Type IIx Fibers

Type IIx fibers are a subtype of fast skeletal muscle fiber defined primarily by predominant expression of the myosin heavy chain IIx, or MyHC-IIx, isoform. They are one of the principal fiber types identified in adult human limb and trunk muscles, alongside Type I and Type IIa fibers. Their myosin is associated with relatively fast contractile kinetics, supporting rapid shortening of the muscle fiber .^1,2

Type IIx fibers generally have lower mitochondrial and oxidative-enzyme content and greater glycolytic capacity than Type I or Type IIa fibers. These characteristics support rapid ATP production and fast contractions but are associated with lower resistance to sustained activity. They are therefore generally described as the fastest-contracting and most glycolytic of the major adult human skeletal muscle fiber types .¹

Type IIx fibers represent a contractile classification rather than a complete description of all of a muscle fiber's properties. Characteristics such as oxidative capacity, fiber size, and force production overlap between fiber types and cannot be predicted perfectly from the myosin isoform alone. Individual fibers can also express more than one myosin isoform, producing hybrid phenotypes such as Type IIa/IIx fibers .^1,2

An important terminology distinction concerns Type IIb fibers. Older literature sometimes called the fastest human fibers Type IIb, but these fibers are now recognized as expressing MyHC-IIx and are correctly termed Type IIx. True MyHC-IIb is expressed in the skeletal muscles of some other mammals but is not considered a normal major fiber type in adult human limb muscle .¹