Participants
Participants comprised 13 healthy adult males with no history of neurophysiological diseases. Mean (± SD) age was 27 ± 11 years, height was 167 ± 2 cm, and weight was 66 ± 6 kg). This study was conducted in accordance with the Declaration of Helsinki. Informed consent to participate in the study was obtained from the participants verbally and in writing. This study was approved by the Ethical Review Board of the Kibi International University (No. 18-37).
Electrical stimulation
A square wave pulse lasting 0.2 ms was generated in the median nerve in the carpal area on the right side (i.e., ipsilateral to the pinching hand). Using a surface electrostimulation apparatus (NM-420S, Nihon Kohden, Japan), the stimulation electrodes were placed 20 mm apart, with the cathode proximal and the anode distal. The stimulation electrodes were secured using a fixation band to regulate their pressure. The minimum intensity of electrostimulation to induce an M-wave was confirmed, and the stimulation region was set appropriately.
F-wave recording and analysis
A surface electromyogram (EMG) during electrostimulation was recorded in the right abductor pollicis brevis using Ag/AgCl bipolar electrodes (5 mm diameter, 20 mm interelectrode distance; Nihon Kohden, Japan) on the same side as the electrostimulation. The recording electrode was attached onto the muscle belly of the abductor pollicis brevis, and the reference electrode onto the first proximal phalanx; both electrodes were fixed with surgical tape. The attachment site for the recording electrode was wiped with alcohol and preprocessed using sandpaper to create an interelectrode resistance < 5 kΩ. An EMG/evoked potential testing device (Neuropack MEB-9404, Nihon Kohden, Japan) was used, and F-wave waveforms were recorded using a band-pass filter of 1.5−3 kHz and a sampling frequency of 10 kHz. The electrostimulation intensity was set to 1.2x, which obtained a maximum M-wave. The stimulation frequency was set to 1 Hz and was carried out for approximately 1 min. The F-wave analysis used the three factors appearance rate, latency, and amplitude F/M ratio during the second half (30 s) of electrostimulation. To determine the percentage of the appearance rate, all identifiable resulting waveforms (total n = 30) on the monitor (500 μV/D) were targeted. For the F-wave amplitude, the average value of the peak-to-peak amplitude for the F-waves was expressed as a ratio compared to the maximum M-wave amplitude. Latency was the average time from electrostimulation until F-wave initiation.
SEP recordings and analysis
Based on the international 10-20 system, SEPs were recorded from the somatosensory area of the right upper arm (C3’: 2 cm posterior from C3) on the stimulation side. The reference electrode was placed at the point Fz. Electrodes were attached onto the skin surface using conductive paste. Interelectrode resistance was set to < 5 kΩ. The electrostimulation intensity was fixed at just above the motor threshold with a repetition rate of 3 Hz. An EMG/evoked potential testing device (Neuropack MEB-9404, Nihon Kohden, Japan) was used, and SEP waveforms were recorded using a band-pass filter of 20 Hz–10 kHz and a sampling frequency of 10 kHz, and 200 responses were averaged. SEP waveforms were evaluated for 100 ms, and evaluations were performed at the time of electrostimulation and 100 ms after stimulation. Epochs with artifacts owing to eye movement or blinking (>± 6 μV from baseline) were automatically excluded before averaging. A plate electrode was used to record the evoked electroencephalogram (Ag/AgCl electrode, NE-132B (Φ 10 mm), Nihon Kohden, Japan). The amplitudes from baseline to N20 and the N20-P25 peak-to-peak amplitude, which are early components after electrostimulation, were analyzed for SEPs. The amplitude of each component was measured from preceding peaks.
Subjective peripheral sensation (SPS)
Prior to the experiment, the SPS threshold on the dorsal surface of the right hand was measured using a Semmes-Weinstein monofilament test (SAKAI Medical Co., Ltd., Tokyo, Japan). After establishing their peripheral sensory thresholds, participants reported the presence or absence of peripheral cutaneous stimulation during the SPS measurements. Monofilament stimulation was carried out using gradually thicker filaments, starting from thin filaments of 0.008 g. Confirmation tests were repeated approximately five times for each intensity, and the filament thickness that could be sensed correctly at a rate of approximately 100% was defined as the SPS threshold. The experimenter lowered the filament vertically onto the hand, removed it, and returned it to its original position within 1 s. The stimulation interval of the filament was random and carried out a total of 20 times. The participants were instructed to give verbal cues when they sensed filament stimulation; afterwards, their accuracy rate was calculated. All measurements were performed by the same experimenter. The filament stimulation site was marked using a marker, so that deviation of a large stimulation site owing to experimenter error did not occur.
Testing procedure
The participants were seated in a chair with both arms placed on armrests with the eyes open to perform a pinching task with the right hand while keeping the arm in a neutral position. Before the task, maximum voluntary isometric force (MVIF) was measured, from which the target tension level was calculated. The participants held the pinch force meter with the thumb and index finger and squeezed with maximal force for 5 seconds in the experimental position. The maximum value of force exertion at 5 seconds was defined as MVIF. The target value was displayed visually on a computer placed 1 m in front of the participant. After sufficient rest of more than 15 minutes, participants maintained 30% MVIF for approximately 2 min. Tension was not exerted in the rest condition. Each measurement item and each condition were randomly set and measured for each subject.
Statistics
All values are shown as the mean ± standard deviation. Comparisons were analyzed using paired t-tests; statistical significance levels were set at < 5% (P < 0.05) with GraphPad Prism Ver 8.3.1 for Macintosh (GraphPad Software, San Diego, CA, USA).