Which characteristic defines the first class lever?

The defining characteristic of a first-class lever is that the fulcrum is positioned between the muscle force and the resistance force. In this type of lever system, the muscle (or effort) applies force on one side of the fulcrum, while the resistance (or load) is located on the opposite side. This setup allows for a mechanical advantage, making it easier to lift or move a load with less effort. Common examples of first-class levers include seesaws and certain movements in the human body, like the action of the neck when tilting the head backward.

Other options describe different lever systems or concepts that do not accurately represent the mechanics of a first-class lever. Understanding this distinction is critical for analyzing body mechanics and improving movement efficiency in fitness training and rehabilitation.