title: Eccentric exercise increases satellite cell content in type II muscle fibers.
abstract: BACKGROUND
  Satellite cells (SCs) are of key importance in skeletal muscle tissue growth, repair, and regeneration. A single bout of high-force eccentric exercise has been demonstrated to increase mixed muscle SC content after 1-7 d of postexercise recovery. However, little is known about fiber type-specific changes in SC content and their activation status within 24 h of postexercise recovery.
  
  
  METHODS
  Nine recreationally active young men (23 ± 1 yr) performed 300 eccentric actions of the knee extensors on an isokinetic dynamometer. Skeletal muscle biopsies from the vastus lateralis were collected preexercise and 24 h postexercise. Muscle fiber type-specific SC content and the number of activated SCs were determined by immunohistochemical analyses.
  
  
  RESULTS
  There was no difference between Type I and Type II muscle fiber SC content before exercise. SC content significantly increased 24 h postexercise in Type II muscle fibers (from 0.085 ± 0.012 to 0.133 ± 0.016 SCs per fiber, respectively; P < 0.05), whereas there was no change in Type I fibers. In accordance, activation status increased from preexercise to 24 h postexercise as demonstrated by the increase in the number of DLK1+ SCs in Type II muscle fibers (from 0.027 ± 0.008 to 0.070 ± 0.017 SCs per muscle fiber P < 0.05). Although no significant changes were observed in the number of Ki-67+ SCs, we did observe an increase in the number of proliferating cell nuclear antigen-positive SCs after 24 h of postexercise recovery.
  
  
  CONCLUSIONS
  A single bout of high-force eccentric exercise increases muscle fiber SC content and activation status in Type II but not Type I muscle fibers.
authors.affiliation: Department of Human Movement Sciences, NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands.
authors.forename: Naomi M
authors.initials: NM
authors.lastname: Cermak
authors.forename: Tim
authors.initials: T
authors.lastname: Snijders
authors.forename: Bryon R
authors.initials: BR
authors.lastname: McKay
authors.forename: Gianni
authors.initials: G
authors.lastname: Parise
authors.forename: Lex B
authors.initials: LB
authors.lastname: Verdijk
authors.forename: Mark A
authors.initials: MA
authors.lastname: Tarnopolsky
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Gibala
authors.forename: Luc J C
authors.initials: LJ
authors.lastname: Van Loon
mesh_terms: D000704:Analysis of Variance; D001706:Biopsy; D015444:Exercise / Q000502:physiology*; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000166:cytology*; D010101:Oxygen Consumption / Q000502:physiology; D032496:Satellite Cells, Skeletal Muscle / Q000166:cytology*; D055815:Young Adult