Which muscle fiber type is more reliant on glycolysis and has lower mitochondrial density?

• A. Slow-twitch (Type I)
• B. Fast-twitch (Type II)
• C. Both have high mitochondrial density
• D. Fast-twitch (Type II) relies more on glycolysis and has lower mitochondrial density

Answer: (D)

Fast-twitch fibers are built for quick, powerful contractions and rely on glycolysis to generate ATP rapidly. Because glycolysis is an anaerobic process, these fibers don’t depend as much on mitochondria for their energy, so they typically have fewer mitochondria than slow-twitch fibers. This combination explains why they can fire fast and hard but fatigue sooner. In contrast, slow-twitch fibers are optimized for endurance and rely on oxidative metabolism, supported by a high density of mitochondria. So the statement that fast-twitch fibers rely more on glycolysis and have lower mitochondrial density fits their metabolic profile.