Tonic, slow-twitch muscle fibers are best described as which type?

• A. Type I
• B. Type II
• C. Type IIa
• D. Type IIx

Answer: (A)

The key idea is distinguishing slow-twitch, endurance-oriented fibers from fast-twitch ones. Tonic, slow-twitch fibers are best described as Type I because they contract slowly and rely on aerobic metabolism, which makes them highly resistant to fatigue. They’re rich in mitochondria and myoglobin, have a dense capillary network, and are innervated by small motor neurons—features that support sustained, endurance-type activity like maintaining posture. In contrast, fast-twitch fibers (Type IIa and IIx) contract quickly and generate more force but fatigue rapidly; Type IIa is more oxidative-glycolytic and Type IIx is more glycolytic. So, for tonic, slow-twitch characteristics, Type I fits best.