What type of contraction occurs when the muscle lengthens while in action?

The type of contraction that occurs when the muscle lengthens while in action is known as an eccentric contraction. During an eccentric contraction, the muscle generates force while simultaneously being elongated, which often happens when a muscle is controlled as it resists an opposing force or when it's acting to decelerate a movement.

For example, when you lower a weight under control, the biceps muscle is lengthening while still producing force to control the descent. This type of contraction is crucial for various activities, providing controlled movements and helping to absorb impact, which can protect joints and prevent injury.

To provide additional context, isometric contractions occur when the muscle generates force without changing length, meaning the muscle remains the same length throughout the contraction. Concentric contractions, in contrast, involve the muscle shortening while generating force, such as when lifting a weight. Isokinetic contractions involve muscle contractions at a constant speed throughout the range of motion, typically measured with specialized equipment. Each type of contraction plays a distinct role in movement and strength training, but in this case, the key feature of lengthening during muscle action identifies eccentric contraction as the correct answer.