title: Input-output characteristics of soleus homonymous Ia afferents and corticospinal pathways during upright standing differ between young and elderly adults.
abstract: OBJECTIVE
  This study investigated the effects of ageing on the excitability of soleus homonymous Ia afferents and corticospinal pathways during bipedal upright standing.
  
  
  METHODS
  The input-output relations for the Hoffmann (H) reflex and motor-evoked potential (MEP) were computed for the soleus in response to electrical nerve stimulation and transcranial magnetic stimulation, respectively, in young (n = 16) and elderly (n = 16) adults. In subsets of subjects, the input-output relations were compared between normal and supported upright standing, and corticomotoneuronal excitability was assessed during upright standing with an H-reflex conditioning method. For the H-reflex and MEP threshold, maximal amplitude (Hmax , MEPmax ) and the slope of the input-output relation (Hslope , MEPslope ) were measured and normalized to the corresponding M-wave value.
  
  
  RESULTS
  In normal standing, the Hmax /Mmax [mean (SD); young: 48.3 (14.2)%; elderly: 17.3 (6.4)%] and Hslope /Mslope were significantly (P < 0.05) lower in elderly than in young adults, whereas the MEPmax /Mmax [young: 13.6 (7.5)%; elderly: 24.5 (12.8)%] and MEPslope /Mslope were greater in elderly adults (P < 0.05). The Hslope /Mslope and MEPslope /Mslope decreased and increased, respectively, from supported to normal standing for both age groups but more so in elderly adults. Furthermore, the conditioned H reflex was greater (P < 0.05) in elderly [175.1 (34.3)%] than in young adults [141.8 (29.8)%] during normal standing.
  
  
  CONCLUSIONS
  This is the first study that clearly indicates lower efficacy of Ia afferents to discharge spinal motor neurones accompanied by greater corticospinal excitability in elderly adults, suggesting an increased contribution of the descending drive in controlling soleus activity during upright standing with ageing.
authors.affiliation: Laboratory of Applied Biology, ULB Neuroscience Institute (UNI), Université Libre de Bruxelles (ULB), Brussels, Belgium.
authors.forename: S
authors.initials: S
authors.lastname: Baudry
authors.forename: F
authors.initials: F
authors.lastname: Penzer
authors.forename: J
authors.initials: J
authors.lastname: Duchateau
mesh_terms: D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006181:H-Reflex / Q000502:physiology; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology; D011187:Posture / Q000502:physiology*; D011712:Pyramidal Tracts / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation; D055815:Young Adult
keywords: H reflex; ageing; balance; motor-evoked potential