Biophysics: Levers in the Human Body
In this topic we'll apply concepts of forces and motion to parts of the human body. We begin with a deeper exploration of levers. Engineers recognize different classes of levers, and this classification applies to the various mechanisms in the body, such as the arm lifting a weight in what is known as a bicep curl.
Vitruvian Man, by Leonardo da Vinci. Source: Wikipedia
Classes of levers
There are three classes of levers. They are categorized by what is in the middle of the lever: the fulcrum, the resistance (or load), and the effort.
A class 1 lever has the fulcrum in the middle and the load and effort on either end. The fulcrum does not have to be geometrically in the middle of the fulcrum; the important thing is that it is between the load and effort. This seesaw is an example of a class 1 lever.
A screwdriver and a crowbar can both be used as first-class levers. A scissors is really a double lever — the fulcrum is the rivet in the middle, the effort force is applied with the fingers, and the load force is what is cut.
A class 2 lever has the resistance or load in the middle, the fulcrum at one end and the effort at the other. An example of a class 2 lever is a wheelbarrow, where the front wheel is the fulcrum.
A wheelbarrow is a second-class lever. Both a nutcracker and a hinged car door are examples of second-class levers. On the car, the hinge is the fulcrum, the effort is applied at the handle near the edge of the door, and the resistance is the weight of the door itself.
A class 3 lever has the effort in the middle, the fulcrum at one end and the load at the other. An example of a class 3 lever is a broom.
A broom is a third-class lever. Tweezers and tongs are pairs of third-class levers with the same fulcrum. A fishing rod can be used as a third-class lever.
What class lever is it? The mnemonic = FRE
One way to remember the classes of lever is to think “FRE” or “free,” as in: “I want to be free of confusion about levers.”
- The F stands for fulcrum, in the middle of a class 1 lever (e.g., seesaw).
- The R stands for resistance (which is the same thing as the load), and it is in the middle of a class 2 lever (e.g., wheelbarrow).
- The E stands for effort, which is in the middle of a class 3 lever (e.g., broom).
Characteristics of class 1 levers
First-class levers always change the direction of the force. In other words, if the effort is “down,” the load moves “up.”
First-class levers can be used to affect the force on the load, the distance through which the load moves, and the speed with which it moves. If the fulcrum is close to the load and far from the effort, the force is increased but the effort must move through a greater distance or with a greater speed to move the load.
Fulcrum close to the load in a class 1 lever
If, on the other hand, the fulcrum is close to the effort, the force is not as much increased but the load moves through a greater distance or with a greater speed.
The mechanical advantage of a first class lever can be greater than 1 or less than 1, depending on the location of the fulcrum relative to the load and effort.
Fulcrum closer to the effort in a class 1 lever
Characteristics of class 2 levers
A second-class lever does not change the direction of the force (if the effort force points “up,” the load moves “up”).
The second-class lever always confers a mechanical advantage because the “effort arm” or distance from the fulcrum to the effort is greater than the “load arm” or distance from the fulcrum to the load.
A second-class lever. Source: http://etc.usf.edu Links to an external site.
Characteristics of class 3 levers
Like a second-class lever, a third-class lever does not change the direction of the force.
The interesting thing about third-class levers is that they do not confer a mechanical advantage. The mechanical advantage of a third-class lever is less than 1!
What, then, is the use of third-class levers? They always produce a gain in the speed (or distance covered per unit time) of the load. Sometimes the gain in speed of the load is useful in itself.
A third-class lever. Source: http://etc.usf.edu Links to an external site.
Levers in the body
The bones in the human body act as levers, with the joints fulfilling the role of pivot points. The muscles provide the effort, and the weights of segments of the body — or external weights — provide the load.
The human body provides examples of first, second, and third-class levers. First and third class levers are the most common in the body.
As we saw in the last section, a characteristic of third-class levers is that they confer no mechanical advantage. And the first-class levers in the body often operate with a mechanical advantage less than 1. The human body is built for speed, rather than mechanical advantage!
First-class levers in the human body
An example of a first-class lever is provided by the head, top of the spine, and neck muscles. The fulcrum of this system is the joint between the occipital bone at the base of the skull and the atlas, the first vertebra of the neck. The weight of the head is like the load, tending to rotate the head forward and down (as one might move if looking through a microscope or writing at a desk). The neck extensor muscles exert the effort to hold the head up.
Second-class levers in the human body
When you do a press-up from the floor, your head, neck, trunk, and legs form a lever that has the balls of the feet as fulcrum. The action of the arms raises the load. This is an example of a second-class lever, with the effort at one end, the load in the middle, and the fulcrum at the other end.
A push-up turns the head, neck, trunk, and legs into a second-class lever.
Third-class levers in the human body
A biceps curl is an example of a third-class lever. The load is the weight held in the hand, the fulcrum is the elbow joint and the effort is provided by the bicep muscles of the arm.
The contraction of the muscles in the upper arm pulls the lower arm up. The muscles move a short distance compared to the end of the lever (the lower arm). The speed of movement in the lower arm is helpful for throwing a ball or swinging a tennis racket.
A biceps curl is an example of a third-class lever.
Joints in the body
Joints (also called articulations) — where two or more bones come together — hold the bones together while giving mobility.
We will briefly consider three types: hinge joints, sliding joints, and ball-and-socket joints.
A hinge joint allows a bone to rotate about one axis, just as a door can open and close by swinging about a vertical axis.
The knuckles of the fingers and between the radius and the ulna in the arm are one kind of hinge joint in the body. Many sources consider the knee and ankle joints to be hinge joints, but these do allow some motion side-to-side as well as around the main axis.
Hinge joints in the body are also called ginglymus joints.
A gliding (or sliding) joint is one in which relatively flat bone surfaces, lubricated with synovial fluid (the biological equivalent of motor oil), slide relative to each other.
The eight carpal bones in the wrist (arranged in two adjacent rows of 4 carpals) form an example of a gliding joint, the mid-carpal joint.
Diagram showing the carpal bones of the wrist. Source: Wikipedia
A ball and socket joint, sometimes called a ball joint, allows the segment that is connected to the ball to rotate in many directions. Ball and socket joints are the most mobile in the body. For example, the arm, which is connected to the shoulder in a ball and socket joint, can rotate vertically (from pointing straight up to pointing straight down), or horizontally (from pointing straight ahead to pointing diagonally behind a person) and around other axes too.
Watch the video below of a ball rotating in a socket:
The hip and shoulder joints are examples of ball-and-socket joints.
The shoulder joint is between the rounded head of the humerus arm bone (the "ball" part) and the concave surface of the scapula (the "socket" part). The concave surface of the scapula is called the glenoid.
The hip joint is between the pelvis and the femur bone in the leg.
The "socket" part of the joint is a rounded cavity in the pelvis.
- The socket in the pelvis is called the acetabulum, which means "little vinegar cup" in Latin. (Vinegar is acetic acid, or acetum in Latin.) The ancient Romans probably kept such an acetabulum on the dining table for dipping food into.
The "ball" part of the joint is at the head of the femur, above a narrower "neck." About half of the ball fits into the acetabulum.
- For more on joints, including types of joints not mentioned here, see the Anatomy Coloring Workbook, by I. Edward Alcamo and John Bergdahl.
Hip joint. Source: Wikipedia