What is it called when a muscle contracts but lengthens?

When a muscle contracts while lengthening, this is known as eccentric action. During an eccentric contraction, the muscle generates force while being elongated, which typically occurs when the muscle is resisting an external force or controlling the descent of a movement. For example, when you lower a weight in a bicep curl, the biceps muscle is actively contracting while slowly lengthening, allowing for a controlled descent.

This type of muscle action is important for various athletic movements and helps prevent injury by stabilizing joints and managing excessive force. It plays a significant role in activities involving deceleration, such as running downhill or landing from a jump, where the muscle must absorb the impact while still working against gravity.