Which principle states that once the necessary threshold of stimulation is reached, all muscle fibers in one motor unit will contract?

The concept that describes how, once the necessary threshold of stimulation is met, all muscle fibers within a single motor unit will contract is known as the all-or-none principle. This principle highlights that muscle fibers either respond completely or not at all to a stimulus. In practical terms, if the stimulus is strong enough to reach the threshold, then all the muscle fibers in that motor unit will contract simultaneously. If it does not reach the threshold, no fibers in that motor unit will contract.

This principle is foundational in understanding how muscles operate during contractions. It emphasizes the binary nature of muscle fiber activation, providing a clear mechanism for how motor units contribute to muscle force production. Understanding this principle is essential for fitness instructors as it informs how they design training programs and understand muscle response during exercise.