Which of the following best describes muscle fibers' response when a threshold stimulus is not met?

When muscle fibers are subjected to a stimulus that does not meet the threshold required for contraction, they do not contract at all. This phenomenon is fundamental to the all-or-nothing principle of muscle physiology, where a stimulus must reach a certain level of intensity (the threshold) for the muscle fibers to generate a contraction. If the stimulus is insufficient, there is no activation of the muscle fibers, and therefore, no tension or contraction is produced. This principle helps ensure that muscle contractions are efficient and controlled, responding only to appropriate levels of stimulation.

In the context of muscle functioning, other responses such as weak contractions, partial contractions, or states of fatigue do not occur because they all imply some level of response by the muscle fibers, which is only possible once the threshold is surpassed.