Motor imagery while viewing self-finger movements facilitates the excitability of spinal motor neurons

Action observation (AO) and motor imagery (MI) are potential methods to improve various motor functions. AO and MI facilitate corticospinal excitability. Additionally, the combination of AO and MI (AO + MI) facilitates greater corticospinal excitability than AO alone. However, it is unclear whether AO + MI facilitates the excitability of spinal motor neurons better than AO alone. Thus, in this study, we compared the excitability of spinal motor neurons between AO + MI and AO conditions using F-waves. We recruited healthy adult volunteers (n = 31) for F-wave analysis. We recorded F-waves during relaxation to determine the baseline level (Rest), followed by measurement during AO and AO + MI. To counterbalance the order effect, 15 participants performed AO first, while the others performed AO + MI first. F-wave persistence was significantly increased during AO and AO + MI compared to Rest. The F-wave/M-wave amplitude ratio was significantly increased during AO + MI compared to Rest. The present results suggest that the combination of AO and MI facilitates the excitability of spinal motor neurons compared to Rest, whereas AO alone does not.