<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 length changes but the muscle tension remains the same . 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> <P> In vertebrates, skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons to produce muscle contractions . A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time . Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory . The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials . In skeletal muscles, muscle tension is at its greatest when the muscle is stretched to an intermediate length as described by the length - tension relationship . </P>

What does a muscle do when it contracts
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