Physical activity modulates corticospinal excitability of the lower limb in young and old adults

Aging is associated with reduced neuromuscular function, which may be due in part to altered corticospinal excitability. Regular physical activity (PA) may ameliorate these age-related declines, but the influence of PA on corticospinal excitability is unknown. The purpose of this study was to determine the influence of age, sex and PA on corticospinal excitability by comparing the stimulus-response curves of motor evoked potentials (MEP) in 28 young (22.4±2.2 yrs, 14 women, 14 men) and 50 old adults (70.2±6.1 yrs, 22 women, 28 men) who varied in activity levels. Transcranial magnetic stimulation was used to elicit MEPs in the active vastus lateralis muscle (10% maximal voluntary contraction, MVC) with 5% increments in stimulator intensity until the maximum MEP amplitude. Stimulus-response curves of MEP amplitudes were fit with a 4-parameter sigmoidal curve, and the maximal slope calculated (slope_max). Habitual PA was assessed with tri-axial accelerometry, and participants categorized into either those meeting current PA guidelines for optimal health benefits (>10,000 steps•day^-1, high-PA, n=21) or those not meeting guidelines (<10,000 steps•day^-1, low-PA, n=41). The MEP amplitudes and slope_max were greater in the low-PA compared with the high-PA group (P<0.05). Neither age nor sex influenced the stimulus-response curve parameters (P>0.05) suggesting that habitual PA influenced the excitability of the corticospinal tract projecting to the lower limb similarly in both young and old adults. These findings provide evidence that achieving the recommended PA guidelines for optimal health may mediate its effects on the nervous system by decreasing corticospinal excitability.

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