## 4 Discussion  
The dynamic evaluation of the ANS based on HRV analysis is important in the treatment of complications caused by ANS abnormalities [18]. As a diagnostic parameter for measuring imbalance in the ANS, the HRV analysis method for measuring cardiovascular ANS activities by observing the R-R interval variations on the electrocardiogram (ECG) has been used for the clinical evaluation of the ANS conditions of patients, to determine the balance between the sympathetic and parasympathetic nervous systems and the activation level, as well as for evaluating diagnostic markers for a given disease [19]. HRV is a determinant factor for evaluating the scope of changes in the ANS based on the beat-to-beat interval changes. Heartbeat is determined by the ANS regulatory function of the sinoatrial node, for the maintenance of homeostasis of the body and for spontaneous activity [28]. Sensory nerves aroused by electrical stimulation are known to influence the cardiovascular system via the ANS [19]. The results of the present study support previous research that found significant HRV immediately after HF-LI and LF-HI electrical stimulations [23]. However, [PERSON] et al. [29] showed decreased activity in the sympathetic nerves after electro-acupuncture stimulation, which was accompanied by a decrease in HRV and pain in healthy men. The reduction of HRV immediately after HF-HI electrical stimulation in the present study is similar to previous research results, confirming the reliability of HRV measurement results based on electrical stimulation variables [30]. It should be noted that the results of this study could not directly demonstrate whether the electrical stimulation induces HRV via changes in the ANS. Because spectral analysis of heart rate and blood pressure changes are indices induced by the sympathetic nervous system, [PERSON] et al. [31] questioned the validity of the use of these indicators alone for the assessment of cardiovascular regulation. Although we were able to determine that HF-HI electrical stimulation decreased HRV, the significant increase in the pain threshold observed also raises the same questions as in previous research, because it could be explained by a simple change in the ANS. [PERSON] et al. [17] proposed that detailed research should be performed to elucidate the pathological mechanism of central sympathetic nerve activity and to develop an effective treatment strategy. Pain types are assessed based on the theory that pain uses a specific pathway in which a single nerve cell forms a pattern within a complex neural network, taking information from multiple nerve cells [32]. The stimulation currents are normalized so that the pain control effect due to electrical stimulation is not affected by changes in tissue resistance; therefore, reliable measurement values can be obtained by maintaining a current that is invariant to changes of resistance [33]. However, electrical stimulation can also render negative results, regardless of pain control effects, such as changes in blood flow or increased release of neurotransmitters, such as bradykinin and histamine, due to pain induction and increased sympathetic nerve activity [34; 35]. This is why the mechanism for pain control can be explained based on frequency-intensity current parameters [36]. [PERSON] et al. [37] claimed that when HF-HI electrical stimulation collides with stimulation from peripheral areas to block pain transmission from peripheral areas, the conductivity speed of the peripheral nerves is reduced, and the temporary block of afferent conductance in A-\(\delta\) and C fibers acts to cause pain. The results of the present study also confirmed this mechanism: Although there