title: Motor unit survival in lifelong runners is muscle dependent.
abstract: A contributing factor to the loss of muscle mass and strength with adult aging is the reduction in the number of functioning motor units (MUs). Recently, we reported that lifelong competitive runners (master runners = ~66 yr) had greater numbers of MUs in a leg muscle (tibialis anterior) than age-matched recreationally active controls. This suggested that long-term exposure to high levels of physical activity may limit the loss of MU numbers with adult aging. However, it is unknown if this finding is the result of long-term activation of the specifically exercised motoneuron pool (i.e., tibialis anterior) or an overall systemic neuroprotective effect of high levels of physical activity.
  
  
  OBJECTIVE
  The purpose was to estimate the number of functioning MUs (MUNEs) in the biceps brachii (an upper body muscle not directly loaded by running) of nine young (27 ± 5 yr) and nine old (70 ± 5 yr) men and nine lifelong competitive master runners (67 ± 4 yr).
  
  
  METHODS
  Decomposition-enhanced spike-triggered averaging was used to measure surface and intramuscular EMG signals during elbow flexion at 10% of maximum voluntary isometric contraction.
  
  
  RESULTS
  Derived MUNEs were lower in the biceps brachii of runners (185 ± 69 MUs) and old men (133 ± 69 MUs) than the young (354 ± 113 MUs), but the old and master runners were similar.
  
  
  CONCLUSIONS
  Although there were no significant differences in MUNE between both older groups in the biceps brachii muscle, with the number of subjects tested here, we cannot eliminate the possibility of some whole-body neuroprotective effect. However, when compared with the remote biceps muscle, a greater influence on age-related spinal motoneuron survival was found in a chronically activated MN pool specific to the exercised muscle.
authors.affiliation: Canadian Centre for Activity and Aging, School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada.
authors.forename: Geoffrey A
authors.initials: GA
authors.lastname: Power
authors.forename: Brian H
authors.initials: BH
authors.lastname: Dalton
authors.forename: David G
authors.initials: DG
authors.lastname: Behm
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Doherty
authors.forename: Anthony A
authors.initials: AA
authors.lastname: Vandervoort
authors.forename: Charles L
authors.initials: CL
authors.lastname: Rice
mesh_terms: D000200:Action Potentials; D000328:Adult; D000368:Aged; D001132:Arm; D004576:Electromyography; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation*; D012420:Running / Q000502:physiology*; D014022:Tissue Survival / Q000502:physiology*; D055815:Young Adult