Final answer:
Slow-twitch muscle fibers are darker due to high levels of myoglobin, which stores oxygen, and their abundance of mitochondria, facilitating sustained aerobic activity and energy production.
Step-by-step explanation:
The darker color of slow-twitch muscle fibers is primarily due to the presence of the pigment myoglobin. Myoglobin binds with oxygen and stores it within the muscle fiber, allowing for sustained aerobic activity. Furthermore, these muscle fibers contain a large number of mitochondria, which are involved in aerobic metabolism, aiding in long-duration contractions.
When engaging in endurance activities such as long-distance running, slow-twitch fibers are predominantly utilized. They are known for their ability to produce a significant amount of ATP through aerobic pathways, resulting in less fatigue during prolonged muscle activity. The abundance of both myoglobin and mitochondria within these fibers contributes to their reddish hue as it contrasts with the paler fast-twitch fibers which have less myoglobin.