*2.3.2. Influence of the imagined muscle contraction strength on the spinal motor neuron excitability*

In our previous works, the relative value of the persistence, F/M amplitude, and latency were similar among all MI conditions. It is suggested that the imagined muscle contraction strength may not affect the spinal motor neuron excitability. There are several previous researches investigated the spinal motor neuron excitability during MI at different imagined muscle contraction strengths. Bonnet et al. [32] reported that the amplitude of H-reflex was significantly greater during MI of ankle plantar flexion at 2 and 10% than that at rest. Additionally, the amplitude of H-reflex during MI was similar between 2% MI and 10% MI condition. Hale et al. [33] also reported that the amplitude of H-reflex during MI of ankle plantar flexion was similar among five (i.e., 20, 40, 60, 80, and 100% MVC) MI conditions. Similarly, Aoyama and Kaneko [34] reported that the amplitude of H-reflex during MI was similar between 50% MI and 100% MI condition. In actual motion, the spinal motor neuron excitability was increased linearly with the muscle contraction strength [27]. Described in the introduction, MI is the mental rehearsal of a movement without any overt movement [1]. One possibility is the contribution of neural mechanism which inhibits actual movement and muscle contraction during MI. Park and Li [35] reported that the amplitude of MEPs during MI of finger flexion and extension at 10, 20, 30, 40, 50, and 60% MVC was significantly greater than that at rest. However, the amplitude of MEPs during MI was similar among all six MI conditions. Further, in an event-related potential study, the magnitude of primary motor cortex activity during MI did not correlate with the imagined muscle contraction strength, although activities of the supplementary motor and premotor area during MI were strongly correlated with it [36]. The supplementary motor and premotor area have crucial roles in larger force generation [37], motor planning, preparation, and inhibition [38, 39]. Thus, the supplementary motor and premotor area may inhibit the actual muscle activity depending on the muscle contraction strength. Because these areas also are connected directly to primary motor cortex, inhibitory inputs from the supplementary motor and premotor area may suppress any additional excitation of primary motor cortex conferred by MI with high imagined contraction strength. Furthermore, the spinal motor neuron excitability during MI is thought to be affected by central nervous system via the corticospinal and/or extrapyramidal tract. The degree of the spinal motor neuron excitability during MI at various imagined muscle contraction strengths may be modulated by both excitatory and inhibitory inputs from the central nervous system.

#### **2.4. Conclusion**

**Rest 50% MI post**

**Rest 100% MI post**

**50% MI condition 100% MI condition**

Persistence (%) 50.8 ± 21.7 88.2 ± 13.2\*\* 48.3 ± 19.9 F/M amplitude ratio (%) 1.71 ± 0.89 3.96 ± 4.56\*\* 1.29 ± 0.56 Latency (ms) 25.5 ± 1.40 24.9 ± 1.91 25.3 ± 1.29

Persistence (%) 60.8 ± 24.9 91.9 ± 7.58\*\* 60.7 ± 21.5 F/M amplitude ratio (%) 1.32 ± 1.12 3.57 ± 4.67\*\* 1.39 ± 1.25 Latency (ms) 25.2 ± 1.32 24.8 ± 1.31 25.2 ± 1.40

Relative values of persistence 2.04 ± 1.17 2.06 ± 1.71 Relative values of F/M amplitude ratio 2.75 ± 2.04 2.53 ± 1.76 Relative values of latency 0.98 ± 0.06 0.99 ± 0.03

excitability during MI with holding the sensor of a pinch meter was significantly greater than that during MI without holding the sensor [20]. Consequently, it is suggested that tactile and proprioceptive perceptions during MI while holding the sensor facilitated the spinal motor

In our previous works, the relative value of the persistence, F/M amplitude, and latency were similar among all MI conditions. It is suggested that the imagined muscle contraction strength may not affect the spinal motor neuron excitability. There are several previous researches investigated the spinal motor neuron excitability during MI at different imagined muscle contraction strengths. Bonnet et al. [32] reported that the amplitude of H-reflex was significantly greater during MI of ankle plantar flexion at 2 and 10% than that at rest. Additionally, the amplitude of H-reflex during MI was similar between 2% MI and 10% MI condition. Hale et al. [33] also reported that the amplitude of H-reflex during MI of ankle plantar flexion was similar among five (i.e., 20, 40, 60, 80, and 100% MVC) MI conditions.

*2.3.2. Influence of the imagined muscle contraction strength on the spinal motor neuron* 

neuron excitability cooperatively with MI-activated pathways.

**Table 8.** Comparison of F-wave parameters between 50% MI and 100% MI condition.

*excitability*

\*\**p* < 0.01; significant difference between rest and 50% MI trial.

60 Evolving BCI Therapy - Engaging Brain State Dynamics

**Table 6.** Changes in F-wave parameters under 50% MI condition.

\*\**p* < 0.01; significant difference between rest and 100% MI trial.

**Table 7.** Changes in F-wave parameters under 100% MI condition.

Our previous woks showed significant increase of the spinal motor neuron excitability during MI of isometric thenar muscle activity. However, the imagined muscle contraction strength was not involved in change of the spinal motor neuron excitability.
