What physiological response happens when muscle fibers are stimulated through exercise?

When muscle fibers are stimulated through exercise, they undergo a process in which the cells contract and produce motion. This contraction is the result of physiological mechanisms involving the interaction of proteins, such as actin and myosin, within the muscle cells. When nerves signal the muscle fibers, calcium ions are released, which facilitate these contractions, allowing the muscle to shorten and generate force. This is fundamental to how exercise leads to movement, enabling activities such as running, lifting, or any other physical activity.

The other options reflect incorrect physiological responses related to muscle fibers during exercise. Cells do not shrink in size as a response to stimulation, nor do they stop reproducing; in fact, the demands of exercise can stimulate growth and adaptation in muscle cells. While cells do require more oxygen during exercise to meet increased energy demands, the direct response being asked about in the context of muscle stimulation is the contraction that produces motion.