Lengthening of the muscle fibers under contraction is what?

The correct answer is the lengthening of muscle fibers under contraction is described as an eccentric contraction. In this type of contraction, the muscle generates force while it is being elongated, which can occur, for example, when lowering a weight. This process is crucial for activities that require deceleration and helps in controlling movements, such as walking downhill or lowering a heavy object.

Eccentric contractions are important for muscle growth and injury prevention, as they create tension in the muscle fibers while they are lengthening. This contrasts with concentric contractions, where muscles shorten while developing tension, and isometric contractions, where muscle length does not change despite the application of force.

The concept of isothermic contraction does not apply in this context since it pertains to a different aspect of muscle physiology that relates to temperature rather than the dynamics of muscle length during exercise. Understanding these different types of muscular contractions is vital for training programming and injury management in fitness and rehabilitation settings.