Final answer:
Type IIx fibers fatigue the fastest among the three human muscle fiber types because they rely on anaerobic glycolysis, have less myoglobin, and fewer mitochondria, which limits their endurance for sustained activity.
Step-by-step explanation:
The correct answer to the student's question, "Type IIx fibers" is b. fatigue fastest of the 3 human muscle fiber types. This is because Type IIx fibers, also known as Type IId or FG fibers, are fast glycolytic fibers that primarily use anaerobic glycolysis to generate ATP, which allows for rapid and forceful muscle contractions. However, they are not equipped to sustain long-term activity due to their low content of mitochondria and myoglobin, leading them to fatigue quickly.
Type IIx fibers have a large diameter with high amounts of glycogen, facilitating their role in producing quick, powerful movements for short durations. They are known as "white muscle" due to their pale color, as opposed to the "red muscle" (Type I) fibers, which are rich in myoglobin and mitochondria. Unlike the assertions in options a, c, and d, Type IIx fibers are recruited last, not first, have a low myoglobin content, and contain fewer mitochondria compared to other muscle fiber types.