## 4 Heart Rate Variability (HRV) Study
### Results
#### 4.3.2 Frequency Domain Parameters  
The HRV features in the frequency domain are visualized in Figure 9a. The power spectral density in the VLF range showed an increase in the sitting position during water immersion (767.4 ms\({}^{2}\)) compared to the position outside the water (590.8 ms\({}^{2}\)), which was statistically not significant. However, in the supine position, the power spectral density was lower during the immersed condition than at the poolside. The VLF showed a value of 442.4 ms\({}^{2}\) in the water in contrast to outside the water with 523.8 ms\({}^{2}\).  
In the low-frequency range, the power density spectrum showed a different behavior compared to the very low-frequency range. With 831.6 ms\({}^{2}\) the measure for the LF range outside the water was increased compared to the immersed condition with 698.6 ms\({}^{2}\). In the supine position, both conditions, in and outside the water, did not differ significantly. The measurements at the poolside showed a LF of 654.6 ms\({}^{2}\), whereas during the immersed condition the value was 680.8 ms\({}^{2}\).  
The power spectral density in the HF range was significantly decreased (\(p=0.0022\)) during sitting outside the water in contrast to the water immersion. The HF value for the outside condition was  
Figure 9: Frequency domain HRV parameters as a mean over all subjects. In (**a**), the changes in the very low frequency power (VLF), the low frequency (LF), and high frequency (HF) power as well as the total power (TP) are visualized between the outside-the-water (bright colors) and immersed condition (dark colors). On the left side of each subfigure the changes in the sitting position and on the right the changes in the supine position are given. In (**b**), the differences in the ratio LF/HF are visualized.  
188.5 ms\({}^{2}\), unlike during immersion 711.9 ms\({}^{2}\). In the supine position, this measure showed reverse behavior. Outside the water, the HF power spectral density was 699.4 ms\({}^{2}\) and in the water 583.7 ms\({}^{2}\).  
Even the total power over all frequency ranges demonstrated such behavior in the VLF and the LF ranges. During sitting, the TP value was significantly lower (\(p=0.0022\)) in the non-immersed condition (1610 ms\({}^{2}\)) compared to the measurements inside the water (2178 ms\({}^{2}\)). Also in the case of the power spectral density for the total power, a reverse characteristic in the supine position compared to the sitting position could be observed. During the immersion, the value was lower (1707 ms\({}^{2}\)) in contrast to the non-immersed condition (1878 ms\({}^{2}\)).  
The ratio between the low-frequency and the high-frequency range (see Figure 9b) in the sitting position indicated a significantly higher value outside the water compared to immersion (\(p=0.0022\)). The ratio between the LF and the HF range in the non-immersed sitting position was 6.20, during immersion 1.15. In the supine position, the differences were not that large. For the measurements at the poolside, the ratio was slightly higher with 1.76 compared to during the immersion with 1.32.  
Table 2 shows a summary of the evaluated results as a mean over all subjects in the frequency domain.