Slow-twitch fibers, also known as type I fibers, are muscle fibers that are designed for endurance activities. They contract slowly and can sustain activity for long periods without fatigue. These fibers are rich in mitochondria, which help produce energy through aerobic metabolism, making them ideal for activities like long-distance running or cycling. These fibers are also highly vascularized, meaning they have a good blood supply, which aids in oxygen delivery. Athletes who engage in endurance sports often have a higher proportion of slow-twitch fibers, allowing them to perform better in activities that require stamina rather than quick bursts of speed, such as marathons or triathlons.