AFAA Personal Trainer Certification Practice Exam

Which term describes muscle fibers that are slow to fatigue?

1. Fast twitch fibers

2. Intermediate fibers

3. Slow twitch fibers

4. Type IIx fibers

The correct answer is: Slow twitch fibers

The term that describes muscle fibers that are slow to fatigue is slow twitch fibers. These fibers, also known as Type I fibers, are characterized by their endurance and ability to sustain prolonged activity without tiring quickly. They have a high capacity for oxidative metabolism, which means they rely primarily on aerobic processes to generate energy. This feature allows them to maintain muscle contractions over extended periods, making them ideal for activities such as long-distance running or cycling. In contrast, fast twitch fibers (Type II fibers) are designed for short bursts of activity and fatigue more quickly. They rely on anaerobic metabolism, which provides energy quickly but not for long durations. Intermediate fibers possess characteristics of both slow and fast twitch fibers, allowing for a mix of endurance and power, but they do not have the same capacity for prolonged activity as slow twitch fibers. Type IIx fibers are a specific type of fast twitch fiber, which are highly powerful but fatigue quickly, further emphasizing the capacity for endurance found in slow twitch fibers.