Which muscle fiber type is mainly used during endurance running?

Type I fibers are primarily utilized during endurance running due to their characteristics that make them well-suited for prolonged activity. These muscle fibers are also known as slow-twitch fibers and are designed for endurance rather than explosive power. They have a high density of mitochondria, which allows for efficient aerobic metabolism by utilizing oxygen to produce energy over long periods. Type I fibers also have a rich supply of blood vessels, enhancing their ability to deliver oxygen and remove waste products during extended periods of exertion, such as running long distances.

In contrast, Type II fibers, which include fast-twitch fibers, are geared towards short bursts of speed and strength, relying more on anaerobic metabolism for quick energy surges but fatiguing faster. Other fiber types, such as Type III or Type IV, are less commonly referenced in standard muscle fiber categorizations, which generally only distinguish between Type I and Type II. This further solidifies the distinction of Type I fibers as the primary choice for endurance activities like running.