## 5 Discussion  
In this work, an ECG sensor for underwater measurements was designed. Our previous current-based concept [18; 19] was implemented and extended. Originally intended for the measurement of muscle activity in electromyograms, we showed that the concept works equally well for the measurement of the electrocardiogram in underwater conditions. In order to evaluate the new developed sensor during real applications, the sensor was tested in different conditions: outside and inside the water, as well as during rest and movements. The results of the small bandwidth configuration showed an almost noiseless signal quality, also under immersed conditions. Looking at the direct signal output of the ADC, we noticed a much higher signal amplitude of the R peak in the QRS complexes acquired by the current-based circuit when the electrodes and the subject were fully head-out immersed in water. We suspect that this is due to the high conductivity of the water surrounding the metal plate electrodes on the subject's body which increased the current flow into the electrode interface substantially. Looking at different placements of the measurement electrode of this monopolar recording system, we found different configurations to work very well, particularly with the measurement electrode placed directly above the heart on the chest-skin at the third intercostal space between the sternum and the midclavicular line (configuration II in Figure 4). At several placement configurations, we did not record the electrical heart activity in a noticeable magnitude; for example, if both the measurement and the ground electrode are placed on the abdomen on the left and right lateral sides, respectively. We assume this has to do with disturbances in the electrical field of the heart activity inside the body. In the scope of this work, we did not research this further. Even though the QRS complexes can be clearly identified in the current-based ECG signal, it is not entirely understood how this method allows an evaluation of all the parameters defined for the standard ECG and in particular, for abnormality detection [28]. Since with our monopolar \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **Sitting** & **Sitting** & **Supine** & **Supine** \\  & **Outside Water** & **Inside Water** & **Outside Water** & **Inside Water** \\ \hline VLF [ms\({}^{2}\)] & 590.8 & 767.4 & 523.8 & 442.4 \\ LF [ms\({}^{2}\)] & 831.6 & 698.6 & 654.6 & 680.8 \\ HF [ms\({}^{2}\)] & 188.5 & 711.7 ** & 699.4 & 583.7 \\ TP [ms\({}^{2}\)] & 1611 & 2178 ** & 1878 & 1707 \\ LF/HF & 6.20 & 1.15 ** & 1.76 & 1.32 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Frequency domain HRV parameters as a mean over all subjects. system, only the entire summed-up electrical field of the heart (i.e., the changes in it) is measured, and not a particular directional derivation of it, it is not trivial to translate specific definitions of pathological electrical activity, seen in certain derivations of the potential ECG, to the current-based system. This needs further research. The proposed current measurement technology was implemented into the FitnessSHIRT platform (electrical back-end) as introduced by the Frauenhofer IIS (Erlangen, Germany) [21]. However, we did not use, or test this to its full potential. The FitnessSHIRT platform's core element, the shirt itself with integrated tissue-based electrodes, could not be used in our study due to restrictions in the manufacturing process of the initial prototype circuit of the system. Furthermore, we decided that for prototype usage, it would be more suitable to have external electrode connectors available to allow for very similarly designed experiments between our circuit and Gold-standard reference sensors, which are also cable-based. In this way, we were able to ignore the influence of the entire cabling in both systems, as they had the same length and quality for all experiments. Also, the sensors (reference and current-based) could be held in the same locations (at the poolside).