title: Human skeletal muscle: transition between fast and slow fibre types.
abstract: Human skeletal muscles consist of different fibre types: slow fibres (slow twitch or type I) containing the myosin heavy chain isoform (MHC)-I and fast fibres (fast twitch or type II) containing MHC-IIa (type IIA) or MHC-IId (type IID). The following order of decreasing kinetics is known: type IID > type IIA >> type I. This order is especially based on the kinetics of stretch activation, which is the most discriminative property among fibre types. In this study we tested if hybrid fibres containing both MHC-IIa and MHC-I (type C fibres) provide a transition in kinetics between fast (type IIA) and slow fibres (type I). Our data of stretch activation kinetics suggest that type C fibres, with different ratios of MHC-IIa and MHC-I, do not provide a continuous transition. Instead, a specialized group of slow fibres, which we called "transition fibres", seems to provide a transition. Apart of their kinetics of stretch activation, which is most close to that of type IIA, the transition fibres are characterized by large cross-sectional areas and low maximal tensions. The molecular cause for the mechanical properties of the transition fibres is unknown. It is possible that the transition fibres contain an unknown slow MHC isoform, which cannot be separated by biochemical methods. Alternatively, or in addition, isoforms of myofibrillar proteins, other than MHC, and posttranslational modifications of myofibrillar proteins could play a role regarding the characteristics of the transition fibres.
authors.affiliation: Department of Sports Medicine, Prevention and Rehabilitation, Paracelsus Medical University Salzburg, Lindhofstr. 20, 5020, Salzburg, Austria. d.neunhaeuserer@salk.at
authors.forename: Daniel
authors.initials: D
authors.lastname: Neunhäuserer
authors.forename: Michaela
authors.initials: M
authors.lastname: Zebedin
authors.forename: Magdalena
authors.initials: M
authors.lastname: Obermoser
authors.forename: Gerhard
authors.initials: G
authors.lastname: Moser
authors.forename: Mark
authors.initials: M
authors.lastname: Tauber
authors.forename: Josef
authors.initials: J
authors.lastname: Niebauer
authors.forename: Herbert
authors.initials: H
authors.lastname: Resch
authors.forename: Stefan
authors.initials: S
authors.lastname: Galler
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D006801:Humans; D007700:Kinetics; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000032:analysis
chemical_list: D018995:Myosin Heavy Chains