What describes an isotonic contraction?

An isotonic contraction is characterized by the muscle actively contracting and changing its length while generating force. This type of contraction is essential during movements like lifting weights or performing any physical activity that requires joint movement.

When a muscle shortens, it typically results in the weight being lifted or moved, which is a fundamental aspect of exercise and many daily activities. This action allows for movement around a joint, which is why isotonic contractions are key in dynamic physical activities.

In contrast, when a muscle lengthens while contracting, as mentioned in another option, it refers to an eccentric contraction, which is not the definition of isotonic. The option that mentions the muscle not contracting at all pertains to a static situation, like maintaining a posture without movement, while the last option describes an isometric contraction, where the muscle maintains a constant length without movement. Thus, the definition of isotonic specifically highlights the muscle’s ability to contract and shorten, which makes the answer correct.