In which type of contraction does the muscle length remain unchanged?

Isometric contractions are characterized by the muscle generating force without changing its length. During this type of contraction, while the muscle fibers are activated, they do not shorten or lengthen, which means the joint angle and muscle length stay constant. This is often observed in exercises where a muscle holds a position, such as holding a plank or pushing against an immovable object. The stability and force generation without movement are essential in many strength training and rehabilitation exercises.

In contrast, isotonic contractions involve a change in muscle length, either through concentric contraction (where the muscle shortens as it contracts) or eccentric contraction (where the muscle lengthens while still resisting force). Both of those types involve movement of the joints and muscle length changes, distinguishing them from isometric contractions.