Input-output characteristics of soleus homonymous Ia afferents and corticospinal pathways during upright standing differ between young and elderly adults

AimThis study investigated the effects of ageing on the excitability of soleus homonymous Ia afferents and corticospinal pathways during bipedal upright standing. MethodsThe 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 (H-max, MEPmax) and the slope of the input-output relation (H-slope, MEPslope) were measured and normalized to the corresponding M-wave value. ResultsIn normal standing, the H-max/M-max [mean (SD); young: 48.3 (14.2)%; elderly: 17.3 (6.4)%] and H-slope/M-slope were significantly (P<0.05) lower in elderly than in young adults, whereas the MEPmax/M-max [young: 13.6 (7.5)%; elderly: 24.5 (12.8)%] and MEPslope/M-slope were greater in elderly adults (P<0.05). The H-slope/M-slope and MEPslope/M-slope 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. ConclusionThis 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.