Slow-twitch fibers are described as which color?

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

Answer: (B)

Slow-twitch fibers are built for endurance and rely on aerobic metabolism. They contain a high amount of myoglobin, many mitochondria, and a dense capillary network to supply oxygen. Myoglobin stores and transport oxygen in muscle cells and has a red color, so muscles rich in slow-twitch fibers look deep red. This is why deep red is the best description. In contrast, fast-twitch fibers have less myoglobin and are often described as pale or white because they rely more on anaerobic energy and have fewer capillaries, which is why colors like pale red or white aren’t typical for slow-twitch. The color blue isn’t commonly used to describe muscle fiber types.