Figure 47.11 Fast- and Slow-Twitch Muscle Fibers (A) Fast-twitch fibers generate a large force quickly but fatigue rapidly. Slow-twitch fibers generate smaller forces, and do so more slowly, but can sustain them for a longer time. (B) The skeletal muscles in these three panels show slow-twitch fibers as red and fast-twitch fibers as blue. Arrows represent the duration (length) and speed (thickness) of running.