Levers

For performing movements bones and muscles form levers at the joints.
The force generated by muscle contraction pulls or lifts bone through tendons. Movement occurs along the joint, hence the bones actually act as lever having 3 parts: (i) fulcrum (F), the fixed point, (ii) point of  effort (E), and (iii) the point of Resistance (R) or load.

Fig. Various classes or orders of lever formed by joints

In the body joint acts as F, point of attachment of muscle to bone is E.

There are 3 classes of lever:

1^st class lever — F comes between E and R, very rare in body, one such example is head resting on the vertebral column.

2^nd class lever — R comes between F and E, only few in the body e.g., raising body on the toes.

3^rd class lever — E remains in between the F and R, the most common type lever in the body e.g., flexing fore arm at elbow joint.

• First Order Levers. They are levers in which fulcrum lies in the middle, weight (resistance) at one end and force (or energy) at the other end. Such a joint occurs between occipital bone and atlas vertebra of neck. The joint functions as fulcrum. Head is the weight or resistance. Muscles of the back of neck provide force or energy.
• Second Order Levers. They are levers in which fulcrum is at one end, weight or resistance in the middle and energy or force at the other end. Human body resting on toes form the second order lever where toes at one end function as fulcrum, resistance or weight over the front part of heel while force or energy is provided by calf muscles.
• Third Order Levers. They are those levers in which force or energy is in the middle with weight or resistance at one end and fulcrum over the other end. Lifting of a book or any other load involving flexing movement over the elbow joint represents the third order lever. Here end of arm with a load functions as weight or resistance, elbow joint as fulcrum and biceps muscles attached to radius functions as force or energy.