## 4 Discussion
### Agreement Between HR\({}_{\text{chest}}\) and HR\({}_{\text{watch}}\)
In a study with a similar setup to ours, in which [PERSON] and [PERSON] [13] do not report any specific watch placement and fit, ICC values as low as 0.2 (at 61-70% HRmax) to 0.44 (at 81-90% HRmax) are reported. The sensor pressure on the skin and the watch placement are stated to affect its accuracy [24; 34]. That is the reason why the brand has specific guidelines for watch placement and fit. In the present study, all of the brand's guidelines were followed. Nevertheless, the wristwatch still performed less well than reported on land. A mean bias of \(-26.5\) with an LoA of \(-73\) and \(20\) for the whole test is in tandem with the low accuracy reported for the wristwatch PPG during swimming. The sensor placement limitation can be indirectly improved by placing a free PPG sensor on the upper arm that transmits in real time to the watch (as the latter is restricted to wrist use), as, in the upper limbs, proximal readings are more accurate than distal ones [34]. Other studies have also reported a lack of accuracy of wrist-based PPG while swimming whether watch placement is reported or not [35; 36]. It should be made clear that none of the studies state that the watches or the technology are to blame; nor does the present one. The explanations put forward by all studies are similar: the presence of water between the sensors and the skin and the heavy arm movements seem to be major factors impairing the HR readings. To surpass the water constraint, the only reliable solution available for PPG devices seems to be to cover the sensor to avoid any contact with the water. Skin pigmentation has been suggested to potentially affect optical HR readings [21; 37; 38]. The participants in the present study were all within the Type 3 skin tone type according to the Fitzpatrick skin tone scale (due to the non-presence of individuals of other races in the class), thus no skin-tone-related bias should be expected. A limitation of the present study may come from the swimming skill of the participants. Despite making all efforts to mitigate the water flow between the sensors and the skin (for example, the adoption of tethered swimming and the placement and fit of the wristwatch), a poor technique might have negatively impacted the wristwatch readings specifically. The participants in this study were attending the second academic year of the sports course. Few had previous contact with swimming classes and the majority were beginners. Beginner swimmers have a rougher technique, which may affect negatively the readings at wrist level. For example, during the catch phase, swimming books suggest there should be approximately a \(90^{\circ}\) angle at the elbow [39], which, in turn, represents a \(90^{\circ}\) angle with the water flow. Since the wristwatch sensor is located perpendicularly to the arm, any deviation from this \(90^{\circ}\) angle at the elbow will increase the flow of water parallel to the arm and, therefore, increase the likelihood of water interfering with the readings. However, [PERSON] and [PERSON] [13] evaluated \"well-trained competitive swimmers\" and the accuracy was still far from desirable.