The three classes of levers are first-class, second-class, and third-class levers. They differ according to the relative positions of the fulcrum, effort, and load.
In human movement, a lever is usually a bone, the fulcrum is the joint or axis of rotation, the effort is the force produced by a muscle, and the load is the resistance being moved. The order of these three components determines the lever class.
| Lever class | Arrangement and mechanical effect | Example in movement or sport |
|---|---|---|
| First class | The fulcrum is between the effort and load. Mechanical advantage may be greater than, equal to, or less than one, depending on arm lengths. | Elbow extension by the triceps, such as during a throw |
| Second class | The load is between the fulcrum and effort. The effort arm is longer than the load arm, producing a mechanical advantage greater than one. | Plantar flexion when rising onto the toes; the ball of the foot is the fulcrum, body weight is the load, and the calf muscles provide effort through the Achilles tendon |
| Third class | The effort is between the fulcrum and load. The effort arm is shorter than the load arm, producing a mechanical advantage less than one. | Elbow flexion by the biceps when performing a curl |
Mechanical advantage can be expressed as:
Second-class levers favour force production, whereas third-class levers favour speed and range of motion. Most levers in the human body are third class because rapid, extensive movement is often more useful than minimizing muscular effort.
A common misconception is that lever class depends on the type of joint. It depends instead on the arrangement of the fulcrum, effort, and load in a particular movement.
For IB SEHS B.1.4, be prepared to identify each component in a sporting action, classify the lever, and explain its mechanical advantage. State the arrangement explicitly rather than relying only on a memorized example.