Which type of muscle fiber is least resistant to fatigue?

Type IIb muscle fibers are most accurately characterized as being least resistant to fatigue. These fibers, also known as fast-twitch fibers, are specialized for explosive, high-intensity activities that require quick bursts of power, such as sprinting or heavy lifting. Their primary energy source is anaerobic metabolism, which allows for rapid energy production but also leads to a quicker accumulation of lactic acid and other metabolic byproducts. This accumulation contributes to fatigue, causing these fibers to tire much faster compared to other types.

In contrast, Type I fibers, which are slow-twitch, are designed for endurance and can sustain activity over longer periods without fatigue due to their efficient use of oxygen. Type IIa fibers possess qualities of both Type I and Type IIb fibers, offering a moderate resistance to fatigue while still allowing for powerful contractions, but they do not fatigue as quickly as Type IIb fibers do.

It's important to understand the differentiating characteristics of these muscle fibers when training or performing physical activities, as the type of muscle fiber predominating will influence performance, endurance, and overall fitness outcomes.