The 3 Classes of Levers

    Understanding how lever systems work in the human body is fundamental to kinesiology!

    What is a Lever?

    Let's talk about levers and how we use these levers in everyday life and inside our bodies to produce movement, increase force, and make directional changes. Levers are one of the simplest forms of a machine. A lever is a simple machine that allows you to gain a mechanical advantage in moving an object or in applying a force to an object.

    A lever is a simple machine that consists of 4 parts:

    1. 1The Fulcrum — known as the pivot point, axis, or balance point
    2. 2The Resistance — or load
    3. 3The Effort Force
    4. 4The Lever Arm — divided into the Force Arm and Resistance Arm
    Diagram showing the 4 parts of a lever: Fulcrum, Resistance, Effort, and Lever Arm
    The 4 parts of a lever system

    Mechanical Advantage Formula

    The mechanical advantage is determined by dividing the force arm by the resistance arm:

    Force Arm ÷ Resistance Arm = Mechanical Advantage

    Equilibrium is achieved when: Effort × Force Arm = Resistance × Resistance Arm

    Watch the Video Lesson

    1st Class Lever

    Fulcrum in the Middle

    The first class lever is set up with the Fulcrum in the middle with the Effort and Resistance on the sides.

    Function

    Change Directions

    Mechanical Advantage

    Can be <1 or >1

    1st Class Lever diagram showing fulcrum in the middle

    Real World Example: The Seesaw

    A comparable real world item would be the seesaw. The Fulcrum is in the middle, with each person acting as the resistance or effort force depending on which one is changing direction.

    Seesaw example with Patty at 200lbs and Sara at 100lbs - unbalanced
    Patty (200lbs) and Sara (100lbs) at equal distances - unbalanced

    For example, we have Patty at 200lbs and Sara at 100lbs. In order for Sara @ 100lbs to move Patty @ 200lbs, Sara would have to increase the mechanical advantage >1 — at least 2 times the distance from the fulcrum.

    Seesaw balanced with Sara at increased distance from fulcrum
    By increasing her distance, Sara gains mechanical advantage to balance Patty
    2nd Class Lever

    Resistance in the Middle

    The 2nd class lever is set up with the Resistance in the middle of the effort force and the fulcrum.

    Function

    Increase Force (ROM sacrificed)

    Mechanical Advantage

    Always >1

    2nd Class Lever diagram showing resistance in the middle

    Real World Example: The Wheelbarrow

    The common real world item that uses this lever would be a wheelbarrow. The wheel is acting as the fulcrum, with the rock being the resistance in the middle, and the effort force would be the human force.

    Wheelbarrow example showing 8ft force arm divided by 2ft resistance arm equals 4
    8ft Force Arm ÷ 2ft Resistance Arm = 4 (Mechanical Advantage)

    Imagine this rock weighed 100lbs, and the force arm is twice as long as the resistance arm — the 100lbs would feel like 50lbs. If the force arm was 4× as long, the 100lbs would feel like 25lbs! However, the longer the force arm, the less ROM you will get.

    Diagram showing loss of range of motion in 2nd class lever
    Increased force comes with decreased range of motion
    3rd Class Lever

    Effort in the Middle

    The final lever is the 3rd class lever. The Effort is located in the middle of the resistance and the fulcrum.

    Function

    Gain Distance/Speed

    Mechanical Advantage

    Always <1

    Real World Example: Hotdog Tongs

    The common real world item would be hotdog tongs. The fulcrum would be where the tongs connect, the effort is applied to the center of the tongs, with the hotdog being the resistance.

    Hotdog tongs as a 3rd class lever example
    Tongs: Effort in the middle between fulcrum and resistance

    💡 This is the most common lever used in the human body!

    Study Tip: Remember "FRE"

    A good way to remember which lever is which is the word FREE, but with one E. So spell it F-R-E:

    F
    1st Class
    Fulcrum in middle
    R
    2nd Class
    Resistance in middle
    E
    3rd Class
    Effort in middle
    FRE memory tip diagram for remembering lever classes

    Levers in the Human Body

    Quick Reference Summary

    Lever ClassMiddle ComponentFunctionMech. AdvantageExample
    1st ClassFulcrumChange Direction<1 or >1Seesaw, Head/Spine
    2nd ClassResistanceIncrease Force>1 (always)Wheelbarrow, Heel Raise
    3rd ClassEffortGain Distance<1 (always)Tongs, Elbow Joint