## 3 Results  
Figure 3 represents the mean HR and standard deviation across 30 participants during the cycling protocol, as measured by the three devices, Garmin Strap, Coospo Strap, and Coospo Armband, over a normalised timeline. The mean HR data are shown as a line for each device, while the shaded intervals represent the standard deviation across participants. Both chest straps (Garmin and Coospo) present comparable HR profiles, whereas the Coospo Armband exhibits occasional divergence, particularly during higher-intensity periods. Effort and recovery stages were segmented and analysed separately from the continuum signal for inter-device comparison. Figure 4 represents the comparison between the two devices from the seven effort stages (upper plot) and the seven recovery stages (lower plot). ANOVA and Tukey's post-hoc comparison confirmed an overestimation in HR measurement during effort stages in the Coospo Armband device compared to Garmin and Coospo Strap (+5 bpm, \(p\) < 0.05). No differences were found between the three devices during recovery stages, nor between Garmin Strap and Coospo Strap during effort. Figure 3: Mean heart rate (line) over the normalised time of the 30 participants measured with Coospo Strap (blue), Garmin Strap (green), and Coospo Armband (red). Heart rate standard deviation (shaded interval) was used for all participants during the cycling session, the colour referencing of each device being maintained. The overlapping of colours represents the coincidence between signals with their respective standard deviation, while the non-overlapping visualises a lack of agreement between signal data. Table 1 provides a detailed summary of the HR responses for each device across the seven effort and recovery stages, showing the mean HR values, coefficient of variation (CV), and intraclass correlation coefficients (ICC) with their 95% CI for each paired comparison. This table provides quantitative metrics highlighting each device's accuracy and consistency under different exercise conditions. The ICC values indicate a strong agreement between the Garmin and Coospo chest straps (ICC = 0.6-1.0). However, lower ICC values between the Coospo Armband and both chest straps (ICC = 0.10-0.77) reflect the higher measurement of discrepancies, particularly during effort stages. Moreover, the level of agreement (LoA) between devices is represented in Figure 5A-C by three Bland-Altman plots. The first one (A) compares Garmin Strap vs. Coospo Strap, where a bias = 0 is observed, with a LoA of \(\pm\)2.5 ppm, indicating a high LoA between devices. In the Coospo Armband vs. Garmin Strap comparison, central plot (B), a bias near 0 was also obtained, but with a much higher upper and lower confidence interval (\(\pm\)9.5 ppm). In addition, a tendency for values outside the upper confidence interval during effort stages is observed. Finally, the right plot (C), with the Coospo Armband vs. Coospo Strap comparison, confirms a plot distribution similar to that of (B) with the same interpretation. In this sense, [PERSON]'s sensitivity to outliers needs to be mentioned, as they can skew the interpretation of agreement by expanding the LoA and thus reducing the perceived interchangeability of devices. This effect is particularly relevant when comparing the Coospo Armband to chest straps, as the differences in sensor placement and technology may lead to occasional outlier readings, especially during rapid heart rate fluctuations. Figure 4: Heart rate measurement comparison considers devices during the seven effort and recovery stages. Boxplots represent the mean heart rate (bpm) of the 30 participants recorded from Garmin Strap (green), Coospo Strap (blue), and Coospo Armband (red) across seven effort (**top panel**) and recovery (**bottom panel**) stages. The centre line of each box represents the median heart rate, while the box edges indicate the interquartile range (IQR). Annotations above the relevant intervals indicate differences (\(p<0.05\)) between device measurements.