Participants. Thirteen healthy individuals (11 males and 2 females, median age: 24, Nationality: Japanese, Nepalese and Chinese) who were unaccustomed to electrical stimulation participated in the experiment (Table 2). Eleven participants were recruited from Tokyo Institute of Technology and two postgraduate students were recruited from other universities. The Human Subject Research Ethics Review Committee at Tokyo Institute of Technology approved the study protocol, and the approval number is 2020255. All research was performed in accordance with relevant guidelines/regulations, and written informed consent was obtained from each participant prior to the initiation of the experiment. No adverse events were associated with this study. Furthermore, the study was a basic experiment conducted on human subjects and was not a clinical trial.
Table 2
Characteristics of the participants.
Participant | Gender | Age | Width of Palm [cm] | Forearm Circumference [cm] | Voltage of Stimulus Pulse [V] |
00 | Male | 23 | 7.5 | 26 | 33.75 |
01 | Male | 30 | 8.5 | 25 | 33.75 |
02 | Male | 70 | 8.0 | 26 | 45 |
03 | Female | 23 | 7.0 | 25 | 48.75 |
04 | Male | 24 | 7.0 | 23 | 30 |
05 | Male | 30 | 8.0 | 26 | 37.5 |
06 | Female | 43 | 7.5 | 23.5 | 45 |
07 | Male | 31 | 8.0 | 26.5 | 30 |
08 | Male | 23 | 7.5 | 21 | 22.5 |
09 | Male | 24 | 8.5 | 25 | 37.5 |
10 | Male | 23 | 8.5 | 26.5 | 41.25 |
11 | Male | 24 | 8.5 | 28.5 | 37.5 |
12 | Male | 37 | 8.0 | 23.5 | 37.5 |
Electrical stimulation apparatus. We designed an original electric circuit referring ‘wavEMS’31 and newly made circuit boards for the electrical stimulators that were used in the experiments. No commercially available electrical stimulators exist that can be programmed to change the shape of the stimulus pulse to work with external sensors and vary the interference conditions of the stimulation pulses. The performance of the apparatus used in this study is based on a rated voltage of ±60 V and a rated current of 60 mA. Additionally, referring to IEC 60601-2-10:2012, in the event of any issues exceeding the rated current during the experiment, the fuse would physically be blown to ensure the safety of the participants.
For the experiment, two sets of parameters for the stimulus pulse were prepared. One was a sinusoidal biphasic pulse with a duty ratio of 2:1 and the other was a rectangular biphasic electrical stimulation pulse with a duty ratio of 8:1. In this study, we treated the latter as a set of parameters that are conventionally used in EMS.
Regarding the electrodes, we used four gel electrodes (Axelgaard Manufacturing Co., Ltd.), which are considered to be the least painful among the surface electrodes29. The size of each electrode was 50 mm \(\times\) 10 mm.
Experimental apparatus. Because the only muscles in the forearm that rotate it inwards and outwards are the pronator teres muscle and the supinator muscle, the proposed method was evaluated by measuring the inward and outward forearm torque using a six-axis force sensor (055YA251, Leptrino Inc.) (Figure 3). When the elbow is fixed so that the palm is perpendicular to the ground, the forearm can be rotated by up to 90° inward and outward, respectively. Therefore, we used a six-axis force sensor to measure how the deep muscle generated the torque through electrical stimulation from a reference position where the palm was perpendicular to the ground. As part of the experiment, we designed a hand holder and attached it to the six-axis sensor. The hand holder was designed so that movements other than that of the palm (such as the elbow and fingertips) were not measured as noise. Additionally, the palm and hand holder were fixed with Velcro so that each participant could rapidly remove them in case of an emergency. An arm cover (SK-491, Komine Co., Ltd.) was used to maintain a horizontal position of the subject's forearm, and it had two additional functions: it allowed for the fixing angle of the elbow and the distance from the hand holder to the elbow to be varied, depending on the participant. The force sensor was attached directly to the steel plate with screws. The arm cover was attached to the aluminium frame and the height was adjusted, then the aluminium frame was attached to the steel plate with screws and the steel plate was fixed to the desk in a vice.
Procedures. The experiment was conducted in four stages: calibration, investigation of the response to conventional EMS, investigation of the response to EMS with/without interference and measurement of the MVIC. The calibration determined the placement of the electrodes and the strength of the voltage. Regardless of which hand is dominant, the right forearm is farther from the heart than the left forearm, thus, the right forearm of the participants was electrically stimulated during the experiment. The arrangement of the four electrodes (two pairs) was chosen so that the proposed method (EdMS) would be the most effective. We were careful to note that as the joints moved, the relative positions of the muscles and skin changed32.
Therefore, based on preliminary experiments, the most reproducible arrangement was the one adopted for the proposed placement of the electrodes (Figure 2). Noting that the angle of the elbow affected the ease of stimulation, we also adjusted the angle of the device's arm cover so that the subject's response would be the strongest. The voltage was adopted as the maximum value at which the subject did not feel any discomfort and was fixed at this value throughout the experiment unless any issues arose.
After the calibration was completed, the participants placed their right forearm on the experimental device. They received stimulation pulses of conventional EMS parameters from a pair of electrodes for 1 second and the forearm torque was measured for 2 seconds simultaneously as the stimulation was initiated. Three measurements were taken under each condition, and because there were two pairs of electrodes, a total of six measurements were taken for each subject (Figure 4). The timing of the stimulation was informed by voice so that the subject would not be startled and make unnecessary movements.
Once the investigation of the response to conventional EMS was completed, the interference-free (0 Hz beat frequency) electrical stimulation was analysed. There are two types of interference-free electrical stimulation: (i) a single sinusoidal stimulation pulse, and (ii) two sinusoidal stimulation pulses of the same frequency. In the case of (i), electrical stimuli of 100 Hz, 4,000 Hz and 10,000 Hz were applied independently from each pair of electrodes three times (18 times in total), and the torque was measured (Figure 4). In the case of (ii), the same three frequencies as in (i) were applied simultaneously from two pairs of electrodes three times (9 times in total), and the torque was measured.
After (i) and (ii) were completed, the interference conditions were compared. In the interference condition, the upper electrode pair applied stimulation pulses at the reference frequency, and the lower electrode pair applied stimulation pulses at a frequency that deviated from the reference frequency by a beat frequency. We used the same reference frequencies as in (i) and (ii) (100 Hz, 4,000 Hz and 10,000 Hz) and the beat frequencies used were 20 Hz, 40 Hz, 80 Hz, 160 Hz, 320 Hz, 640 Hz, 1,280 Hz and 2,560 Hz. Under the conditions of one reference frequency and one beat frequency, electrical stimulation was applied three times and the torque was measured (72 times in total) (Figures 5, 6 and 7).
As a comparison of the interference conditions had not been conducted previously, the participants were asked to complete a questionnaire for each condition to obtain additional information. Four questions were asked based on the intensity of the pain, the type of pain, sensations other than pain and the sense of agency during electrical stimulation. The participants were asked to respond to the FRS regarding the intensity of the pain, part of the McGill pain questionnaire (MPQ) regarding the type of pain, an open-ended description regarding the sensations other than pain and the numeric rating scale (NRS) regarding the sense of agency.
Finally, the spontaneous pronation torque of the forearm was measured. No electrical stimulation was applied to the participants. Each participant placed his or her right arm on the experimental apparatus and waited for the sound to be generated at random intervals. When the sound was generated, the participant grasped the handle and pronated it with maximum force by pushing the forearm inwards until the sound faded. The participants were reminded not to use their biceps. The measurements were taken five times per participant, and the maximum torque value obtained was considered to be the MVIC.
Data processing. The data from the six-axis force sensors were initially processed using our original software. When measuring the torque via electrical stimulation, the force sensor values that were measured simultaneously while the stimulation pulse was applied were considered as the baseline and subtracted from the measured forces and torques. Similarly, when measuring the MVIC, the force sensor value that was measured simultaneously as a sound was heard was taken as zero-based. During the experiment, the data were obtained for the forces in the three axes and the torques around the three axes. After the experiment, the values of the rotational torques of the forearm were calculated using a statistical analysis programme that we created. The point of action where the participant's force was applied to the handle was set at the centre of the palm, and the force sensor measured the torque and translational force. The value of the rotational torque of the forearm was calculated for each participant by subtracting the torque due to the translational force from the torque around the axis parallel to the forearm. The distance of the torque due to the translational force was calculated as the distance between the centre of the participant's palm and the centre of the force sensor. Due to a large variation in the torque data between the participants, the index (%MVIC) that normalised the mean value of the torque value via the electric stimulation by MVIC was calculated (Figure 8). The average value of the torque due to electrical stimulation was calculated from 0.55 seconds to 1.05 seconds after the electrical stimulation was applied.
Statistical analyses. A two-way repeated-measures ANOVA was performed on the time-series data of the torque measured during electrical stimulation to investigate whether a particular combination of reference and beat frequencies was efficient at stimulating the pronator teres muscle. For each of the other conditions of electrical stimulation, a one-way repeated-measures ANOVA was used to assess whether there was a dominant combination. A two-sided paired t-test was used to show the superiority of each of the EdMS conditions compared with the other parameters. Regarding the FRS of the questionnaire, a non-parametric test (Wilcoxon rank-sum test) was used. We considered p < 0.05 to be statistically superior.