Final answer:
The class of skeletal muscle fibers that produces low force over long periods is the Type I (Slow-twitch oxidative) fibers. They are equipped with a high content of myoglobin and mitochondria and utilize aerobic metabolism, providing them the endurance needed for activities like long-distance running.
Step-by-step explanation:
The class of skeletal muscle fibers that produces low amounts of force for long periods of time is the Type I (Slow-twitch oxidative) fibers. These fibers are known for their ability to sustain aerobic activities, such as long-distance running, by primarily using aerobic respiration to produce ATP. They are dense in capillaries and are rich in mitochondria and myoglobin, granting them a higher resistance to fatigue. Type I fibers are distinct for their red color due to the presence of myoglobin, which aids in oxygen transport and storage within the muscle, enabling these fibers to support endurance activities.
Type II fibers can be further subdivided into Type IIa (Fast-twitch oxidative-glycolytic) and Type IIb (Fast-twitch glycolytic), with Type IIa being capable of both aerobic and anaerobic metabolism and Type IIb being primarily anaerobic and more prone to fatigue. While Type IIa fibers still contain a good number of mitochondria and myoglobin, Type IIb fibers are less dense in these components and are known as 'white' muscle due to the paler tissue color. Neither of these fast-twitch fibers are the correct answer to this question, as they both generate higher tension contractions that can fatigue more quickly compared to slow-twitch fibers.