**1. Introduction**

Motor imagery (MI) is defined as an active process during which a specific motor action is reproduced within working memory without any overt movement [1]. MI is considered a potential tool for improvement of motor function in rehabilitation. Indeed, MI has been shown to improve various motor functions. Yue and Cole [2] reported that muscle strength of little finger abduction was significantly increased after MI training for 4 weeks. Additionally, muscle strength of ankle dorsiflexion was significantly increased after MI training for 4 weeks [3]. Also, Guillot et al. [4] reported that muscle flexibility was improved after MI of stretching for 5 weeks.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Immediate enrollment in rehabilitation programs for functional reorganization should be important to obtain better outcomes [5]. Specifically, Motor-evoked potentials (MEPs) amplitude, an index of corticospinal excitability, was decreased in post-stroke [6]. However, MEPs amplitude was increased in patients who have functional motor recovery [7]. Additionally, spinal motor neuron excitability was significantly reduced in the post-stroke acute phase [8]. Thus, facilitating the corticospinal excitability, including the spinal motor neuron excitability, should be needed for post-stroke patients whom have motor impairment.

**2. The spinal motor neuron excitability during MI at various** 

the spinal motor neuron excitability during MI by using the F-wave [24–26].

**2.1. The spinal motor neuron excitability during MI at 10, 30, 50, and 70% MVC**

We previously reported that when the subject performed MI of isomeric thenar muscle activity at 50% MVC, the spinal motor neuron excitability was significantly increased than at rest [20]. In actual motion, Suzuki et al. [27] reported the spinal motor neuron excitability was increased linearly with muscle contraction strength. If MI and motor execution share common neural networks, the spinal motor neuron excitability will be increased linearly with imagined muscle contraction strength. Then, we investigated the spinal motor neuron excitability during MI at various imagined muscle contraction strengths. Specifically, we adopted the 10, 30, 50, and 70% MVC for imagined muscle contraction strength. In this research, we assessed

The Application of Motor Imagery to Neurorehabilitation

http://dx.doi.org/10.5772/intechopen.75411

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Ten healthy volunteers were participated in this research (5 males, 5 females; mean age = 28.7 ± 4.5 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.

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.

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.,

**imagined muscle contraction strengths**

*2.1.1. Purpose*

*2.1.2. Materials*

*2.1.3. F-wave recording procedure*

*2.1.4. Experimental protocol*

Filter bandwidth was ranged from 20 Hz to 3 kHz.

Numerous neurophysiological studies using positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and near infrared spectroscopy (NIRS) have demonstrated that MI and motor execution activate similar brain activation patterns [9–13]. Specifically, primary motor cortex, supplementary motor area, premotor area, somatosensory area, prefrontal cortex, parietal lobule, cingulate area, cerebellum, and basal ganglia were activated during MI and motor execution. Thus, MI shares common neural substrates with motor execution. When transcranial magnetic stimulation (TMS) was delivered over the primary motor cortex during MI, MEPs amplitude obtained from corresponding muscle was significantly increased relative to rest level [14–16]. The increase of MEPs amplitude during MI indicates that MI facilitates corticospinal excitability. Thus, MI can facilitate the central neural function.

However, previous studies have shown various patterns in the spinal motor neuron excitability during MI using the F-wave and H-reflex as indices of spinal reflex excitability [17–19]. Taniguchi et al. [17] reported that the F-wave amplitude was significantly decreased after volitional relaxation for 3 h. When subjects did MI of thumb abduction during volitional relaxation simultaneously, the F-wave amplitude was maintained at before volitional relaxation level. Whereas, Kasai et al. [18] reported that the H-reflex amplitude was unchanged during MI of wrist flexion movement. Oishi et al. [19] also reported that there was decline of H-reflex amplitude during MI of speed skating. Our laboratory previously investigated the spinal motor neuron excitability during MI of isometric thenar muscle activity at 50% maximal voluntary contraction (MVC) for 1 min using the F-wave [20]. The F-wave is a compound action potential resulting from re-excitation ("backfiring") of an antidromic impulse following distal electrical stimulation of motor nerve fibers at the anterior horn cells [21–23]. The F-wave measured during MI at 50% MVC for 1 min was significantly increased than that at rest. Thus, we concluded that MI of isometric thenar muscle activity can increase the spinal motor neuron excitability.

We are aiming to find the way of MI obtained most beneficial effect. In order to do that, it is important to assess the spinal motor neuron excitability concurrent with the central nervous system. We think that facilitating the spinal motor neuron excitability will be required for improvement of motor function. Because, described above, the facilitation of the corticospinal excitability including the spinal motor neuron excitability is needed for recovery of motor function. In this chapter, we would like to introduce our previous works about the spinal motor neuron excitability during MI of isometric thenar muscle activity. In the first half of this chapter, we described about the spinal motor neuron excitability during MI of isometric thenar muscle activity at various imagined muscle contraction strengths. In the second half of this chapter, we described about the autonomic nervous system during MI. At the end of chapter, we discuss about how apply MI to neurorehabilitation using brain-machine interface (BMI).
