During a sprint, fast-twitch (type IIX) muscle fibers are recruited. Compared to slow-twitch (type I) fibers, fast-twitch fibers differ in which key metabolic and mechanical property?

• A Higher mitochondrial density and myoglobin content than slow-twitch fibers
• B Greater capillary density and resistance to fatigue due to oxidative metabolism
• C Lower twitch force and shorter contraction time than slow-twitch fibers
• D Higher myosin ATPase isoform activity (MyHC-IIX), faster cross-bridge cycling, greater reliance on anaerobic glycolysis (higher glycolytic enzyme content and glycogen stores), and lower fatigue resistance ✓

Explanation

Type IIX (fast fatigable) muscle fibers express the fastest myosin heavy chain isoform (MyHC-IIX) with high ATPase activity, enabling rapid cross-bridge cycling and fast contractile velocity. They are optimized for anaerobic glycolysis: high glycogen content, high phosphocreatine stores, high glycolytic enzyme activities (phosphofructokinase, LDH), but low mitochondrial density and low oxidative capacity. This provides rapid, high-force output but leads to rapid fatigue due to lactate accumulation and PCr depletion. Type I (slow oxidative) fibers have the opposite profile: slow MyHC-I, abundant mitochondria, high myoglobin and capillary density, oxidative metabolism, and fatigue resistance. Type IIX produce higher peak forces and faster twitch times than type I.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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