MUSCLE PHYSIOLOGY Human skeletal muscle: transition between fast and slow fibre types Daniel Neunhäuserer & Michaela Zebedin & Magdalena Obermoser & Gerhard Moser & Mark Tauber & Josef Niebauer & Herbert Resch & Stefan Galler Received: 14 January 2011 /Revised: 15 February 2011 /Accepted: 17 February 2011 /Published online: 1 March 2011 # Springer-Verlag 2011 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. Keywords Skeletal muscle . Muscle fibre types . Myosin heavy chain . Stretch activation . Muscle mechanics . Human muscle fibres . Muscle fiber . Skeletal muscle fiber . Kinetics Introduction Skeletal muscles are composed of fibres with different properties to fulfil various functional needs. Contraction properties of fibres vary depending on differences in the kinetics of Ca 2+ activation, energy metabolism and iso- forms of myofibrillar proteins. For classification of fibre types, morphological, metabolic and contraction properties have been used. During the last decades, the classification based on the isoforms of myosin heavy chain (MHC) D. Neunhäuserer : J. Niebauer Department of Sports Medicine, Prevention and Rehabilitation, Paracelsus Medical University Salzburg, Lindhofstr. 20, 5020 Salzburg, Austria D. Neunhäuserer e-mail: d.neunhaeuserer@salk.at M. Zebedin : M. Obermoser : S. Galler (*) Department of Cell Biology, University of Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria e-mail: stefan.galler@sbg.ac.at G. Moser Department of ENT (Ear, Nose and Throat), Paracelsus Medical University Salzburg, Müllner Hauptstr. 48, 5020 Salzburg, Austria e-mail: g.moser@salk.at M. Tauber : H. Resch Department of Traumatology and Sports Injuries, Paracelsus Medical University Salzburg, Müllner Hauptstr. 48, 5020 Salzburg, Austria M. Tauber e-mail: mark.tauber@atos-muenchen.de Pflugers Arch - Eur J Physiol (2011) 461:537–543 DOI 10.1007/s00424-011-0943-4