What separates Type I, IIa, and IIx fibers
Skeletal muscle is a mix of slow and fast fibers classified by their myosin heavy chain. Type I (slow-twitch) fibers are smaller, rich in mitochondria and capillaries, and highly fatigue resistant, making them the workhorses of posture and endurance. Type IIa (fast oxidative-glycolytic) fibers are larger, contract faster, and sit in the middle: powerful but with reasonable fatigue resistance. Type IIx fibers are the fastest and most forceful but fatigue quickly and rely heavily on anaerobic glycolysis.
Most human muscles contain a blend, and the exact proportions vary by muscle and by person. The popular idea that you can count your fiber percentages without a biopsy is not reliable, performance tests only give rough hints.
How training shifts the balance
Fiber types exist on a continuum, and training nudges fibers along it rather than wholesale converting slow to fast. Endurance work pushes IIx toward the more oxidative IIa profile, while heavy or explosive work tends to favor IIa characteristics. True conversion of Type I to Type II is largely not supported by current evidence; what changes most is fiber size and the IIx-to-IIa balance.
This is general educational information. Individual response to training varies, and biopsy-level claims about your own fiber makeup should be treated cautiously.