What type of muscle contraction occurs when there is constant tension, but the muscle fiber length shortens?

The type of muscle contraction that involves constant tension while the muscle fibers shorten is known as concentric contraction. During this process, the muscle generates enough force to overcome a resistance and thus reduces in length. This typically occurs when lifting an object against gravity or during the curling of a weight where the muscle is actively contracting and shortening to lift the load.

In contrast, eccentric contraction refers to the lengthening of the muscle while it is still under tension, such as when lowering a weight. Isometric contraction occurs when the muscle exerts tension without changing length, meaning the muscle remains static despite the tension. Isotonic contraction is a broader term that can encompass both concentric and eccentric contractions, as it refers to muscle contractions that result in movement. However, since the question specifies shortening of the muscle fibers, concentric contraction is the most precise answer.