What describes the motion of an active muscle while it is lengthening?

Eccentric motion refers to the process where a muscle generates force while it is being lengthened. This occurs during activities where a muscle must control the descent of a load or resist an opposing force. For example, when lowering a weight or slowly bending a knee, the quadriceps are engaged in eccentric contraction as they lengthen under tension to control the movement. This type of muscle action is fundamental in activities such as running downhill or lowering oneself into a squat.

In contrast, concentric motion involves muscular shortening while generating force, isometric motion indicates that the muscle length remains the same while under tension, and passive motion typically refers to movement not caused by muscular contraction, such as when an external force moves a limb. Understanding these types of muscular actions is crucial for assessing movement patterns and creating effective rehabilitation and training programs.