Final answer:
As people age, the number of Type I muscle fibers increases, while the number of Type II muscle fibers decreases, leading to a decrease in muscle strength and quick forceful contractions. The correct option is (a)
Step-by-step explanation:
Age tends to increase the number of Type I fibers and decreases the number of Type II fibers. The correct answer is a) Type I; Type II. As we age, our skeletal muscles undergo a transformation in which the number of slow-twitch, or Type I fibers, increases. These are rich in mitochondria and myoglobin, which enable them to sustain aerobic activity for longer periods. In contrast, the number of fast-twitch, or Type II fibers, tends to decrease. Type II fibers can be further subcategorized into Type IIa, which are aerobic and somewhat resistant to fatigue, and Type IIx/d and Type IIb, which are anaerobic and tend to fatigue more quickly. With advancing age, there is a general shift from these fast-twitch fibers, which are capable of producing quick, forceful contractions, to the more endurance-oriented Type I fibers. This change can affect muscle strength and function, a condition known as sarcopenia.