<P> Muscle contraction is the activation of tension - generating sites within muscle fibers . In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book or a dumbbell at the same position . The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension - generating state . </P> <P> Muscle contractions can be described based on two variables: length and tension . A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same . In contrast, a muscle contraction is isotonic if muscle tension remains the same throughout the contraction . If the muscle length shortens, the contraction is concentric; if the muscle length lengthens, the contraction is eccentric . In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time - varying manner . Therefore, neither length nor tension is likely to remain the same in muscles that contract during locomotor activity . </P>

Tension followed by a release of tension produces
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