title: Age-associated changes in motor unit physiology: observations from the Baltimore Longitudinal Study of Aging.
abstract: OBJECTIVE
  To examine motor unit characteristics (size and firing rate) associated with aging.
  
  
  METHODS
  Cross-sectional, observational.
  
  
  METHODS
  Community.
  
  
  METHODS
  Baltimore Longitudinal Study of Aging participants (N=102), aged 22.2 to 94.1 years, were studied.
  
  
  METHODS
  Not applicable.
  
  
  METHODS
  Surface-represented motor unit size and firing rate were collected from the vastus medialis during knee extension at 10%, 20%, 30%, and 50% of each subject's maximum isometric voluntary contraction (MVC).
  
  
  RESULTS
  MVC declined with older age (P<.0001). Adjusting for differences in MVC, both firing rate and motor unit size per newton force generated began to increase in the 6th decade of life. Motor unit size increased per newton force to a greater extent than firing rate. Those over the age of 75 years also activated significantly larger motor units per unit force (P=.04). Relative to force generated, the average firing rate began increasing at 57.8+/-3.4 years and between 50.2 and 56.4 years (+/-4y) for motor unit size.
  
  
  CONCLUSIONS
  The size of motor units and firing rates used to achieve a given force changes with age, particularly after middle age. Whether these changes precede, follow, or occur concurrent to age-related modifications in muscle structure and contractile properties or sarcopenia is not known.
authors: Clinical Research Branch, National Institute on Aging Intramural Research Program, National Institutes of Health, Baltimore, MD, USA. lingsh@mail.nih.gov
authors: Shari M
authors: SM
authors: Ling
authors: Robin A
authors: RA
authors: Conwit
authors: Luigi
authors: L
authors: Ferrucci
authors: E Jeffrey
authors: EJ
authors: Metter
mesh_terms: D000200:Action Potentials; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D015142:Baltimore; D003430:Cross-Sectional Studies; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology