## 4 Discussion  
The main aim of this research was to compare the HR measurement accuracy of three devices: two chest straps (Garmin HRM-Dual and Coospo H6) and one armband (Coospo HW807) during an intermittent protocol in a controlled laboratory situation (indoor cycling). The authors' hypotheses were confirmed as significant differences were found between the Coospo Armband device and the Garmin and Coospo chest straps, specifically in the effort stage. Considering that the literature has validated chest straps by comparing them with electrocardiograms [12; 13], this research sought to compare two different brands of chest straps (Garmin and Coospo) to find out if there were differences between them and at the same time to verify the LoA between chest straps and an armband (Coospo HW807), a device that is replacing chest straps in performance sports due to its greater comfort in use. No differences were found in either the recovery or effort stages in the Garmin chest strap vs. Coospo chest strap comparison. A high LoA between the two devices was confirmed with a mean error equal to 0 and a difference confidence interval of \(\pm\)2.5 bpm. These findings align with the literature, confirming that chest straps are valid and reliable for measuring HR in different contexts regardless of intensity or effort [12; 13]. Based on the summarised metrics in Table 1, specific device-specific recommendations can be made. The chest straps (Garmin and Coospo), with higher ICC, present greater accuracy and stability during high-intensity stages, likely due to their placement near the heart and reduced susceptibility to device movement. These characteristics make them well-suited for sports or activities with sudden HR fluctuations. In contrast, the Coospo Armband, although slightly less accurate in high-intensity phases, may be preferable in situations where user comfort and less restrictive wearables are prioritised, such as during prolonged moderate-intensity activities or in non-competitive settings. Moreover, as the data have confirmed the accuracy and LoA between the two chest straps, the interpretation of the comparison between the armband and the two chest straps can be unified. In this regard, significant differences were found in the effort stages (+5.34 bpm, \(p<0.05\)), and although the mean bias was close to 0, the error confidence interval was \(\pm\)9.5 bpm, with a tendency towards out-of-interval data in the effort stages. These results are in line with previous studies that found a tendency towards difficulty in recognising changes in intensities in arm bands [24] and provide a possible answer to studies that have found inter-individual differences without particular patterns in daily activity tracking [25]. Focusing on the different devices' technology, research comparing HR measurements from chest straps using electrocardiography and devices using PPG have shown varying levels of accuracy. Both technologies provide similar values during rest and low-intensity activities, but differences become apparent during intense exercise [17; 29]. Although the literature has confirmed a tendency in PPG devices to underestimate HR by 5-10 bpm at high intensities [17; 29], our results have shown a 5 bpm overestimation of the armband in rapid HR increases. These differences may be explained by the difficulty that PG devices encounter in detecting rapid blood flow changes, thus affecting PPG signal quality. Furthermore, ergonomic factors associated with the placement of HR monitoring devices can impact measurement accuracy [30]. Chest straps near the heart typically maintain stable contact with the skin, reducing device movement and improving signal stability during dynamic or high-intensity activities. This placement likely contributes to more consistent readings across varying exercise intensities. In contrast, armband monitors, such as the Coospo Armband, are more susceptible to displacement or slight shifts due to arm movement, resulting in temporary signal loss or delayed response to rapid HR changes [24; 25].