weight,

which in this case is a pivot point around the ball of the foot.

And this is the most mechanically advantageous type

of lever system.

And we'll come on for the the reasons as to why that is after

we've introduced our last lever system,

or class of lever system, should I say,

which is the third class.

So as there are three components that we,

considering within our lever systems, the effort,

the fulcrum, and the load,

the third class lever system is the combination that we just

haven't seen yet.

So this is where the effort sits in between the fulcrum

and the load itself.

Okay?

So imagine and this is the most common type of lever system we

would see in the human body when we think about rotating a

body segment about a particular joint.

If we look at a bicep curl as a simple example,

imagine we have a extended arm position here and we flex the,

forearm up towards the shoulder,

so we bring our palm up.

This is a nice example of a third class lever system.

We have the fulcrum which sits at the elbow joint here.

The effort of the biceps is just past our pivot point as

the tendon of the bicep runs onto the forearm,

so the effect of that force is applied around here.

And then further away from that fulcrum

is our weight or our load or the resistance that we're trying to move.

If we're unloaded,

then the center of mass of the forearm and hand system is

gonna be where the resistance is placed,

so somewhere around here.

If we're holding a heavier weight or a dumbbell in our palm,

it's gonna shift that position of the load much further

towards the hand and much more distally.

So when we contract the biceps,

the line of action is approximately here,

which sits in front of the fulcrum,

but the load is even further away than that.

So we have a situation where we have fulcrum effort load,

effort sits in the middle.

And this is the least advantageous of our lever systems,

and it's probably a nice way now to start talking about that

mechanical advantage in a bit more detail.

So the efficiency of a lever relates to the relative

positions of the effort and the load,

with regard to our fulcrum.

Okay?

And the reason why we have different advantages,

is is because it can be very useful,

to turn smaller forces into what can be much larger

turning effects or those talks or moments we spoke about at

the start of the video.

When we think about advantage or mechanical advantage within lever systems,

there's two key components that we now need to consider.

It's the effort arm and what is known as the load arm.

And the ratio of those will determine the advantage of that

lever system, and we want it to be in favour of the effort arm.

To explain what those arms are in a bit more detail,

it's quite simply the distance from that effort or the load to the fulcrum.

So how far away from our fulcrum point is the effort being applied?

How far from that fulcrum point is the load being applied?

That determines the the arms or the,

the the effort arm and the load arm.

And as I said,

the the distance of those in relation to each other will

determine the mechanical advantage of that lever system.

So let's think about a very common example that we face

most days of our lives,

to bring this to life a little bit more.

So opening a door.

If we think about opening a door,

we have the door on its hinges,

and those hinges are effectively the fulcrum.

If you try to push open a door by pushing right next

to the hinges, it's gonna be very difficult to open that door.

You're gonna have to apply a lot more force.

You're gonna have to put in a lot more effort to open that door.

If we move a little bit further across,

I will say all the way to the other side where we place the

handle of a door, there's a really good reason for why that

is, is it becomes a lot more easy to open that door.

We need a lot less force to push a door open,

and we can feel that.

If you go and find your nearest door, push against the hinges,

push on the handle,

the handle is gonna be much easier to open the door.

And the reason why is we're increasing that effort arm,

which provides a mechanical advantage.

The load is always staying in the same place.

We're not changing the material properties of the door,

but what we're doing is increasing the distance of our

effort force to the fulcrum,

which provides a mechanical advantage.

Okay?

If we think about this then in terms of our lever systems

within the human body, we've got a nice comparison between,

the second and third class levers that we've just spoke

about with our examples of raising our toes versus

performing a bicep curl.

The second class lever system, so the standing on tiptoes,

is the most mechanically advantageous lever system that we have.

And this is because regardless of the minor details

of those effort arms and the load arms specifically,

the construction of a second class lever always puts the

effort further away to the fulcrum than the load.

Because remember, with a second class lever system,

the load sits in the middle of the fulcrum and the effort.

So the effort is always going to be further away than the fulcrum.

The further away it is in relation to the load, the more

mechanical advantage we have.

So with our plantar flexor example,

we have the muscular effort that sits at the the the back

of the ankle joint.

Our, fulcrum was right onto the ball of the foot and the load was

somewhere in the middle where our center of mass acts.

So the gastrocnemius can produce a certain amount of

force, and it can raise our center of mass.

So for me, ninety approximately ninety kilograms,

I can raise my center of mass just by using my gastrocnemius muscles.

There's no way that I could do a ninety kilogram bicep curl,

especially with one arm.

Absolutely no chance.

And partly of that would be due to the mechanical properties of

the muscles themselves,

but let's assume that my biceps and gastroc could produce a

similar amount of force.

The reason why it would become a lot more difficult to move

ninety kilograms with a bicep curl is because of the

disadvantaged third class lever system that operates around the elbow joint.

Lever

joint.

And it's the same for all third class levers.

The construction of that third class lever puts the effort,

between the fulcrum and the load.

So no matter what those specific distances are that

determine the effort and the load arm,

the load is always going to be further from the

fulcrum than the effort just because of the way the third

class lever system is designed.

So to try and move ninety kilograms with my

biceps is gonna require a lot more force to create the

turning torque required to perform that movement.

So third class lever systems are the least advantageous,

second class systems are the most advantageous,

and first class systems can sit somewhere in the middle.

And, again, the first class system is fulcrum in the

middle, effort, and load either side.

And it really then does come down to the distances of those effort and load,

which determine the effort arm and the load arm.

That really determines how advantageous that lever system is.

If we can increase the effort arm and keep the load arm the

same, we'll have more advantage.

If we can decrease the load arm and keep the effort arm the

same, we'll have some advantage.

And, again, if we come back to our seesaw example, if we had

two people of exactly the same weight sat on either side of a

fulcrum, it's gonna stay still.

If we were to keep those people exactly the same, so exactly

the same weight, but we increase the length of the

effort side of our seesaw,

then it will start to tilt in that favor.

And the same thing would happen if we start to decrease the

distance that this person sat to the fulcrum,

which is effectively, let's say, our load arm.

We'll start to see the same movement apply.

And that's because of,

that turning effect of those forces that we started the video with.

We can have a given amount of force,

but what happens with our lever systems is it turns that force

into a turning effect.

So we can have the same way or the same force for our effort and our load,

but the distance that sits in relation to our fulcrum will

determine if that object moves or how much of a turning effect

we get from that lever system.