Answer
There are three classes or orders of the levers system of movement. These are the first, second and third class of levers. The factors use to classify levers are the effort or power (E); the fulcrum or axis (F) and the resistance/load/ weight (R). In the first class of levers the fulcrum is positioned between the resistance and the effort--RFE; in the second class the resistance comes between the the effort and the fulcrum (ERF). In the third order the effort is applied between the fulcrum and the resistance(FER).
Work Step by Step
An example of the first class of levers in the body (EFR) is the movement of the head back and forth in nodding on the atlanto-occipital joint In this example the head is the resistance or weight; the fulcrum is the atlanto-occipital joint , and the effort is supplied by back muscles such as the trapezius.
An example of the second class of levers is provided by the action of standing on tip-toe. In the case the fulcrum is at the metatarso-phalangeal joints at the ball of the foot. the resistance is the weight of the body and the effort is exerted by the surae muscles ( gastrocnemius and soleus) on the calcaneus through the Achilles tendon. --EFR.
The third class of levers is exemplified in the body by the action of the flexing of the elbow joint. In this action the effort is supplied by the biceps brachii( and other fore arm flexors); the resistance is the weight of the forearm and any weight that may be in the hand . The fulcrum is the elbow joint-- FER
