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The purpose of muscle contraction is to cause movement in some part of the body. This is achieved through a system of levers, of which there are three types. Each type of lever has the following:
- A fulcrum – this is the pivot point, which is usually found in the centre of a joint
- A resistance – this is normally body weight or some external object
- An effort – this is a muscular force that moves the resistance.
First class levers
The fulcrum is between the effort and the resistance. An example in everyday life would be a see-saw.
Second class levers
The resistance is between the fulcrum and the effort. This is the most effective lever because a relatively small force can move a larger weight. An example in everyday life would be a wheelbarrow.
Third class levers
The effort is between the resistance and the fulcrum. This is not as efficient as class two levers, but small muscle movements create long lever movement. An example in everyday life would be using a pair of tweezers.
Most levers in the body are third class allowing quick movements. Large muscular forces, however, are needed to carry out these rapid movements. The relative lengths of the force-arm (the distance between the fulcrum and the effort) and load-arm (the distance between the fulcrum and the resistance) mean that muscles usually generate forces far in excess of the loads that are being moved.
Mechanical advantage occurs when the resistance is close to the fulcrum (pivot point) and the effort is applied far from the fulcrum, allowing a small effort exerted over a relatively large distance to move a large load over a small distance.
Mechanical disadvantage occurs when the load is far from the fulcrum and the effort is applied near the fulcrum – the effort applied must be greater than the load to be moved.