ABSTRACT
This study investigated the influence of vision and proprioception on the excitability of direct corticospinal (corticomotoneuronal) pathway to the soleus in young and elderly adults during upright standing. Ten young and 10 elderly adults stood upright on a rigid surface with eyes open or closed, and on foam mat with eyes open. The corticomotoneuronal excitability was investigated by assessing facilitation of the soleus H-reflex induced by subthreshold transcranial magnetic stimulation (TMS). The torque produced by the plantar flexor muscles during a maximal voluntary contraction was also measured. The maximal plantar flexion torque was significantly lower in elderly than in young adults (p<0.05). The activity of leg muscles, recorded by electromyography (EMG) was greater in elderly than in young adults regardless of balance conditions (p<0.05), and greater when standing on foam than in the other conditions (p<0.05), regardless of age. The H-reflex facilitation was greater for elderly [182.9 (45)%] than young adults [130.5 (33.1)%; p<0.05] but did not differ across sensory conditions (p>0.05). However, the amplitude of the H reflex conditioned by TMS relative to the amplitude of the test H reflex ratio was positively associated with EMG activity of the plantar flexor muscles during upright standing (r(2)=0.47; p<0.001). These results indicate that regardless of age the excitability of the corticomotoneuronal pathway is not modulated with changes in the sensory conditions during upright standing. Nonetheless, the corticomotoneural drive to control leg muscle during upright standing increases with the level of soleus muscle activity.
Subject(s)
Aging/physiology , H-Reflex/physiology , Posture/physiology , Proprioception/physiology , Vision, Ocular/physiology , Adult , Aged , Aged, 80 and over , Cerebral Cortex/physiology , Electromyography , Female , Humans , Leg/physiology , Male , Middle Aged , Motor Neurons/physiology , Muscle, Skeletal/physiology , Neural Pathways/physiology , Physical Stimulation , Postural Balance/physiology , Transcranial Magnetic Stimulation , Young AdultABSTRACT
AIM: 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. CONCLUSION: 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.
