title: Can fast-twitch muscle fibres be selectively recruited during lengthening contractions? Review and applications to sport movements.
abstract: Literature examining the recruitment order of motor units during lengthening (eccentric) contractions was reviewed to determine if fast-twitch motor units can be active while lower threshold slow-twitch motor units are not active. Studies utilizing surface electromyogram (EMG) amplitude, single motor unit activity, spike amplitude-frequency analyses, EMG power spectrum, mechanomyographic, and phosphocreatine-to-creatine ratio (PCr/Cr) techniques were reviewed. Only single motor unit and PCr/Cr data were found to be suitable to address the goals of this review. Nine of ten single motor unit studies, examining joint movement velocities up to 225 degrees/s and forces up to 53% of a maximum voluntary contraction, found that the size principle of motor unit recruitment applied during lengthening contractions. Deviation from the size principle was demonstrated by one study examining movements within a small range of low velocities and modest forces, although other studies examining similar low forces and lengthening velocities reported size-ordered recruitment. The PCr/Cr data demonstrated the activation of all fibre types in lengthening maximal contractions. Most evidence indicates that for lengthening contractions of a wide range of efforts and speeds, fast-twitch muscle fibres cannot be selectively recruited without activity of the slow-twitch fibres of the same muscle.
authors.affiliation: Department of Physical Education, Health and Recreation, Western Washington University, Bellingham, WA 98225-9067, USA. gordon.chalmers@wwu.edu
authors.forename: Gordon R
authors.initials: GR
authors.lastname: Chalmers
mesh_terms: D000818:Animals; D004576:Electromyography; D015444:Exercise / Q000502:physiology; D006801:Humans; D009119:Muscle Contraction / Q000502:physiology*; D018656:Muscle Fibers, Fast-Twitch / Q000502:physiology*; D010725:Phosphocreatine / Q000378:metabolism; D010805:Physical Conditioning, Animal / Q000502:physiology; D011999:Recruitment, Neurophysiological / Q000502:physiology*; D013177:Sports / Q000502:physiology*
chemical_list: D010725:Phosphocreatine