## 2 Materials and Methods  
This study involved a parallel design to compare the means of three groups, and the analysis method used for the main evaluation variables was a two-way repeated measures Analysis of Variance (ANOVA). We included 45 healthy students living in Kwangju city, Republic of Korea, in the study. The participants were provided with sufficient explanation about the study, and all voluntarily agreed to participate in the experiment. The sample size was estimated from power calculations using a commercially-available software package (G-Power). Assuming a power of 90%, a type 1 error (\(\alpha\)) of 5%, a type 2 error (\(\beta\)) of 10%, and a confidence interval of 95%, we calculated a sample size of 15 per group [20; 21]. Exclusion criteria were as follows: Medical history and vital signs (blood pressure, pulse, body temperature, respiratory rate) that may be associated with changes in blood flow; conditions that may influence the blood test and experimental results; metal prosthetic implants; neurological or brain disorders; and unsuitability for electrical stimulation intervention or other. In order to control for factors that may influence the results, the participants were instructed to refrain from vigorous exercise that may influence their heart rate variability (HRV). They were also required to abstain from smoking and the drinking of alcohol and coffee, which may influence the autonomic nervous system (ANS), 1 h prior to the experiment. In addition, the participants were given instructions about other factors that may affect the results (e.g., talking, coughing and deep breathing), and were requested to avoid such behaviors during the measurements. Preliminary measurements were conducted, and subjects who had a large HRV that deviated from the standard value due to excessive sweating on the palms, or cold hands, were excluded. The participants were randomly assigned to receive high frequency-low intensity (HF-LI), low frequency-high intensity (LF-HI), or high frequency-high intensity (HF-HI) electrical stimulation (\(n=15\) per group). For the analysis, the three groups were considered to have similar general characteristics (Table 1). A flowchart of the study procedure is shown in Figure 1. The study protocol was approved by the institutional review board of Nambu University in Kwangju, and the participants provided written consent after receiving verbal and written information about the study. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Characteristics** & **HF-LI** & **LF-HI** & **HF-HI** & **F** & **P** \\ \hline \multirow{2}{*}{Sex} & Men & 7 & 8 & 9 & - & \\  & Women & 8 & 7 & 6 & & \\ Age (years) & 22.00 \(\pm\) 1.41 & 22.45 \(\pm\) 1.44 & 22.64 \(\pm\) 1.63 & 0.527 & 0.596 \\ Height (cm) & 166.27 \(\pm\) 7.46 & 167.09 \(\pm\) 8.50 & 170.64 \(\pm\) 8.08 & 0.615 & 0.547 \\ Weight (kg) & 62.55 \(\pm\) 4.55 & 60.82 \(\pm\) 4.08 & 65.09 \(\pm\) 5.93 & 0.512 & 0.640 \\ \hline \hline \end{tabular} Note: Data are expressed as the number or mean \(\pm\) standard deviation; HF-LI: high frequency-low intensity, LF-HI: low frequency-high intensity, HF-HI: high frequency-high intensity.  
\end{table} 
Table 1: General characteristics of the study participants. HF-LI: high frequency-low intensity, LF-HI: low frequency-high intensity, HF-HI: high frequency-high intensity  
The Endomed 582 (Enraf Nonius, The Netherlands) was used as an interferential current (IFC) electrical stimulator. Each intervention lasted 20 min. Pad electrodes (4 x 4 cm, Protens Electrodes, Bio Protech Inc., Korea) were placed on the transverse process at a distance of 2 cm from the left side of the spinous process of the thoracic vertebrae T1-T4. To ensure the psychological stability of the subjects, these subjects were asked to lie down on a bed for 10 min before intervention [11]. The conditions used for the HF-LI, LF-HI and HF-HI groups were 100 bps/10-20 mA, 5 bps/45-50 mA, and 100 bps/80-90 mA, respectively. The amplitude refers to the stimulation's applied wave voltage or current intensity.