Answer
There are three classes (or orders) of levers : first class, second class and third class. The criterion for the classification system is the relative positions of the three factors fulcrum (F), effort (E) and resistance (R).
First Class Levers:
In this class the fulcrum ( fixed point) is between the effort and the resistance
R F E
In the body an example of the first class of levers is the movement of the head at the atlanto-occipital joint ( on the occipital condyles) .
In this case the atlanto - occipital joint is the fulcrum; the trapezius muscle supplies the effort and the head is the resistance or weight. The extension of the elbow joint ( F) by the tricep brachii ( E ) with resistance being the forearm is another example of the first class of levers
Work Step by Step
Second Class of Levers:
In this class of levers the resistance is between effort and the fulcrum : E R F.
An example in the body is standing on tiptoe. In this movement the ball of the foot is the fulcrum (F), the weight of the body is the resistance (R), and the effort is provided by the contraction of the gastrocnemius muscle. Another example is the action of doing push ups: here the toes act as fulcrum, the whole body provides the resistance, and the muscles of both upper limbs provide the effort.
E R F
In the third class of levers the effort is in the middle between the resistance and the effort.--R E F.
An example in the body is the flexing of the elbow joint. In this action the fulcrum is the elbow joint; the the biceps brachii (and brachialis) supply the effort; resistance is the weight of the forearm or anything held by/in the hand.
