Slow-twitch and fast-twitch muscle fibers differ in the way they generate energy and their functions. Slow-twitch fibers, also known as type I fibers, are more endurance-oriented, utilizing oxygen to produce energy efficiently, making them ideal for prolonged activities like marathons. In contrast, fast-twitch fibers, or type II fibers, generate quick bursts of energy but fatigue more rapidly, making them suitable for high-intensity activities such as sprinting or weightlifting. Both fiber types are essential for athletic performance and improving overall fitness.