MHC Fiber Typing

MHC fiber typing is laboratory analysis of muscle fiber composition, quantifying the percentage of slow-twitch (Type I) and fast-twitch (Type II) fibers in a muscle sample. Based on myosin heavy chain (MHC) isoform expression, fibers are classified into Type I (slow-twitch, oxidative), Type IIa (fast-twitch, oxidative-glycolytic), and Type IIx/IId (fast-twitch, glycolytic). Introduced by Bottinelli and colleagues (1994), MHC typing requires muscle biopsy and biochemical analysis. Fiber type composition is partially genetic but trainable; endurance training promotes Type II-to-IIa conversion, while power training promotes Type I-to-IIa transitions in some contexts. Understanding fiber composition informs training prescription and explains performance predispositions.