Which description best characterizes red muscle?

• A. White muscle: mostly fast fibers, pale, limited blood supply.
• B. White muscle: mostly slow fibers, dark.
• C. Cardiac muscle contracts rhythmically.
• D. Red muscle: mostly slow fibers, dark, rich blood supply, large amounts of myoglobin.

Answer: (D)

Red muscle is made up mainly of slow-twitch, oxidative fibers that rely on aerobic metabolism. Because these fibers use oxygen efficiently, the muscles have a rich blood supply and many mitochondria to generate energy for long, endurance activities. The high myoglobin content gives the tissue a dark red color and acts as an oxygen reservoir, supporting sustained contractions without fatiguing quickly. This combination—mostly slow fibers, dark color, abundant blood supply, and ample myoglobin—defines red muscle.

In contrast, white muscle consists largely of fast-twitch glycolytic fibers, which are paler, fatigue more quickly, and have less myoglobin and fewer blood vessels. Cardiac muscle is a separate tissue type that contracts rhythmically to pump blood, rather than functioning as skeletal red or white muscle.