What kind of muscle contraction occurs when muscle length changes under tension?

The correct answer refers to isotonic contraction, which is a type of muscle contraction that occurs when a muscle changes length while generating force or tension. Isotonic contractions are divided into two subcategories: concentric and eccentric contractions. During a concentric contraction, a muscle shortens as it generates force, like when lifting a weight. In contrast, during an eccentric contraction, the muscle lengthens while still under tension, such as when lowering a weight.

The term isotonic specifically denotes the aspect of tension remaining relatively constant while the muscle changes length. This distinguishes isotonic contractions from isometric contractions, where the muscle does not change length at all while maintaining tension. Since isotonic contractions are characterized by the muscle actively changing length while still exhibiting tension, this highlights why it is the correct choice for the question regarding muscle length change under tension.