Final answer:
Type IIa (Fast-twitch oxidative-glycolytic) muscle fibers can produce moderate amounts of force for a moderate period of time through a combination of aerobic and anaerobic respiration.
Step-by-step explanation:
The class of skeletal muscle fibers that produces moderate amounts of force for moderate periods of time is Type IIa (Fast-twitch oxidative-glycolytic). These fibers have fast contractions and primarily use aerobic respiration, just like slow oxidative (SO) fibers, but can switch to anaerobic glycolysis when necessary. Type IIa fibers are rich in mitochondria and myoglobin, and their ability to use both oxidative and glycolytic processes allows them to generate a compromise between power and endurance. This makes them suitable for activities that require a moderate level of force over a moderate duration, such as middle-distance running.