## 4 Discussion
In addition, the RMTgroup showed changes in respiratory function with lower levels of VE/BSA before the 30 s StS test after the intervention period and lower levels of VE/MVV at the end of the test, and changes were also observed in PRQ in three stages. The PRQ (ratio of pulse rate to respiratory rate) is a parameter that captures the complex state of cardiorespiratory interactions, and the changes (increase, decrease) are related to conditions such as body position, anxiety, respiratory, and/or heart disease [42], is dependent on the vagus nerve, which plays a crucial role in maintaining homeostasis between these systems. Respiratory muscle training can positively influence the vagus nerve by increasing parasympathetic activity and improving vagal tone, while this interaction between the respiratory system and the autonomic nervous system (especially through respiratory rhythms) influences the vagal pathways [43]. The respiratory pattern influences cardiac vagal outflow, while the respiratory control system integrates complex signals from the drive to breathe generated by the central pattern with spontaneous breathing, which can be modulated by changes in respiratory cycle duration and HR dynamics [44]. Our results showed that respiratory muscle training improved cognitive function (via the MoCA score) and memory. Our previous study showed that the combination of aerobic exercise-resistance exercise-exercise with new technologies improves blood flow to the brain and has a positive effect on hippocampal function [45]. The current results are probably due to the intervention program with activation of anaerobic metabolism and activation of the lactic acid neutralization mechanism (Haldane effect) and facilitated cardio-pulmonary-metabolic processes, which is consistent with the results of our study and the adjustment of oxygen breathing [46]. A previous study has shown that transcutaneous vagus nerve stimulation correlates with cognitive performance on cognitive tasks and with neural activity in the brain, activating the fusiform region, which is associated with cognitive changes [47]. Vagal activation reduces anxiety levels and increases vagal outflow, which appears to have a greater effect on parasympathetic activity (depending on age), through a process of activating physiological and psychobiological mechanisms [48]. Another outcome of the current study was changes in sleep quality. We found an improvement in sleep quality (via the PSQI score) of \(-2.00\) for the RMTgroup and \(-0.93\) for the CPAPgroup, which is not clinically relevant for the CPAP group according to previous studies [49]. The CPAPgroup showed an increase in the \"waking in the middle of the night or early in the morning\" domain, probably due to lack of familiarity with the CPAP device, such as noise, mask fitting, dry nose or throat, etc., while the RMTgroup decreased the \"cannot breathe comfortably\" domain, results consistent with previous studies [50]. The regulation of breathing during sleep is mainly under the control of chemoreceptors, while the chemoreflex is fully stimulated, increasing sympathetic nervous system tone during both sleep and wakefulness [51]. Vagus nerve stimulation is a neuroregulatory technique in several cognitive domains [52] and is beneficial in sleep disorders [53] and affected from respiratory muscle training [44]. Previous studies have shown that patients with OSA who train the respiratory muscles (main and accessory respiratory muscles) have fewer arousals, fewer periodic limb movements (\(\downarrow\) blood pressure and plasma norepinephrine levels) and better overall sleep quality [54] and reduce stress on the respiratory muscles during intermittent breath-holding during hypoxia reoxygenation, leading to an increase in intrathoracic pressure [11]. The inspiratory muscles play a more important role in this improvement [55] and may be influenced by the Venturi effect--[PERSON]'s principle as derived from computational fluid dynamics and the Starling Resistor model [56].