There are two basic types of skeletal muscle cells, fast twitch and slow twitch. Slow-twitch fibers are designed for endurance and prolonged exertion. When you run a marathon, slow-twitch muscle fibers are the ones that get used the most. Fast twitch muscle fibers are physically larger and are responsible for shorter bursts of exertion. A 100 yard sprint invokes fast twitch muscle fibers almost exclusively. If you train for endurance, your muscles will tend to be smaller but will be able to perform work for much longer periods of time (aerobic). If you train for explosive strength, your muscles will be much larger but will fatigue much more quickly (anaerobic).