Fast-twitch fibers are described as which color?

• A. Deep red
• B. White
• C. Blue
• D. Pale red

Answer: (D)

Color shows how much myoglobin and blood vessels a muscle fiber has. Fast-twitch fibers are built for quick, powerful bursts and fatigue quickly, so they don’t need as much myoglobin or a dense network of capillaries as slow-twitch fibers do. That means they appear paler. The best description for them is pale red: not as red as slow-twitch, but not completely white either. Deep red would suggest lots of myoglobin (typical of slow-twitch), white would be almost no myoglobin (more extreme), and blue isn’t a standard description for muscle fiber color.