The term muscle is used with two distinctly different meanings. In one case, the term is used to designate tissues.

In the other case, the term refers to individual, discrete organs of the body. However, the structure and actions of tissues are often quite different in detail from the structure and actions of organs.

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Tissues of the body are collections of like cells performing a common function. Muscle tissues are specialized to produce tension by contraction. In fact, they function solely by contraction.

As a by-product, muscle tissues also produce heat. Shivering is a state in which the muscles of the body are primarily concerned with producing heat. Shivering involves contractions that are not synchronized and therefore do not produce motion.

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As used in muscle physiology, the term contraction is not necessarily synonymous with the term shortening. Rather, contraction means the production of tension through the interaction of the muscle tissues.