A sprinter uses type IIx (fast-twitch glycolytic) muscle fibers during a 100-m sprint. These fibers fatigue rapidly because they rely primarily on anaerobic glycolysis. The specific biochemical explanation for why these fibers are more fatigue-prone than type I (slow-twitch oxidative) fibers is:

• A Type IIx fibers have higher myosin ATPase activity that depletes ATP faster than it can be resynthesized by any metabolic pathway
• B Type IIx fibers have fewer mitochondria and lower oxidative capacity, relying on lactate production which causes intracellular acidosis and interferes with actin-myosin cross-bridge cycling ✓
• C Type IIx fibers express only one isoform of troponin C that has reduced Ca²⁺ sensitivity causing faster cross-bridge detachment
• D Type IIx fibers have a larger diameter causing diffusion limitation of oxygen even if mitochondria were abundant

Explanation

Type IIx fibers have high myosin ATPase activity (fast cross-bridge cycling) but few mitochondria and limited oxidative capacity. They rely predominantly on anaerobic glycolysis, generating lactate and H⁺. Intracellular acidosis (pH < 6.5) inhibits phosphofructokinase (limiting glycolytic flux), reduces Ca²⁺ sensitivity of troponin C (requiring more Ca²⁺ for the same force), and may impair SR Ca²⁺ release. Accumulation of Pi from ATP hydrolysis also inhibits cross-bridge transitions. Type I fibers, with abundant mitochondria, oxidative enzymes, and myoglobin, sustain aerobic ATP generation and resist acidosis-related fatigue.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.