*2.1.3. F-wave recording procedure*

Participants were in supine position on a bed and instructed to fix one's eye on a pinch meter (Digital indicator F304A, Unipulse Corp., Japan) display throughout the F-wave recording. A Viking Quest electromyography machine ver. 9.0 (Natus Medical Inc., USA) was used for the F-wave recordings. The room temperature was kept at 25°C. The skin was cleaned with an abrasive gel to keep impedance below 5 kΩ. F-waves were recorded from left thenar muscle after stimulating the left median nerve at the wrist. A pair of 10 mm silver EEG cup electrodes (Natus Medical Inc., USA) were placed over the ventral surface of the thumb and base of the first dorsal metacarpal bone. The simulating electrodes comprised a cathode placed over the left median nerve 3 cm proximal to the palmar crease and an anode was placed 2 cm more proximally. Before the F-wave recording, maximal intensity of electrical stimulation was determined by delivering 0.2-ms square-wave pulses of increasing intensity from 0 to 50 mA until eliciting the largest compound muscle action potential (M-wave). Supramaximal electrical stimuli (20% above maximal stimulus intensity) were delivered at 0.5 Hz in each trial. The sensitivity for the F-wave was set at 200 μV/division and a sweep of 5 ms/division. Filter bandwidth was ranged from 20 Hz to 3 kHz.

#### *2.1.4. Experimental protocol*

For the rest trial (rest), F-waves were recorded during relaxation for 1 min. Subsequently, for the motor task, participants learned the isometric thenar muscle activity at 50% MVC (i.e., participants press the sensor of pinch meter by left thumb and index finger at 50% MVC) for 1 min. They were instructed to keep the 50% MVC value (kgf) measured numerically on the display of pinch meter. For the MI trial, participants performed MI of isometric thenar muscle activity at 50% MVC for 1 min. F-waves were recorded during MI (50% MI). Immediately after 50% MI trial (post), F-waves were recorded during relaxation for 1 min. The above process was defined as the MI at 50% MVC condition (50% MI condition). This protocol was repeated for 10, 30, and 70% MI conditions. Each condition was performed randomly on different days.
