## 3 Results
### Results of Individual Studies and Synthesis of Results  
In each of the five studies assessed, frequency domain, i.e., high frequency (HF), low frequency (LF), and the LF/HF ratio, were the main components of HRV with posture. The HF component of the HRV power spectrum reflects parasympathetic activity, and LF reflects a combination of sympathetic and parasympathetic activity [14]. The ratio LF/HF represents the balance between sympathetic and parasympathetic nerves. An increase in the ratio indicates sympathetic activity predominance, and a decrease indicates parasympathetic predominance [13]. The main goal of [PERSON] et al. 's study [11] was to investigate the possible impact of determining HRV in an orthostatic posture compared to sitting. The study found a high and positive correlation between heart rate variability measurements performed while standing and those taken while sitting for autonomic nervous system responses. The study concluded that heart rate variability recordings while sitting may be impacted by orthostatic hypotension; hence, lying down is advised as being the best position for such recordings. Nevertheless, standing, supine, and sitting positions were used to record heart rate variability. To analyze the data, the study calculated the means, standard deviations, repeated measures ANOVA results, and paired \(t\)-test results for each HRV marker in the frequency domain. Statistically significant differences among the three postural positions were observed only for the lower and higher frequency markers, with a \(p\)-value of 0.004 (considered statistically significant if \(p<0.05\)). According to [PERSON] et al. [12], the study findings indicated that sedentary behavior had the strongest association with autonomic cardiac function at rest compared to other lifestyle factors such as sleep quality, caloric expenditure, and light-intensity physical activities. This was evident through alterations in various cardiac parameters, particularly a reduction in heart rate variability (HRV) when transitioning from a resting position to a supine or orthostatic position. A substantial correlation between sedentary behavior and HRV indices was seen in people lying down. Statistical significance was found in all variables between supine and orthostatic postures, with each \(p\)-value being \(<\)0.001 except LF, which had a \(p\)-value of 0.005, and confidence intervals were calculated at 95%. [PERSON] et al. assessed changes in cardiac function in various positions to determine underlying reasons for improving the cardiovascular and autonomic nervous systems. The study assessed HRV in seven positions: neutral trunk posture (supine), posterior extension, forward flexion, left lateral flexion, right lateral flexion, left rotation, and right rotation. In addition to the HF, LF, and LF/HF ratio, the R-R interval was also assessed as a variable for HRV in this study. The R-R of the posterior extension posture was significantly lower compared to forwarding flexion, left flexion, and right flexion postures (\(p=0.047\), \(p=0.026\), \(p=0.025\)), and the RR of the right flexion posture was significantly higher than right rotation posture (\(p=0.020\)). The LF/HF in the neural position of the trunk was significantly lower than the posterior extension, forward flexion, and right rotation postures with \(p=0.037\), 0.041, and 0.008, respectively. The frequency domain index LF/HF analysis showed that the sympathetic and parasympathetic tensions were relatively balanced at rest, and other postures showed a sympathetic dominance, suggesting that the neutral posture of the trunk is the best resting posture. This study also showed that a steady decrease in thoracic volume caused by increased trunk inclination enhanced heart rate variability and autonomic tone. This supports the findings of [PERSON] et al. [12]. [PERSON] et al. [14] compared heart rate variability parameters in three positions: sitting, prone, and supine.