Which contraction type involves no movement and the muscle's length remains unchanged?

The type of contraction that involves no movement and where the muscle's length remains unchanged is known as isometric contraction. During isometric contractions, the muscle exerts force without changing its length, which means that while muscle fibers are activated and generating tension, the overall muscle remains the same length, and there is no joint movement.

This type of contraction is commonly encountered when holding a position, such as during a plank exercise or pushing against an immovable object. It can be beneficial for building muscular endurance and stabilizing joints, as the muscle maintains tension without shortening or lengthening.

In contrast, isotonic contractions involve movement, where the muscle changes length while producing force, typically classified into concentric (muscle shortening) and eccentric (muscle lengthening) contractions. Thus, those options do not fit the criteria described in the question.