A) Contralateral M1 MRBD amplitude against age, this correlation was not significant (R = − 0.27, p = 0.13). B) Significant correlation between ipsilateral M1 MRBD amplitude and age (R = − 0.37, p = 0.04). C) Significant negative correlation between peak frequency of MRBD in contralateral M1 and age (R = − 0.48, p = 0.01). D) Significant negative correlation between peak frequency of MRBD in ipsilateral M1 and age (R = − 0.45, p = 0.01).