Which characteristic allows slow-twitch muscle fibers to be more fatigue resistant?

Slow-twitch muscle fibers, also known as type I fibers, are characterized by their ability to sustain prolonged activity without fatigue. This endurance comes primarily from two significant physiological features: increased capillary density and higher levels of myoglobin.

The increased capillary density ensures that these fibers are well-supplied with oxygen, which is crucial for aerobic metabolism. A rich supply of blood vessels allows for better oxygen delivery to the muscle fibers, promoting sustained energy production during prolonged activities, such as distance running or cycling.

Likewise, myoglobin plays a vital role in oxygen storage and transport within the muscles. Elevated levels of myoglobin enhance the muscle's capacity to store oxygen, facilitating aerobic respiration. With adequate oxygen available, slow-twitch fibers can efficiently metabolize fat and carbohydrates over extended periods, which further contributes to their fatigue resistance.

In contrast, characteristics such as higher fat content, rapid force generation, or decreased reliance on oxygen do not enhance fatigue resistance in slow-twitch fibers. These fibers are specialized for endurance activities, relying heavily on aerobic processes rather than anaerobic ones, which is why their design includes adaptations like increased capillary density and myoglobin content.