Human skeletal muscle: transition between fast and slow fibre types |
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Authors: | Neunhäuserer Daniel Zebedin Michaela Obermoser Magdalena Moser Gerhard Tauber Mark Niebauer Josef Resch Herbert Galler Stefan |
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Affiliation: | 1. Department of Sports Medicine, Prevention and Rehabilitation, Paracelsus Medical University Salzburg, Lindhofstr. 20, 5020, Salzburg, Austria 2. Department of Cell Biology, University of Salzburg, Hellbrunnerstr. 34, 5020, Salzburg, Austria 3. Department of ENT (Ear, Nose and Throat), Paracelsus Medical University Salzburg, M??llner Hauptstr. 48, 5020, Salzburg, Austria 4. Department of Traumatology and Sports Injuries, Paracelsus Medical University Salzburg, M??llner Hauptstr. 48, 5020, Salzburg, Austria
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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. |
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