Which fiber type is the smallest, has slow contraction speed, and is difficult to fatigue?

Which fiber type is the smallest, has slow contraction speed, and is difficult to fatigue?

• A. Type I
• B. Type IIA
• C. Type IIB
• D. Type IIx

Answer: (A)

Muscle fibers differ in size, how quickly they contract, and how quickly they fatigue. The slow-twitch oxidative fibers—Type I—are the smallest in diameter, contract more slowly, and resist fatigue because they rely on aerobic metabolism, with many mitochondria, rich capillary supply, and high myoglobin content. This combination supports sustained, low-intensity activity over long periods. Fast-twitch fibers, by contrast, are larger and contract quickly but tire more rapidly; among them IIx is the fastest and most fatigue-prone, while IIa falls in between. Therefore, the fiber type described—smallest, slow to contract, and fatigue resistant—best matches Type I.

Muscle fibers differ in size, how quickly they contract, and how quickly they fatigue. The slow-twitch oxidative fibers—Type I—are the smallest in diameter, contract more slowly, and resist fatigue because they rely on aerobic metabolism, with many mitochondria, rich capillary supply, and high myoglobin content. This combination supports sustained, low-intensity activity over long periods. Fast-twitch fibers, by contrast, are larger and contract quickly but tire more rapidly; among them IIx is the fastest and most fatigue-prone, while IIa falls in between. Therefore, the fiber type described—smallest, slow to contract, and fatigue resistant—best matches Type I.