Which activity primarily uses slow-twitch muscle fibers?

Slow-twitch muscle fibers, also known as Type I fibers, are designed for endurance and sustained activities. They are highly efficient at using oxygen to generate more fuel for continuous, extended muscle contractions over a long period, which makes them ideal for endurance activities.

Long-distance running primarily engages these slow-twitch muscle fibers because it requires a prolonged and steady output of energy. This type of exercise emphasizes aerobic metabolism, allowing the muscles to work efficiently over extended periods without fatiguing quickly.

In contrast, activities like weight lifting or sprinting rely more on fast-twitch fibers, which are better suited for explosive, high-intensity efforts. These fibers can generate more force but fatigue quickly, making them less effective for endurance tasks. High-intensity interval training combines short bursts of intense effort with brief recovery periods, again utilizing fast-twitch fibers primarily.

Thus, long-distance running is characterized by the predominant use of slow-twitch muscle fibers, making it the correct choice for this question.