Levers in the Human Body

A lever can be broken down into 3 parts. Each of these parts performs a specific function, needed to overcome a force, and cause movement.
These parts are the force arm (FA), resistance arm (RA) and fulcrum (F).

  • The force arm is the length between where a force is applied, and the fulcrum (Effort).
  • The resistance arm is the length between the object/weight/resistant force and the fulcrum (Load).
  • The fulcrum is the point of rotation about which the movement of the lever system takes place.

Types of levers

There are 3 categories of levers, separated by the arrangement of its 3 parts.

Type 1 – FA-F-RA

Consider a see-saw. Two children, on opposite ends of a long beam, that rocks about a central point. The central point is the fulcrum, with the force arm to one side, and the resistance arm on the opposite side of the fulcrum.

Type 2 – FA-RA-F

This is a more common type of lever, in the everyday world. It is, essentially, the opposite of type 3, where the force arm is now longer than the resistance arm, and the resistance arm lies within the length of the force arm. An example of this would be wedging a plank under a heavy rock, and then using t plank to lift and move the rock.

Type 3 – RA-FA-F

This is by far the most common type of lever in the human body. In this type, the force arm and resistance arm are the same side of the fulcrum. But, the resistance arm is the longer of the two. This means the force arm lies inside the length of the resistance arm.

How levers overcome forces

“Give me a lever long enough and I will move the world”

Levers are used to decrease the amount of force needed to overcome a weight, or resistance. The longer the lever, the less force required to move the same weight, or overcome the same resistance. In a lever system, if the force arm and resistance arm are the same length, then a force equal to or greater than the weight (or resistance) is required. If the force arm is the longer of the two, a force less than the weight is required. And, finally, if the force arm is shorter, then the required force to cause movement is greater than the resistance.