title: The decline in skeletal muscle mass with aging is mainly attributed to a reduction in type II muscle fiber size.
abstract: BACKGROUND
  The loss of skeletal muscle mass with aging has been attributed to a decline in muscle fiber number and muscle fiber size.
  
  
  OBJECTIVE
  To define to what extent differences in leg muscle cross-sectional area (CSA) between young and elderly men are attributed to differences in muscle fiber size.
  
  
  METHODS
  Quadriceps muscle CSA and type I and type II muscle fiber size were measured in healthy young (n=25; 23 ± 1 y) and older (n=26; 71 ± 1 y) men. Subsequently, the older subjects performed 6 months of resistance type exercise training, after which measurements were repeated. Differences in quadriceps muscle CSA were compared with differences in type I and type II muscle fiber size.
  
  
  RESULTS
  Quadriceps CSA was substantially smaller in older versus young men (68 ± 2 vs 80 ± 2 cm(2), respectively; P<0.001). Type II muscle fiber size was substantially smaller in the elderly vs the young (29%; P<0.001), with a tendency of smaller type I muscle fibers (P=0.052). Differences in type II muscle fiber size fully explained differences in quadriceps CSA between groups. Prolonged resistance type exercise training in the elderly increased type II muscle fiber size by 24 ± 8% (P<0.01), explaining 100 ± 3% of the increase in quadriceps muscle CSA (from 68 ± 2 to 74 ± 2 cm(2)).
  
  
  CONCLUSIONS
  Reduced muscle mass with aging is mainly attributed to smaller type II muscle fiber size and, as such, is unlikely accompanied by substantial muscle fiber loss. In line, the increase in muscle mass following prolonged resistance type exercise training can be attributed entirely to specific type II muscle fiber hypertrophy.
authors: Department of Human Movement Sciences, NUTRIM School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Centre+, Maastricht, The Netherlands.
authors: Rachel
authors: R
authors: Nilwik
authors: Tim
authors: T
authors: Snijders
authors: Marika
authors: M
authors: Leenders
authors: Bart B L
authors: BB
authors: Groen
authors: Janneau
authors: J
authors: van Kranenburg
authors: Lex B
authors: LB
authors: Verdijk
authors: Luc J C
authors: LJ
authors: van Loon
mesh_terms: D015502:Absorptiometry, Photon / Q000379:methods; D000368:Aged; D000375:Aging / Q000473:pathology* / Q000502:physiology; D001706:Biopsy; D006801:Humans; D006984:Hypertrophy / Q000473:pathology; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000000981:diagnostic imaging / Q000473:pathology* / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000000981:diagnostic imaging / Q000473:pathology / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000473:pathology* / Q000503:physiopathology; D055070:Resistance Training; D055948:Sarcopenia / Q000000981:diagnostic imaging / Q000473:pathology* / Q000534:rehabilitation; D014057:Tomography, X-Ray Computed; D055815:Young Adult