Which type of muscle contraction occurs when a muscle lengthens while maintaining tension?

Eccentric contraction occurs when a muscle lengthens while maintaining tension. This type of contraction typically occurs when a muscle is working against a load that is greater than the force generated by the muscle itself. For example, when lowering a heavy object, the muscle must control the descent by lengthening, consequently producing a force that counters the pull of gravity. This action is essential in various activities such as descending stairs or controlling movements during exercises, as it helps to absorb shock and prevent injury.

In contrast, isometric contractions happen when a muscle generates force without changing its length; concentric contractions involve muscle shortening while generating force; and isotonic contractions refer to movements that involve changes in muscle length with the tension remaining constant, which can include both concentric and eccentric actions. Thus, the distinct behavior of the muscle during eccentric contraction clearly marks it as the correct choice in the context of the question.