Final answer:
Type II muscle fibers are not entirely aerobic. They can be classified into Type IIa, which are aerobic, and Type IIx, which are less reliant on aerobic metabolism. Type IIx fibers are the fastest in humans and can sustain short bursts of anaerobic activity.
Step-by-step explanation:
Type II muscle fibers are not entirely aerobic. They are classified into two subtypes: IIa and IIx. Type IIa muscle fibers are aerobic, rich in mitochondria and capillaries, and appear red. On the other hand, Type IIx muscle fibers are less dense in mitochondria and myoglobin, making them less reliant on aerobic metabolism. These fibers are the fastest in humans and can sustain short bursts of anaerobic activity.