Older adults differentially modulate transcranial magnetic stimulation-electroencephalography measures of cortical inhibition during maximal single-joint exercise

Abstract

The effects of muscle fatigue are known to be altered in older adults, and age-related changes in the brain are likely to be a contributing factor. However, the neural mechanisms underlying these changes are not known. The aim of the current study was to use transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) to investigate age-related changes in cortical excitability with muscle fatigue. In 23 young (mean age ± SD: 22 ± 2 years) and 17 older (mean age ± SD: 68.3 ± 5.6 years) adults, single-pulse TMS-EEG was applied before, during and after the performance of fatiguing, intermittent isometric abduction of the index finger. Motor-evoked potential (MEP) measures of cortical excitability were increased during (estimated mean difference, 123.3%; P < 0.0001) and after (estimated mean difference, 117.5%; P = 0.001) fatigue and this was not different between groups (P > 0.5). For TMS-EEG, the amplitude of the P30 and P180 potentials were unaffected by fatigue in older participants (P > 0.05). In contrast, the amplitude of the N45 potential in older adults was significantly reduced both during (positive cluster: mean voltage difference = 0.7 µV, P < 0.005; negative cluster: mean voltage difference = 0.9 µV, P < 0.0005) and after (mean voltage difference = 0.5 µV, P < 0.005) fatiguing exercise, whereas this response was absent in young participants. These results suggest that performance of maximal intermittent isometric exercise in old but not young adults is associated with modulation of cortical inhibition likely mediated by activation of gamma-aminobutyric acid type A receptors.