How do fast-twitch muscle fibers differ in appearance compared to slow-twitch fibers?

Fast-twitch muscle fibers, also known as Type II fibers, are characterized by their pale color compared to slow-twitch muscle fibers (Type I). This difference in appearance is largely due to the concentration of myoglobin, the protein that binds oxygen in muscle cells, which is higher in slow-twitch fibers. Myoglobin gives these fibers a darker red appearance.

Fast-twitch fibers are designed for rapid and powerful contractions but fatigue more quickly, which correlates with their lighter color. The pale appearance reflects lower levels of myoglobin, fewer mitochondria, and a reliance on anaerobic metabolism for energy, as opposed to the aerobic metabolism predominantly utilized by slow-twitch fibers. Thus, the lighter color of fast-twitch fibers is a visual representation of their specific functional characteristics.