## 4 Discussion
### Agreement Between HR\({}_{\textit{chest}}\) and HR\({}_{\textit{temple}}\)  
Previous studies have reported, during land-based activities, a good agreement between this free PPG sensor and ECG signals with the sensor placed on the upper arm, the forearm, or the temple [11]. A good agreement is also reported with the \"gold standard\" chest straps [21; 40; 41]. The results of the present study are in tandem. The HR\({}_{\text{chest}}\) vs. HR\({}_{\text{temple}}\) analysis showed an R\({}^{2}\) of 0.85 (very high), an ICC of 0.96 (CI = 0.95-0.96; excellent), and a mean bias of \(-1\), falling within the criteria for accuracy in every measure. Furthermore, lower intensities scored lower in terms of both R\({}^{2}\) and ICC, in tandem with the literature for land-based activities [8; 16]. However, an individual analysis of the HR measured at the temple (as seen in Figure 2(A1)) revealed that a misreading often occurred during the first part of the test (usually around 30 s), i.e., at a lower intensity. This was not the case for all participants, but there was a trend. One could argue that the low exercise intensity was the root cause of the misreading, as the literature suggests. However, it was during the rest step (the last one), where the HR fell to pre-test values (or below), that the highest accuracy was observed. This gives the impression that the problem was at the beginning of the test, which coincidently is when lower intensities were attained. In all studies where Bland-Altman plots are presented, their LoA should be interpreted carefully. As pointed out by [PERSON] [42], this kind of plot only defines the interval of agreement; it does not say whether the limits are acceptable or not. According to the same authors, the acceptable LoA must be defined a priori, based on clinical necessity, biological considerations, or other goals. This means that statistical agreement is one thing, and clinical acceptability is another. In the present study, it was defined that \(-10/10\) b\(\cdot\)min\({}^{-1}\) would be a clinically acceptable lower and upper LoA. As such, despite the agreement shown by the Bland-Altman plots (bias < 5 b\(\cdot\)min\({}^{-1}\)), one should not consider the calculated LoA appropriate for any of the cases presented in Figure 2. Instead, consider the proposed \(-10/10\) b\(\cdot\)min\({}^{-1}\) (red lines in Figure 2). Thus, upon closer inspection of the Bland-Altman plots from the HR\({}_{\text{chest}}\) vs. HR\({}_{\text{temple}}\) comparison, one can notice that in all plots, the vast majority of the data points fall within the \(-10/10\) b\(\cdot\)min\({}^{-1}\) range. As a specific example, looking at the overall plot of HR\({}_{\text{chest}}\) vs. HR\({}_{\text{temple}}\) (Figure 2(A5)) the mean bias is \(-1\), which is very close to zero, but the computed LoA are still larger than 10 b\(\cdot\)min\({}^{-1}\) (\(-17.5\) and 15.4 to the lower and upper LoA, respectively). This was due to the previously reported higher inaccuracy of the first 30 s readings. Notwithstanding, most of the values represent differences between \(-10\) and 10 b\(\cdot\)min\({}^{-1}\), which agree with the proposed LoA (red lines in Figure 2). In this study, all devices were started with the subject seated (but not at rest) on the pool's side wall, fully equipped, just a few seconds before the test start. Thus, there is the possibility that if the temple sensor had been turned on a bit earlier, or if a dry warmup had been performed right before the test start, the values would probably have been more accurate. This multi-site PPG sensor (in the present study placed at the temple) provides some solutions to the limitations of wrist-based HR measurement, which might justify its results. From a location perspective, it is placed in a site with less movement than anywhere in the upper or lower arm; thus, there are fewer artifacts to be corrected and fewer reading interruptions.