## 4 Discussion
### Agreement Between HR\({}_{\text{chest}}\) and HR\({}_{\text{watch}}\)  
On land, wristwatches from this brand have demonstrated a good agreement with ECG signals in various sports and activities [28; 29]. However, in the present study, the wristwatch underperformed in every established criterion (stepwise and in the whole test), as seen in Table 2. The literature reports various criteria for accuracy: the presence of an association above 0.90 (r \(>\) 0.90 or R\({}^{2}\) \(>\) 0.81) and a bias (mean difference) of \(<\)3 b\(\cdot\)min\({}^{-1}\)[17], \(<\)5 b\(\cdot\)min\({}^{-1}\)[30], or \(<\)6 b\(\cdot\)min\({}^{-1}\)[31] are two of the most used criteria. In the present study, an association of r \(>\) 0.90 (R\({}^{2}\) \(>\) 0.81), a bias of \(<\)5 b\(\cdot\)min\({}^{-1}\), and an ICC \(>\) 0.90 were used as cut-off points for accuracy and, as such, the measurement of HR by the wristwatch was deemed as inaccurate. In the aquatic environment, one of the studies conducted to assess the agreement between wrist-based and chest-based HR readings reported ICC values ranging from 0.2 (at 61-70% HRmax) to 0.44 (at 81-90% HRmax). This is corroborated by their Bland-Altman plot, where a bias of \(-\)9.51 and an LoA of approximately \(-\)53 and 34 denote a low agreement between the methods. In the present study, a mean bias of \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Test Step**} & \multicolumn{3}{c}{**HR\({}_{\text{chest}}\times\) HR\({}_{\text{temple}}\)**} & \multicolumn{3}{c}{**HR\({}_{\text{chest}}\times\) HR\({}_{\text{watch}}\)**} \\ \cline{2-7}  & **R\({}^{2}\)** & **ICC (95\% CI)** & **Mean Bias (95\% CI)** & **R\({}^{2}\)** & **ICC (95\% CI)** & **Mean Bias (95\% CI)** \\ \hline
1 st step (0–30 s) & 0.37 & 0.71 & \(-\)8.7 & 0.01 & 0.03 \({}^{\dagger}\) & \(-\)48.1 \({}^{\dagger}\) \\  & & (0.53–0.81) & (\(-\)9.9—39.7) & 0.01 & (\(-\)0.04–0.11) & (\(-\)49.9—46.3) \\ \hline
2 nd step (30–60 s) & 0.81 \({}^{\star}\) & 0.94 \({}^{\star}\) & \(-\)3.1 \({}^{\star}\) & 0.04 \({}^{\dagger}\) & 0.1 \({}^{\dagger}\) & \(-\)46.4 \({}^{\dagger}\) \\  & & (0.91–0.96) & (\(-\)3.7—2.5) & 0.04 \({}^{\dagger}\) & (\(-\)0.09–0.28) & (\(-\)48.2—44.6) \\ \hline
3 rd step (60–90 s) & 0.84 \({}^{\star}\) & 0.95 \({}^{\star}\) & \(-\)2.1 \({}^{\star}\) & 0.06 \({}^{\dagger}\) & 0.13 \({}^{\dagger}\) & \(-\)42.2 \({}^{\dagger}\) \\  & & (0.93–0.96) & (\(-\)2.5—1.6) & (\(-\)0.11–0.35) & (\(-\)44.0—40.4) \\ \hline Rest step (90–150 s) & 0.93 \({}^{\star}\) & 0.98 \({}^{\star}\) & 1.6 \({}^{\star}\) & 0.04 \({}^{\dagger}\) & 0.26 \({}^{\dagger}\) & \(-\)16.1 \({}^{\dagger}\) \\  & & (0.97–0.99) & (1.3–1.8) & & (\(-\)0.09–0.39) & (\(-\)17.5—14.7) \\ \hline Whole test (0–150 s) & 0.85 \({}^{\star}\) & 0.96 \({}^{\star}\) & \(-\)1.0 \({}^{\star}\) & 0.23 & 0.42 \({}^{\dagger}\) & \(-\)26.5 \({}^{\dagger}\) \\  & & (0.95–0.96) & (\(-\)1.3—0.8) & & (\(-\)0.13–0.68) & (\(-\)27.3—25.6) \\ \hline \end{tabular} \({}^{\star}\) Values that fall within the highest level of the effect size and/or met the established criterion for agreement.  
\({}^{\dagger}\) Values that fall within the “weak” or “poor” level of the effect size or did not meet the established criterion for agreement.  
\end{table} 
Table 2: Coefficient of determination, intraclass correlation coefficient, and mean bias between devices in each test step and in the whole test. Also on land, there are reports of reduced accuracy at higher running velocities, meaning that with quicker and wider arm movements, the accuracy falls [8; 14; 17; 20; 32]. In fact, this impairment is acknowledged by the brand, as on their website one can find a disclaimer warning that wrist-based optical heart rate measurement is \"not necessarily accurate in sports where you move your hands vigorously\" [33]. Thus, a low accuracy during front crawl swimming was somewhat expected not only because of the arm movements but also because of the impact with the water during the entry phase of the arms.