Spinal and corticospinal pathways are differently modulated when standing at the bottom and the top of a three-step staircase in young and older adults

Purpose: This study investigated the modulation of spinal (group Ia afferents) and corticospinal pathways when young (22.7 ± 1.3 years) and older adults (72.2 ± 7.9 years) stood at the bottom and at the top of a three-step staircase equipped with force platforms. Method: Changes in submaximal H-reflex amplitude (H) and slope of the H-reflex input–output relation (spinal pathway), and in amplitude of motor-evoked potentials (MEP) triggered by transcranial magnetic stimulation (corticospinal pathway) at two intensities (1.1× and 1.2× motor threshold) were recorded in soleus when subjects stood as steady as possible downstairs and upstairs. The centre of pressure (CoP) excursion was analyzed in the time and frequency domains in both conditions. Results: Regardless of age, the mean CoP velocity was greater when standing upstairs (11.1 ± 3.5 mm s) than downstairs (9.0 ± 2.3 mm s; p = 0.002). The CoP power spectral density (PSD) in the 0–0.5 Hz band was greater upstairs than downstairs (+18.4%; p = 0.03) whereas PSD in the 2–20Hz frequency band was lesser (−41%) upstairs than downstairs (p < 0.001), regardless of age. In both groups, the H amplitude (−30.6%; p < 0.001) and slope of H-reflex input–output relation (−10.2%; p = 0.002) were lesser when standing upstairs than downstairs, whereas no significant difference was observed in MEP amplitude and silent period between balance conditions (p > 0.05). Conclusion: These results indicate a lower dependence on spinal pathway to control soleus motor neurones when standing upstairs than downstairs accompanied by a change in postural control. This suggests that healthy older adults preserved their ability to adjust postural control to environmental demands.