Which characteristic is exhibited by type I muscle fibers?

Type I muscle fibers are characterized primarily by their slow-twitch nature and their ability to sustain prolonged activity. These fibers are rich in mitochondria and myoglobin, which enhance their capacity for aerobic metabolism. As a result, they are better suited for endurance activities, allowing them to perform over extended periods without tiring quickly. This characteristic makes them particularly beneficial for athletes engaged in long-distance running, cycling, and other endurance sports.

In contrast, the other choices highlight attributes of different muscle fiber types. Fast-twitch fibers (Type II) are known for generating quick, powerful bursts of energy but do so at the cost of rapidly becoming fatigued. High power output and low endurance correspond to these fast-twitch muscles, which are ideal for short, explosive movements rather than sustained exercise. Therefore, the defining trait of Type I fibers is their slow fatigue rate, enabling endurance performance.