ACSM Personal Trainer Certification Practice Exam

Which type of muscle fibers are classified as aerobic, slow twitch, and red twitch?

1. Type I Fibers

2. Type IIA Fibers

3. Type IIB Fibers

4. Type III Fibers

The correct answer is: Type I Fibers

Type I fibers, often referred to as slow-twitch muscle fibers, are characterized by their reliance on aerobic metabolism for energy production. This means they use oxygen to generate ATP, making them more efficient for sustained, endurance-type activities. These fibers are rich in mitochondria and myoglobin, giving them a reddish color and enhancing their ability to utilize oxygen. Due to their endurance capabilities, Type I fibers are typically utilized in activities such as long-distance running or cycling, where aerobic energy production is essential. Their slow contraction speed is beneficial for activities requiring stamina rather than power or explosive strength. In contrast, Type IIA fibers, while still aerobic, have a greater capacity for anaerobic metabolism and can generate force more quickly, making them suitable for both endurance and moderate-intensity activities. Type IIB fibers, on the other hand, are classified as fast-twitch fibers and rely primarily on anaerobic metabolism, producing quick bursts of power but fatigued rapidly. Type III fibers do not have a classification in the traditional Type I, IIA, or IIB framework used in muscle fiber classification. Thus, Type I fibers are indeed the correct identification for aerobic, slow twitch, and red twitch muscle fibers.