This is the third lesson of Mechanical Principles, part of our full Exercise Therapy Video Lessons course for BPT 1st year, and in today’s lesson, we’ll cover levers and pulleys. These fall under the category of simple machines.
So, friends, let’s start with a lever.
Lever
A lever is a rigid bar capable of movement around a fixed point called a fulcrum.
A lever has two main components: a rigid bar and a fulcrum. The fulcrum is a fixed point around which the rigid bar moves. It can either rotate or move up and down.
Moving a lever requires two forces. One force is called effort, and the other is called weight (or load). It is only with the help of both forces that any work is done.
So, when is work done?
Work
If we apply effort at one point, and the force of that effort acts at another point where the weight is located, then we call it work done.
For example, in this picture, this little girl is trying to move a stone with a lever. The lever here is the rigid bar, with the help of which she is trying to move this large stone.
The small stone placed beneath the bar acts as a fulcrum. It acts as a fixed point.
So, when this girl applies effort here, i.e., presses it downward, only then will the weight experience any action or motion. Only then can we call it work done.
Components of a lever
So, what are the components of a lever?
It has three important components: effort, weight (load), and the fulcrum. The fulcrum is the fixed point.
- The distance from the fulcrum to the effort point is called the effort arm.
- The distance from the fulcrum to the weight is called the weight arm (or load arm).
Later, we’ll learn about the role of the effort arm and the weight arm in mechanical advantage.
Levers in the human body
Any movement that occurs in the joints of the human bodyโlike moving the elbow or bending the headโhappens through levers. We can understand body movements through the concept of levers.
Look at a simple figure of an arm with the humerus bone, the radius and the ulna bone. You can also see the biceps muscle.
If we understand this in the concept of a lever, then the elbow joint acts as a fulcrum.
Due to the action of the biceps, our elbow flexes. The action of the biceps becomes the effort part of the lever, and the insertion point of the biceps muscle on the proximal part of the radius bone becomes the point of effort. Finally, the weight of the forearm becomes the weight part of the lever.
So, it is a lever with all three components: fulcrum, effort, and weight. In this way, we can understand any motion in our body by means of a lever.
Types of lever
There are three types of levers: first-class, second-class, and third-class. Let’s understand each one.
First-class lever
In a first-class lever, the fulcrum lies between the weight and the effort.
A classic example is a seesaw (or crowbar). The fulcrum is in the middle, effort is applied on one side, and the load is on the other side.
It is categorised as a first-class lever because the fulcrum is present between the effort and the weight.
First-class lever in the human body
We also have first-class levers in our bodies. The best example is the nodding of the head (atlanto-occipital joint).
The nodding of the head results from the effort of the muscles at the back of the neck. These muscles insert on the occipital region of the skull, which becomes the point of effort.
The weight of the head falls anterior to the neck (the line of gravity of the head becomes the point of weight). The atlanto-occipital joint acts as the fulcrum over which the nodding movement takes place.
To sum up: effort acts on the back of the head, weight falls in front of the neck, and the joint acts as the fulcrum lying between them. That is why it is a first-class lever.
Another excellent clinical example of a first-class lever is the hip joint during single-leg stance.
Second-class lever
In a second-class lever, the weight (load) is placed between the fulcrum and the effort. A common example is a wheelbarrow.
Second-class lever in the human body
When we stand on our toes (heel raise / plantarflexion), that is a classic example of a second-class lever.
Here is how it works:
โข Fulcrum โ the balls of the feet (metatarsal heads)
โข Weight / Load โ the weight of the body acting through the ankle joint
โข Effort โ the force of the calf muscles (gastrocnemius and soleus) transmitted through the Achilles tendon, which inserts on the calcaneus (heel bone)
The load lies between the fulcrum and the effort. That is why rising onto the toes is a second-class lever.
There are other examples in the body, but we will not cover them all here to keep the lesson focused.
Third-class lever
In a third-class lever, the effort lies between the fulcrum and the weight.
Interestingly, most levers in the human body are third-class levers. This arrangement favours speed and range of motion over force. Even though the mechanical advantage is less than 1 (we have to apply more effort than the load), we gain greater movement and velocity at the distal end โ which is very useful for everyday activities and sports.
A good example is elbow flexion while holding a dumbbell.
Elbow flexion occurs due to the contraction of the biceps muscle. The biceps inserts just proximal to the radius bone, so the point of effort is close to the elbow. The weight of the forearm plus the dumbbell acts as the load, and the elbow joint acts as the fulcrum.
In a simple line diagram, the effort lies between the fulcrum and the weight โ making it a third-class lever.
Mechanical advantage of a lever
Whether a lever has a mechanical advantage or not depends on the relative lengths of the arms.
Mechanical advantage (MA) = Effort arm รท Weight arm (Load arm).
Quick summary by class:
- First-class lever โ MA can be greater than 1, equal to 1, or less than 1, depending on the relative lengths of the arms.
- Second-class lever โ MA is always greater than 1 (effort arm is longer than weight arm). These levers favour force.
- Third-class lever โ MA is always less than 1 (effort arm is shorter than weight arm). These levers favour speed and range of motion. Most body levers fall into this category.
Example calculation:
Suppose the length of the effort arm is 8 metres and the length of the weight arm is 2 metres. MA = 8 รท 2 = 4. The mechanical advantage is 4. The longer the effort arm compared with the weight arm, the greater the mechanical advantage and the easier the task becomes.
In the real-life example of the girl moving a stone with a lever, refer back to the image at the beginning of the article.
The longer the distance between the fulcrum and the point where she applies effort (effort arm), relative to the weight arm, the greater the mechanical advantage.
Pulleys
Now let’s move on to pulleys and their mechanical principles. A pulley consists of a wheel with a grooved rim through which a rope or cable passes.
If you have lived in a village, you have likely seen the system for drawing water from a well using a pulley. A rope runs through the groove of the pulley. On one side is a bucket; on the other side, a person pulls the rope. This rope-and-pulley system increases the efficiency of the work by changing the direction of force and, in some arrangements, reducing the effort required.
Types of pulley
There are two main types of pulleys:
- Fixed pulley
- Movable pulley
Fixed pulley
In a fixed pulley, the position of the pulley remains fixed (as in the well example). A bucket hangs on one side and a person pulls downward on the other side of the rope.
A fixed pulley mainly changes the direction of the force. The magnitude of the force required remains roughly the same, but the new direction makes the task more convenient for the body.
Other common examples include flagpoles and certain construction crane systems. In these cases, the weight itself does not change; only the direction of the effort changes, making the task easier to perform.
Movable pulley
A movable pulley system usually involves two pulleys: one fixed and one movable. One end of the rope is fixed and effort is applied to the free end. With a single movable pulley, the effort required to lift the load is reduced by approximately half.
For example, if you are lifting a 10 kg weight, the effort required can drop to about 5 kg because the weight is shared between two segments of the rope.
This reduction in effort is the main advantage of a movable pulley.
Key Takeaways
- A lever consists of a rigid bar, a fulcrum, effort, and load (weight).
- There are three classes of levers, classified by the relative positions of fulcrum, effort, and load.
- Most levers in the human body are third-class levers. They sacrifice force for greater speed and range of motion.
- Second-class levers (e.g., rising on toes) always provide mechanical advantage greater than 1.
- Fixed pulleys change the direction of force; movable pulleys reduce the effort required.
In the next chapter, we will explore the pendulum. We hope you found this lesson interesting.
FAQ
First-class levers have the fulcrum between the effort and the load (e.g., nodding of the head at the atlanto-occipital joint). Second-class levers have the load between the fulcrum and the effort (e.g., standing on toes, with the metatarsal heads as the fulcrum). Third-class levers have the effort between the fulcrum and the load (e.g., elbow flexion) and are the most common type found in the human body.
The author is a physiotherapist who has been practising for the last 17 years.ย He holds a Bachelor's in Physiotherapy (BPT) from SVNIRTAR (Swami Vivekananda National Institute of Rehabilitation and Research), one of the prestigious physiotherapy schools in India.
Whatever he learns dealing with his patient, he shares it with the world through blogs and e-books. He also owns a YouTube channel, "Sunit Physiotherapist" with over 8 lakh active subscribers. Here, he shares everything he gets to learn serving the patient.
