*3.1.4. Experimental protocol*

For the rest trial (rest), the ANS activity was recorded during relaxation for 5 min. The European Society of Cardiology and the North American Society of Pacing and Electrophysiology recommend 5 min recordings for heart rate variability analysis [43]. 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 10% MVC for 5 min. The ANS activity was recorded during MI (10% MI) and immediately after 10% MI trial (post) for 5 min respectively. The above process was defined as the MI at 10% MVC condition (10% MI condition). The ANS activity recording under 50% MI condition was performed using the same protocol as 10% MI condition. These conditions were performed randomly on different days.
