The type of contraction in which the muscle fibers do not shorten is called

Isometric contraction

Explanation:

The length of the muscle is changed during contraction only when the muscle tension during contraction can outstand the resistance exerted by the object to be lifted.  

During isometric contractions, the muscle tension is not sufficient to exceed the resistance of the object. Hence, there is no change in the length of the muscle. The isometric contraction does not bring about any body movements.  

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