Final answer:
No, fibers within a motor unit are not different types; they are uniform in type, either slow-twitch or fast-twitch. Fast-twitch muscle fibers have large glycogen reserves, which enable them to perform quick, powerful contractions.
Step-by-step explanation:
Motor units within skeletal muscles are comprised of fibers that are similar in type, so the answer is No. Muscle fibers within a single motor unit are not different types; they are uniform in type, either slow-twitch or fast-twitch. However, it is important to note that within the entire muscle, there can be a mixture of motor units, some consisting of slow-twitch fibers and others consisting of fast-twitch fibers. For instance, the muscles of a professional sprinter are most likely to have about 80 percent fast-twitch muscle fibers and 20 percent slow-twitch muscle fibers, due to the high demand for rapid and powerful contractions, whereas the muscles of a professional marathon runner are most likely to have the opposite proportion with approximately 80 percent slow-twitch muscle fibers for endurance.
Regarding the characteristics of fast-twitch fibers, option c. Fast fibers have large glycogen reserves is true. This attribute allows them to produce a quick burst of energy through anaerobic metabolism, which is beneficial in activities requiring powerful and rapid muscle contractions. In contrast, slow-twitch fibers, also known as slow oxidative fibers, contain more mitochondria and rely on aerobic respiration for energy, making them more suitable for sustained, long-duration activities like distance running.