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Levers and Lever Systems
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Levers, Axes and Planes of Movement
In this course, Dr Adam Brazil (University of Bath) explores axes and planes of movement. In the first lecture, we think about levers and lever systems, reviewing all three classes of lever. In the second and final lecture, we think about the six degrees of freedom in the physical world, looking at the three planes of movement and the three axes of rotation. Photo by Scott Broome on Unsplash.
Levers and Lever Systems
In this lecture, we think about levers and lever systems, focusing in particular on: (i) understanding the purpose of a lever system being to create movement about a turning point, or fulcrum, which is a joint in the body; (ii) the four key features of a lever system being the rigid structure, the fulcrum, the load force and the effort force; (iii) a first class lever system being one which has a fulcrum in the centre and a rigid structure either side, with opposing effort and load forces on each side; (iv) an example of a first class lever system in the body being raising the head using the spine; (v) a second class lever system being one where the load sits in between the fulcrum and the effort; (vi) an example of a second class lever system in the body being standing up onto toes from being flat footed; (vii) a third class lever system being one where the effort sits between the load and the fulcrum; (viii) an example of a third class lever system in the body being a bicep curl; (ix) the fact that the efficiency of a lever depends on the relative positions of the effort and load to the fulcrum; (x) using the placement of a hand when pushing open a door to illustrate how this relative position impacts efficiency; (xi) comparing the efficiencies of the second and third class lever system examples of standing onto toes with a bicep curl.
Hi, everyone. My name is doctor Adam Brazil.
00:00:06I'm a lecturer in sports biomechanics at the University
00:00:09of Bath, specifically within the department for health.
00:00:12Most of my research is based within the sports biomechanics realm,
00:00:15so trying to understand how we can achieve higher performance
00:00:19through biomechanical analysis and specifically the
00:00:22application of biomechanics to the realm of of physical
00:00:25preparation, so strength and conditioning training,
00:00:29trying to understand how biomechanics can inform,
00:00:32better training, regimes for for athletes.
00:00:35And most of my teaching is within the realm of
00:00:40biomechanics as well.
00:00:42So I teach across first and second year biomechanics,
00:00:43with our students here on the sport and exercise science
00:00:46course and the health and exercise science course,
00:00:48and I do some additional teaching within strength and
00:00:50power development, and coordination studies as well.
00:00:53So what I'm gonna talk to you today about,
00:00:57being levers and lever systems, does,
00:00:59is involved within some of our first and second year, cohort,
00:01:02material at the university.
00:01:07So I'm in a good position to talk to you about those today.
00:01:09If we think about levers, what they are designed to do
00:01:12generally within mechanics,
00:01:16but more specifically within the human body is to help us to create movement.
00:01:19Okay?
00:01:23So when we contract our muscles, a linear force is
00:01:24developed, and the effect of that linear force about a
00:01:29fulcrum, a pivot point, which is typically our joints,
00:01:32creates a turning force, which is known as a joint torque or a moment.
00:01:36And it's that torque or that moment that is the the effect
00:01:40that a turning effect of a force that's applied.
00:01:43And levers help us to achieve a turning effect,
00:01:46which then allows us to rotate segments about our joints that
00:01:50help us to produce movements, within our body.
00:01:54And then that allows us to move and perform tasks in a whole
00:01:57host of sport, exercise, going about our daily lives
00:02:01type of activities.
00:02:05So when it comes to lever systems, there are four key
00:02:07features of levers that we need to think about.
00:02:11The first of which is a rigid structure. Okay?
00:02:15So this could be a rigid object, for example, a seesaw,
00:02:18which we'll probably come back to a few times within this
00:02:22session to provide an example.
00:02:25Within the human body, those rigid structures are our bones.
00:02:27Okay? So we need something that we're trying to move.
00:02:31And as I said, within the human body,
00:02:35they're they're our bones.
00:02:36Then the next key feature of a lever system is a fulcrum,
00:02:38which is essentially a pivot point,
00:02:42which allows that rigid structure to move around.
00:02:44And, again, if we're thinking about the human body,
00:02:48those fulcrums are our joints, okay,
00:02:50which allow rigid structures such as, let's say,
00:02:53the forearm forearm here to rotate about that joint.
00:02:56So the fulcrum provides the axis of rotation,
00:02:59for that turning effect to happen.
00:03:03Then we have two forces, which essentially oppose one another.
00:03:05One is known as the load or the resistance.
00:03:11So that is some kind of weight that provides resistance that
00:03:14we need to move.
00:03:17And the other is the effort,
00:03:19which is typically the force that we're applying to try and
00:03:21move the rigid object about that fulcrum point.
00:03:25Okay?
00:03:28So the effort most typically within our human system would
00:03:29be the force produced by our muscles that then pulls onto a
00:03:33tendon, which attaches to a bone,
00:03:36which tries to move the bones around our joints.
00:03:38And if we have those four, components of our lever,
00:03:41and we take the resistance or the load, the effort,
00:03:46and the fulcrum, they're the three key,
00:03:50key aspects of that lever system that we can manipulate
00:03:53that helps us to define different classes of levers,
00:03:57different types of levers,
00:04:00and also to consider the mechanical advantage of
00:04:01different lever systems.
00:04:04Okay. So now we've introduced the features of a lever system.
00:04:08What we'll go on to now is to talk through the different
00:04:12types of levers, which are known as lever classes.
00:04:14There are three classes of lever, and they,
00:04:17dependent on the position of the effort, the load,
00:04:20and the fulcrum in relation to one another.
00:04:24So we'll start from the first and work our way through to the third.
00:04:26The first class of lever system,
00:04:30kind the classic seesaw example.
00:04:32So we have a fulcrum point in the middle,
00:04:34and we have a rigid structure which spans either side of that fulcrum.
00:04:35And then on either side of the fulcrum,
00:04:38we have the effort and the load or
00:04:41the resistance, and the fulcrum is therefore in the middle of those two compro
00:04:48those two components.
00:04:53We have the effort on one side, fulcrum in the middle,
00:04:54and load on the opposite side.
00:04:57So as I've said, a classic seesaw example,
00:04:59fulcrum in the middle,
00:05:02and the seesaw will be moving up and down based on how much
00:05:03weight, is provided on each side.
00:05:06Within the human body, first class lever system would be,
00:05:08for example, raising our neck.
00:05:12In this scenario, we have the neck joint as the fulcrum,
00:05:14which sits in the middle.
00:05:18We have the weight of our head, which we're trying to move,
00:05:20which sits in front of the fulcrum point.
00:05:23And the muscles that are trying to provide the effort or are
00:05:25providing the effort sit behind the joint on the rear side of the body here.
00:05:29And when those muscles contract, they provide an effort force,
00:05:34which pulls the head around the fulcrum to lift our neck.
00:05:37Okay? So that's an example of a first class lever system.
00:05:42And we'll talk about mechanical advantage as we go forwards in
00:05:45the talk, but this, in terms of advantage,
00:05:48sits somewhere in the middle and is dependent on the
00:05:50positions, of that effort and that load.
00:05:54A second class lever is where the load is placed in the
00:05:57middle between the effort and the fulcrum.
00:06:00So in this example, we can imagine a wheelbarrow.
00:06:04So we have the fulcrum, which is the wheel. Okay?
00:06:08And that's the pivot point that we're trying to lift the
00:06:12wheelbarrow around.
00:06:14We come back a little bit,
00:06:15and we have the resistance or the load,
00:06:17which is what we've put into the wheelbarrow itself,
00:06:19which provides the weight of the wheelbarrow that we're trying to lift.
00:06:22And then if we come back a bit further,
00:06:26we have the handles of the wheelbarrow,
00:06:28which is where we apply our effort to lift the wheelbarrow itself.
00:06:30Okay.
00:06:35So we have this system where the load sits between the and
00:06:35the effort this time.
00:06:39From a human body perspective,
00:06:41we can consider standing on our tiptoes as an example of a
00:06:44second class lever if we simplify things.
00:06:48Imagine the fulcrum in this scenario is the ball of our foot,
00:06:51and we are pivoting around the ball of our foot to raise our
00:06:55heels, off the ground.
00:06:58We then have the load or the resistance,
00:07:00which is our body weight in this scenario.
00:07:03That sits behind the fulcrum,
00:07:05around about probably in line with the ankle joint,
00:07:09maybe just slightly in front depending on where our center
00:07:11of mass is placed.
00:07:14And then behind the ankle joints,
00:07:16so even further away from that fulcrum,
00:07:17are going to be our plantar flexor muscles,
00:07:20so our gastrocnemius and our soleus.
00:07:23And when those muscles contract,
00:07:26they're gonna provide an effort force that's going to,
00:07:28aim to raise the heel,
00:07:31and that's gonna allow us to raise the center of mass into
00:07:33the air pivoting around the ball of the foot,
00:07:36which is the fulcrum.
00:07:39So in this scenario, we have the load in the middle.
00:07:40The body weight sits between the effort provided by the
00:07:43gastrocnemius on the posterior aspect of the calf and the
00:07:46fulcrum which sits in front of that body weight,
00:07:50which in this case is a pivot point around the ball of the foot.
00:07:53And this is the most mechanically advantageous type
00:07:57of lever system.
00:07:59And we'll come on for the the reasons as to why that is after
00:08:01we've introduced our last lever system,
00:08:04or class of lever system, should I say,
00:08:07which is the third class.
00:08:09So as there are three components that we,
00:08:10considering within our lever systems, the effort,
00:08:13the fulcrum, and the load,
00:08:16the third class lever system is the combination that we just
00:08:17haven't seen yet.
00:08:20So this is where the effort sits in between the fulcrum
00:08:21and the load itself.
00:08:26Okay?
00:08:28So imagine and this is the most common type of lever system we
00:08:28would see in the human body when we think about rotating a
00:08:32body segment about a particular joint.
00:08:34If we look at a bicep curl as a simple example,
00:08:37imagine we have a extended arm position here and we flex the,
00:08:40forearm up towards the shoulder,
00:08:44so we bring our palm up.
00:08:46This is a nice example of a third class lever system.
00:08:48We have the fulcrum which sits at the elbow joint here.
00:08:51The effort of the biceps is just past our pivot point as
00:08:56the tendon of the bicep runs onto the forearm,
00:09:00so the effect of that force is applied around here.
00:09:02And then further away from that fulcrum
00:09:06is our weight or our load or the resistance that we're trying to move.
00:09:09If we're unloaded,
00:09:13then the center of mass of the forearm and hand system is
00:09:14gonna be where the resistance is placed,
00:09:17so somewhere around here.
00:09:20If we're holding a heavier weight or a dumbbell in our palm,
00:09:21it's gonna shift that position of the load much further
00:09:24towards the hand and much more distally.
00:09:27So when we contract the biceps,
00:09:30the line of action is approximately here,
00:09:32which sits in front of the fulcrum,
00:09:34but the load is even further away than that.
00:09:36So we have a situation where we have fulcrum effort load,
00:09:39effort sits in the middle.
00:09:42And this is the least advantageous of our lever systems,
00:09:44and it's probably a nice way now to start talking about that
00:09:49mechanical advantage in a bit more detail.
00:09:52So the efficiency of a lever relates to the relative
00:09:55positions of the effort and the load,
00:09:59with regard to our fulcrum.
00:10:03Okay?
00:10:05And the reason why we have different advantages,
00:10:05is is because it can be very useful,
00:10:08to turn smaller forces into what can be much larger
00:10:11turning effects or those talks or moments we spoke about at
00:10:16the start of the video.
00:10:19When we think about advantage or mechanical advantage within lever systems,
00:10:22there's two key components that we now need to consider.
00:10:26It's the effort arm and what is known as the load arm.
00:10:29And the ratio of those will determine the advantage of that
00:10:33lever system, and we want it to be in favour of the effort arm.
00:10:36To explain what those arms are in a bit more detail,
00:10:41it's quite simply the distance from that effort or the load to the fulcrum.
00:10:44So how far away from our fulcrum point is the effort being applied?
00:10:49How far from that fulcrum point is the load being applied?
00:10:54That determines the the arms or the,
00:10:57the the effort arm and the load arm.
00:11:00And as I said,
00:11:02the the distance of those in relation to each other will
00:11:03determine the mechanical advantage of that lever system.
00:11:07So let's think about a very common example that we face
00:11:11most days of our lives,
00:11:16to bring this to life a little bit more.
00:11:18So opening a door.
00:11:20If we think about opening a door,
00:11:21we have the door on its hinges,
00:11:23and those hinges are effectively the fulcrum.
00:11:25If you try to push open a door by pushing right next
00:11:27to the hinges, it's gonna be very difficult to open that door.
00:11:32You're gonna have to apply a lot more force.
00:11:35You're gonna have to put in a lot more effort to open that door.
00:11:37If we move a little bit further across,
00:11:42I will say all the way to the other side where we place the
00:11:45handle of a door, there's a really good reason for why that
00:11:49is, is it becomes a lot more easy to open that door.
00:11:51We need a lot less force to push a door open,
00:11:55and we can feel that.
00:11:57If you go and find your nearest door, push against the hinges,
00:11:58push on the handle,
00:12:02the handle is gonna be much easier to open the door.
00:12:03And the reason why is we're increasing that effort arm,
00:12:06which provides a mechanical advantage.
00:12:09The load is always staying in the same place.
00:12:11We're not changing the material properties of the door,
00:12:12but what we're doing is increasing the distance of our
00:12:13effort force to the fulcrum,
00:12:16which provides a mechanical advantage.
00:12:18Okay?
00:12:24If we think about this then in terms of our lever systems
00:12:25within the human body, we've got a nice comparison between,
00:12:29the second and third class levers that we've just spoke
00:12:34about with our examples of raising our toes versus
00:12:37performing a bicep curl.
00:12:40The second class lever system, so the standing on tiptoes,
00:12:42is the most mechanically advantageous lever system that we have.
00:12:46And this is because regardless of the minor details
00:12:50of those effort arms and the load arms specifically,
00:12:53the construction of a second class lever always puts the
00:12:57effort further away to the fulcrum than the load.
00:13:01Because remember, with a second class lever system,
00:13:04the load sits in the middle of the fulcrum and the effort.
00:13:07So the effort is always going to be further away than the fulcrum.
00:13:11The further away it is in relation to the load, the more
00:13:15mechanical advantage we have.
00:13:19So with our plantar flexor example,
00:13:21we have the muscular effort that sits at the the the back
00:13:23of the ankle joint.
00:13:27Our, fulcrum was right onto the ball of the foot and the load was
00:13:28somewhere in the middle where our center of mass acts.
00:13:32So the gastrocnemius can produce a certain amount of
00:13:35force, and it can raise our center of mass.
00:13:38So for me, ninety approximately ninety kilograms,
00:13:41I can raise my center of mass just by using my gastrocnemius muscles.
00:13:45There's no way that I could do a ninety kilogram bicep curl,
00:13:49especially with one arm.
00:13:54Absolutely no chance.
00:13:55And partly of that would be due to the mechanical properties of
00:13:57the muscles themselves,
00:14:02but let's assume that my biceps and gastroc could produce a
00:14:02similar amount of force.
00:14:03The reason why it would become a lot more difficult to move
00:14:03ninety kilograms with a bicep curl is because of the
00:14:06disadvantaged third class lever system that operates around the elbow joint.
00:14:09Lever
00:14:12joint.
00:14:14And it's the same for all third class levers.
00:14:17The construction of that third class lever puts the effort,
00:14:19between the fulcrum and the load.
00:14:23So no matter what those specific distances are that
00:14:25determine the effort and the load arm,
00:14:28the load is always going to be further from the
00:14:29fulcrum than the effort just because of the way the third
00:14:32class lever system is designed.
00:14:34So to try and move ninety kilograms with my
00:14:35biceps is gonna require a lot more force to create the
00:14:43turning torque required to perform that movement.
00:14:47So third class lever systems are the least advantageous,
00:14:50second class systems are the most advantageous,
00:14:54and first class systems can sit somewhere in the middle.
00:14:57And, again, the first class system is fulcrum in the
00:15:00middle, effort, and load either side.
00:15:03And it really then does come down to the distances of those effort and load,
00:15:06which determine the effort arm and the load arm.
00:15:10That really determines how advantageous that lever system is.
00:15:13If we can increase the effort arm and keep the load arm the
00:15:17same, we'll have more advantage.
00:15:21If we can decrease the load arm and keep the effort arm the
00:15:22same, we'll have some advantage.
00:15:26And, again, if we come back to our seesaw example, if we had
00:15:28two people of exactly the same weight sat on either side of a
00:15:32fulcrum, it's gonna stay still.
00:15:36If we were to keep those people exactly the same, so exactly
00:15:39the same weight, but we increase the length of the
00:15:43effort side of our seesaw,
00:15:48then it will start to tilt in that favor.
00:15:50And the same thing would happen if we start to decrease the
00:15:52distance that this person sat to the fulcrum,
00:15:55which is effectively, let's say, our load arm.
00:15:57We'll start to see the same movement apply.
00:16:00And that's because of,
00:16:02that turning effect of those forces that we started the video with.
00:16:04We can have a given amount of force,
00:16:08but what happens with our lever systems is it turns that force
00:16:11into a turning effect.
00:16:14So we can have the same way or the same force for our effort and our load,
00:16:15but the distance that sits in relation to our fulcrum will
00:16:19determine if that object moves or how much of a turning effect
00:16:23we get from that lever system.
00:16:27
Cite this Lecture
APA style
Brazil, A. (2024, September 10). Levers, Axes and Planes of Movement - Levers and Lever Systems [Video]. MASSOLIT. https://massolit.io/courses/levers-axes-and-planes-of-movement
MLA style
Brazil, A. "Levers, Axes and Planes of Movement – Levers and Lever Systems." MASSOLIT, uploaded by MASSOLIT, 11 Nov 2024, https://massolit.io/courses/levers-axes-and-planes-of-movement