During which type of contraction does a muscle elongate under tension?

Eccentric contraction occurs when a muscle elongates while under tension, which often happens when the muscle is actively resisting a force. This type of contraction typically takes place during movements where the muscle is controlling the speed of a motion or working against gravity, such as lowering a weight or descending stairs. For instance, when performing a bicep curl, as you lower the weight back down, your biceps are eccentrically contracting to control the movement.

This is in contrast to concentric contraction, where the muscle shortens while generating force, and isometric contraction, where the muscle generates force without changing length. There is also static contraction, which generally refers to the absence of movement altogether while maintaining tension. Therefore, eccentric contraction is characterized specifically by the elongation of the muscle under load, making it the correct answer.