Which type of muscle fibers are fatigue-resistant, O2 rich, best for endurance activities, lactate intolerant, aerobic, and fuel from blood nutrients?

• A. Fast twitch (white fibers)
• B. Slow twitch (red fibers)
• C. Intermediate twitch fibers
• D. Glycolytic fibers

Answer: (B)

Slow-twitch fibers are built for prolonged, steady work. They’re packed with mitochondria and myoglobin, giving them a rich blood- and oxygen-access system. This enables oxidative metabolism—using oxygen to burn glucose and fatty acids delivered by the bloodstream to produce ATP. Because of this aerobic setup, they resist fatigue and don’t accumulate lactate quickly during endurance efforts. They contract more slowly, but can sustain activity for long periods, which fits endurance activities perfectly. The energy they rely on comes mainly from blood-borne nutrients, rather than quick bursts of glycolytic energy. That combination—fatigue resistance, high oxygen availability, aerobic metabolism, and reliance on blood nutrients—best matches slow-twitch, red fibers.