## 4 Discussion
\begin{table}
\begin{tabular}{l l l c c c c c} \hline \hline \multirow{2}{*}{**Variable**} & \multirow{2}{*}{
\begin{tabular}{c} **Group (\(\mu\) = 15** \\ **Per Group** \\ \end{tabular} } & \multicolumn{3}{c}{**Time From Electrical Stimulation**} & \multicolumn{3}{c}{**F (p)**} \\ \cline{3-8}  & & **Before** & **Immediately** & **Thirty Minutes** & & & \\  & & **After** & **After** & & & & \\ \hline HRV & HF-LI & 69.94 \(\pm\) 6.88 & 72.54 \(\pm\) 6.10 & 70.99 \(\pm\) 6.28 & & & \\ (Hz) & LF-HI & 65.88 \(\pm\) 2.90 & 64.2 \(\pm\) 3.31 & 65.69 \(\pm\) 3.22 & & & \\  & HF-HI & 66.46 \(\pm\) 3.29 & 66.43 \(\pm\) 3.19 & 66.72 \(\pm\) 4.92 & & & \\ \hline Pain threshold & HF-LI & 0.92 \(\pm\) 0.24 & 1.07 \(\pm\) 0.32 & 1.09 \(\pm\) 0.24 & & & \\ (mA) & LF-HI & 0.90 \(\pm\) 0.27 & 1.23 \(\pm\) 0.34 & 1.31 \(\pm\) 0.24 & & & \\  & HF-HI & 1.09 \(\pm\) 0.28 & 1.73 \(\pm\) 0.59 & 1.68 \(\pm\) 0.40 & & & \\ \hline \hline \end{tabular} Note: Data are expressed as mean \(\mu\) standard deviation; HF-LI: high frequency-low intensity, LF-HI: low frequency-high intensity, HF-HI: high frequency-high intensity, \(\cdotp<0.05\).  
\end{table} 
Table 2: Changes in heart rate variability (HRV) and pain threshold. was a significant increase in the pain threshold upon HF-HI electrical stimulation compared to the other conditions, there was also a shorter pain duration. By contrast, the effects of low-frequency electrical stimulation on pain were not local, had a long duration and were sometimes accompanied by sleepiness or calmness [38]. When performing electrical stimulation according to frequency, [PERSON] et al. [8] reported that a frequency of 2 Hz was effective in relieving acute pain, and that 1-10 bps had stimulating effects on the motor nerves and tissues, leading to an increase in strong vasodilatory substances. Moreover, the combination of LF and HI electrical stimulation, which shows an increase in deep pain and endorphin secretion, had relatively slower pain induction but longer pain duration; therefore, this type of stimulation is often used for chronic pain [39]. On the other hand, it has been reported that HF and LI transcutaneous electrical nerve stimulation (TENS) treatment activates afferent nerve tissues with a larger diameter, and the sensation threshold emerges rapidly, but the duration is short [5]. The present study also showed a significant effect of the HF-LI and LF-HI conditions on the pain threshold immediately after electrical stimulation, but the results after 30 min showed there were significant changes only in the LF-HI group, indicating maintenance of the pain control period immediately after, and 30 min after, electrical stimulation. [PERSON] et al. [40] showed that high-frequency electrical stimulation causes the release of \(\beta\)-endorphin by inhibiting serotonin, which has a top-down control effect, and the blocking of harmful information suppresses the pain-related activity of nerve cells within the spinal cord, leading to a reduction in pain and a decrease in the length of pain persistence. We consider that the difference in the pain threshold duration observed in the present study is due to the same mechanism. [PERSON] et al. [41] reported that the intensity of the electrical stimulation acts as an important factor in the pain threshold increase. [PERSON] & [PERSON] [42] reported that subjects receiving strong, painless, electrical stimulation had a greater increase in their pain threshold. [PERSON]'s research also indicated a significant effect in segmented stimulation at high intensity [43]. This could occur because when stimulation signals are sent to the central nerves by the physiological transmission systems for electrical signals, endogenous opioid secretion is increased, and \(\upmu\) receptors in the spinal cord are activated, producing a pain effect, blocking the pain transmitted to the higher central nervous system, and activating descending pain functions to suppress pain [44].