Lever systems in the human body

Summary

This video explains the mechanics of lever systems in the human body. It covers the basic components of a lever, including the effort force, resisting force, and the fulcrum, and how they relate to mechanical advantage. The video explores the three types of levers—first-class, second-class, and third-class—detailing their functions and examples in both machines and the human body. It also highlights the importance of levers in enhancing force, changing direction, and gaining distance, while emphasizing the prevalence of third-class levers in the body for movement speed and precision at the cost of force.

Takeaways

• 😀 A lever is a simple machine with two forces (effort and resistance) acting around a pivot (fulcrum).
• 😀 The mechanical advantage of a lever is determined by the ratio of the effort arm to the resistance arm.
• 😀 Levers are classified into three types: first-class, second-class, and third-class, each with distinct functions.
• 😀 First-class levers change the direction of effort force, and their mechanical advantage can be greater or less than one.
• 😀 An example of a first-class lever in the body is the splenius muscle, which helps extend the head across the atlanto-occipital joint.
• 😀 Second-class levers increase the force, with the resistance located between the effort and the fulcrum, such as a wheelbarrow.
• 😀 In second-class levers, the mechanical advantage is always greater than one, but the range of motion is sacrificed to gain force.
• 😀 A rare example of a second-class lever in the body is the plantar flexion of the foot at the ankle joint.
• 😀 Third-class levers increase the range of motion, with the effort located between the fulcrum and the resistance, like a hammer pulling out a nail.
• 😀 Third-class levers are the most common in the body, allowing for precision and speed of movement, though they sacrifice force.

Q & A

What is a lever and how does it work?

A lever is a simple machine that consists of two forces—an effort force and a resisting force—acting around a pivot or fulcrum. The mechanical advantage is determined by the ratio of the distance from the effort force to the fulcrum (force arm) to the distance from the resisting force to the fulcrum (resistance arm).

What determines the mechanical advantage of a lever?

The mechanical advantage of a lever is determined by dividing the force arm distance by the resistance arm distance. The location of the forces influences whether the mechanical advantage is greater or less than one.

What are the three main uses of levers?

The three main uses of levers are: 1) Changing the direction of the effort force (seen in first-class levers), 2) Increasing the force (seen in second-class levers), and 3) Gaining distance (seen in third-class levers).

What characterizes a first-class lever?

In first-class levers, the pivot or fulcrum is located between the effort and resistance. The mechanical advantage can be greater or less than one depending on the relative lengths of the force and resistance arms. A teeter-totter is a common example.

How does the splenius muscle act as a first-class lever?

The splenius muscle extends the head across the atlanto-occipital joint, acting as a first-class lever. It balances the head by applying a force across the joint, with the pivot being the joint itself.

What is the function of a second-class lever?

Second-class levers are designed to increase force. In these levers, the resistance is located between the effort and the fulcrum. A wheelbarrow is a typical example. The mechanical advantage is always greater than one.

Why is the second-class lever rare in the human body?

Second-class levers are rare in the human body because muscle insertions are typically close to joints, preventing most muscles from acting in second-class lever systems. However, an example is the plantar flexion of the foot at the ankle joint.

How does a third-class lever work?

In third-class levers, the effort is located between the fulcrum and the resistance. The effort must be of greater magnitude than the resistance, but it moves a shorter distance. This results in a loss of force but a gain in distance, which is ideal for producing speed and range of motion.

Can you provide an example of a third-class lever in the human body?

A common example of a third-class lever in the human body is the action of the biceps muscle when flexing the forearm. The effort is applied between the elbow joint (fulcrum) and the resistance (hand or object in the hand).

Why are third-class levers so common in the human body?

Third-class levers are common in the human body because they allow for greater speed and precision of movement, even at the expense of force. This adaptation is due to the frequent location of muscle insertions near joints, enabling quick and controlled movements.