Which type of muscle action is characterized by movement and a change in muscle length?

The correct choice describes isotonic muscle action, which involves movement and a change in muscle length while contracting. Isotonic contractions can be further classified into two types: concentric and eccentric. During a concentric contraction, the muscle shortens as it generates force, such as when lifting a weight. In contrast, during an eccentric contraction, the muscle lengthens while still under tension, which occurs, for example, when lowering a weight.

This type of muscle action is essential in various physical activities and plays a significant role in enhancing strength and endurance. Understanding isotonic contractions is crucial for fitness instructors, as these movements are commonly incorporated into exercise routines to improve performance and prevent injuries.

In contrast, static actions do not involve any change in muscle length or joint movement, while isometric contractions maintain the muscle length without movement. Eccentric, while a form of isotonic contraction, specifically refers to the lengthening of the muscle rather than the overall category that includes both lengthening and shortening movements. Thus, the comprehensive definition aligns with isotonic actions as a whole.