title: Metabolically assessed muscle fibre recruitment in brief isometric contractions at different intensities.
abstract: This study investigated the recruitment of type I, IIA and IIAX fibres after seven isometric contractions at 40, 70 and 100% maximal voluntary knee extension torque (MVC, 1 s on/1 s off). Biopsies of the vastus lateralis muscle were collected from seven subjects at rest and immediately post-exercise. Fibre fragments were dissected from the freeze-dried samples and characterized as type I, IIA and IIAX using mATPase staining. Phosphocreatine (PCr) and creatine (Cr) content were measured in the remaining part of characterized fibres. A decline in the ratio of PCr to Cr (PCr/Cr) was used as an indication of activation. The mean peak torques were, respectively, 39 (2), 72 (2) and 87 (6)% MVC. Cumulative distributions of type I and IIA fibres were significantly shifted to lower PCr/Cr ratios at all intensities (Kolmogorov-Smirnov test, P<0.05). The cumulative distribution of type IIAX fibres showed a significant leftward shift only at 87% MVC ( P<0.05). A hierarchical order of fibre activation with increasing intensity of exercise was found, with some indication of rate coding for type I and IIA fibres. Evidence for activation of type IIAX fibres was only found at 87% MVC.
authors.affiliation: Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT, Amsterdam, The Netherlands.
authors.forename: J G M
authors.initials: JG
authors.lastname: Beltman
authors.forename: A
authors.initials: A
authors.lastname: de Haan
authors.forename: H
authors.initials: H
authors.lastname: Haan
authors.forename: H L
authors.initials: HL
authors.lastname: Gerrits
authors.forename: W
authors.initials: W
authors.lastname: van Mechelen
authors.forename: A J
authors.initials: AJ
authors.lastname: Sargeant
mesh_terms: D000251:Adenosine Triphosphatases / Q000502:physiology; D000328:Adult; D001706:Biopsy; D003401:Creatine / Q000502:physiology; D004576:Electromyography; D005260:Female; D006651:Histocytochemistry; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D007866:Leg / Q000502:physiology; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000201:enzymology / Q000378:metabolism* / Q000502:physiology*; D018482:Muscle, Skeletal / Q000201:enzymology / Q000502:physiology*; D010725:Phosphocreatine / Q000502:physiology
chemical_list: D010725:Phosphocreatine; D000251:Adenosine Triphosphatases; D003401:Creatine