Anterior and posterior muscles can move around a fulcrum at the joint where the skull and first vertebra meet. This configuration is an illustration of a first-class lever.
First class, second class, and third class are the three different categories of levers. The placement of the muscle's insertion (effort) and bones (lever/arm) around the joint varies according to each of these lever classes (fulcrum).
The proportion of the effort arm to the load arm determines how effective a lever is. The distance between the fulcrum and the effort is known as the effort arm (EA); in the body, this is the distance between the joint and the location where the muscle inserts. The load arm (LA), which in the body is the distance between the joint and the laden body part, is the space between the fulcrum and the load.
To learn more about anterior and posterior muscles, refer to:
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