Type I fibers primarily obtain ATP for contraction from aerobic metabolism, with fat and the oxidation of pyruvate via the Krebs cycle being the primary sources, making these fibers more resistant to fatigue. They also contain large numbers of mitochondria, have a dense capillary supply, small cross-sectional area (CSA) and a high myoglobin concentration, explaining their distinctive red color. Type IIx fibers primarily rely on anaerobic metabolism, which includes ATP replenishment via glycolysis and use of phosphocreatine stores. These fibers have very few mitochondria, low capillary supply, are large in CSA, pale in color and fatigue easily. Type IIa fibers show characteristics of both fatigue resistances, yet fast contraction velocities