## 4 Discussion
### Limitations of the Included Studies and Their Synthesis  
Each of the analyzed papers had specific limitations that are acknowledged and taken into account in this review.  
[PERSON] et al. [11] had an imbalanced sample in terms of sex, with a majority of female participants. The study focused solely on an older population (age 70+ years), which may have influenced the results since autonomic control typically decreases with age (add reference). The researchers attempted to control potential confounding variables using an electronic metronome rhythm to standardize breathing patterns, but participants reported experiencing subjective stress during the interventions, which may have influenced the recorded HRV measurements.  
[PERSON] et al. [12] had several methodological limitations. All lifestyle aspects were extracted from self-reported questionnaires, and statistical transformation of the data into Log10 hindered the development of formulas that estimate HRV values from behavioral data. In addition, this study had a limited sample size, making it difficult to generate results. [PERSON] et al. [14] also had a limited sample size and thus could not reveal related differences in autonomic regulation of the heart.  
The timing of interventions in each study varied throughout the day (Table 5). Although this may be considered insignificant, the influence of circadian rhythms should be considered, particularly in studies assessing autonomic function [24]. Furthermore, in [PERSON] et al.'s study [14], the prone-supine and prone-sitting interventions were performed on separate days, not accounting for diurnal variation and the participants' circadian rhythm, affecting baseline measurements. Future research should adopt standardized timing and procedures to address these limitations.  
In terms of the systematic review itself, several limitations should be acknowledged. Firstly, none of the studies included were RCTs, which could provide a better distribution of characteristics among groups. While random allocation and blinding would be challenging due to the nature of the interventions involving posture changes, it is important to consider this limitation. Additionally, the included studies had relatively small sample sizes, which limited the statistical power and generalizability of the findings.  
Secondly, the review included only five studies, which limited the reliability of the synthesized results. The studies exhibited both heterogeneity and homogeneity in various aspects, making it challenging to draw definitive conclusions.  
Lastly, some of the analyzed papers did not primarily focus on measuring HRV in terms of posture changes but used HRV as a measurement tool to compare differences among study populations. This made direct comparisons between the results of the papers difficult.  
These limitations should be considered when interpreting this review's findings. However, they should not significantly impact the primary results of the review. Despite these limitations, the overall synthesis of the findings provides valuable insights into the relationship between posture and heart rate variability and contributes to our understanding of the impact of body position on autonomic function.