title: Evidence for ACTN3 as a Speed Gene in Isolated Human Muscle Fibers.
abstract: OBJECTIVE
  To examine the effect of α-actinin-3 deficiency due to homozygosity for the ACTN3 577X-allele on contractile and morphological properties of fast muscle fibers in non-athletic young men.
  
  
  METHODS
  A biopsy was taken from the vastus lateralis of 4 RR and 4 XX individuals to test for differences in morphologic and contractile properties of single muscle fibers. The cross-sectional area of the fiber and muscle fiber composition was determined using standard immunohistochemistry analyses. Skinned single muscle fibers were subjected to active tests to determine peak normalized force (P0), maximal unloading velocity (V0) and peak power. A passive stretch test was performed to calculate Young's Modulus and hysteresis to assess fiber visco-elasticity.
  
  
  RESULTS
  No differences were found in muscle fiber composition. The cross-sectional area of type IIa and IIx fibers was larger in RR compared to XX individuals (P<0.001). P0 was similar in both groups over all fiber types. A higher V0 was observed in type IIa fibers of RR genotypes (P<0.001) but not in type I fibers. The visco-elasticity as determined by Young's Modulus and hysteresis was unaffected by fiber type or genotype.
  
  
  CONCLUSIONS
  The greater V0 and the larger fast fiber CSA in RR compared to XX genotypes likely contribute to enhanced whole muscle performance during high velocity contractions.
authors.affiliation: Exercise Physiology Research Group, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, Heverlee, Belgium.
authors.forename: Siacia
authors.initials: S
authors.lastname: Broos
authors.affiliation: Sports Medicine Research Laboratory, Luxembourg Institute of Health, Grand-Duchy of Luxembourg, Luxembourg.
authors.forename: Laurent
authors.initials: L
authors.lastname: Malisoux
authors.affiliation: Sports Medicine Research Laboratory, Luxembourg Institute of Health, Grand-Duchy of Luxembourg, Luxembourg.
authors.forename: Daniel
authors.initials: D
authors.lastname: Theisen
authors.affiliation: Exercise Physiology Research Group, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, Heverlee, Belgium.
authors.forename: Ruud
authors.initials: R
authors.lastname: van Thienen
authors.affiliation: Exercise Physiology Research Group, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, Heverlee, Belgium.
authors.forename: Monique
authors.initials: M
authors.lastname: Ramaekers
authors.affiliation: Institute of Neuroscience, Université catholique de Louvain, Louvain-la-Neuve, Belgium.
authors.forename: Cécile
authors.initials: C
authors.lastname: Jamart
authors.affiliation: Exercise Physiology Research Group, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, Heverlee, Belgium.|Institute of Neuroscience, Université catholique de Louvain, Louvain-la-Neuve, Belgium.
authors.forename: Louise
authors.initials: L
authors.lastname: Deldicque
authors.affiliation: Physical Activity, Sports & Health Research Group, Department of Kinesiology, Faculty of Kinesiology and Rehabilitation Sciences, KU Leuven, Heverlee, Belgium.
authors.forename: Martine A
authors.initials: MA
authors.lastname: Thomis
authors.affiliation: Institute of Neuroscience, Université catholique de Louvain, Louvain-la-Neuve, Belgium.
authors.forename: Marc
authors.initials: M
authors.lastname: Francaux
mesh_terms: D000185:Actinin / Q000235:genetics / Q000502:physiology*; D001707:Biopsy, Needle; D005796:Genes / Q000502:physiology; D005838:Genotype; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D018482:Muscle, Skeletal / Q000033:anatomy & histology; D055815:Young Adult
chemical_list: C118456:ACTN3 protein, human; D000185:Actinin