Which fiber-type shift reduces endurance in neck muscles?

• A. Slow twitch -> fast
• B. No change
• C. Increase in mitochondria
• D. Fast -> Slow

Answer: (A)

Endurance in neck muscles depends on having a higher proportion of fatigue-resistant, slow-twitch fibers. Slow-twitch fibers (Type I) are built for sustained, low-intensity activity; they rely on oxidative metabolism, have many mitochondria, and resist fatigue. Fast-twitch fibers (Type II) contract quickly and forcefully but tire more easily because they rely more on glycolytic pathways and have lower oxidative capacity. A shift from slow-twitch toward fast-twitch fibers means more of the muscle is made up of fibers that fatigue faster, so the neck would tire sooner during prolonged tasks like maintaining posture, reducing endurance. Increasing mitochondrial content would boost endurance, a fast-to-slow shift would likewise improve endurance, and no change would leave endurance unchanged. Hence, the transformation from slow-twitch to fast-twitch fibers best explains reduced endurance in the neck.