**2.2. The spinal motor neuron excitability during MI at 50 and 100% MVC**

### *2.2.1. Purpose*

Our previous works [24–26] suggested that MI of isometric thenar muscle activity at 10, 30, 50, and 70% MVC can facilitate the spinal motor neuron excitability. However, the imagined muscle contraction strength did not influence on change of the spinal motor neuron excitability. Whereas, Cowley et al. [29] previously reported that the amplitude of H-reflex during MI of ankle plantar flexion at 100% MVC was significantly greater than that at 50% MVC. Then, we hypothesized the MI of isometric thenar muscle activity at 100% MVC will be greater than that at 50% MVC. In this research, we compared the spinal motor neuron excitability between 50% MI and 100% MI condition [28].

#### *2.2.2. Materials*

Fifteen healthy volunteers were participated in this research (13 males, 2 females; mean age = 25.3 ± 5.0 years). All participants provided informed consent before the study commenced. This research was approved by the Research Ethics Committee at Kansai University of Health Sciences. All recordings were conducted in accordance with the Declaration of Helsinki.

#### *2.2.3. F-wave recording procedure*

The environment and F-wave recording condition was set as previous works [24, 25].

### *2.2.4. 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) and immediately after 50% MI trial (post) for 1 min respectively. The above process was defined as the MI at 50% MVC condition (50% MI condition). F-wave recording under 100% MI condition was performed using the same protocol as 50% MI condition. These conditions were performed randomly on different days.

After all F-wave recordings, F-wave data was analyzed with respect to the persistence, F/M amplitude ratio, and latency.
