Which type of muscle fibers are mainly utilized during endurance activities?

Endurance activities primarily engage slow-twitch (Type I) muscle fibers due to their unique characteristics designed for prolonged, low-intensity effort. These fibers are highly resistant to fatigue, allowing them to sustain activity over longer periods compared to other muscle fiber types. They are rich in mitochondria, which are essential for aerobic metabolism, and contain a high level of myoglobin, facilitating oxygen delivery and storage. This makes slow-twitch fibers particularly efficient for activities such as long-distance running, cycling, and swimming, where endurance is critical.

In contrast, fast-twitch muscle fibers (including Type II) are more suited for short bursts of strength and power because they generate force quickly but fatigue rapidly. These fibers are utilized primarily in high-intensity, short-duration activities. Type III muscle fibers are generally not recognized in the traditional classification of muscle fibers, adding further clarification to why slow-twitch fibers are the correct answer in this context.