## 1 Introduction  
Heart rate (HR) is a widely used and objective internal load variable in the scientific literature, particularly in the fields of physical activity [1], sports performance [2; 3; 4], and health [5]. Monitoring HR at rest, before, during, and after exercise is important for assessing cardiac status, evaluating fatigue, measuring intensity, and managing recovery [6; 7]. In team sports, it allows for individualised training load control, providing coaches with valuable information for optimising training programs during the season [7]. Moreover, high correlation values have been found on comparing HR to metabolic threshold detection, oxygen consumption, and lactate levels, highlighting its use in endurance sports [8; 9; 10]. Considering the importance of HR monitoring in different contexts, many HR devices have been developed in recent years [4]. Chest straps, vests, armbands, and smartwatches are the most commonly used devices in sports performance [4; 11; 12]. Each device has its advantages and disadvantages. Concerning chest straps, studies have demonstrated their validity in comparison to an electrocardiogram [12; 13], making them the most widely used devices, even though athletes have reported comfort issues so highlighting the need to look for other alternatives. Vests that sought to unify compatibility with multi-sensor devicesplaced in the interscapular area [12; 14] were seen as an alternative, but the reference literature has shown lower accuracy and a higher mean absolute percentage error in comparison to chest straps [12]. As for smartwatches, there is a wide variety of devices, and extensive sources in the literature have focused on studying their accuracy and establishing expert consensus [11; 13; 15; 16]. However, smartwatches are banned in most team sports; so, armbands have emerged as an excellent alternative to chest straps. However, to the authors' knowledge, little research has analysed HR armband accuracy in sports contexts. Recent studies have compared the accuracy of various HR monitoring devices. Chest straps monitors show greater accuracy than wrist-worn optical devices across various activities and populations [12; 17; 18]. Wrist-worn optical HR monitors show good accuracy in healthy individuals during various activities, achieving data coverage between 92% and 98% and mean absolute errors not exceeding 3 bpm [19]. However, the performance of the devices varies depending on the type of exercise; some perform better during cycling, while others were more effective during treadmill use [18]. In cardiac rehabilitation patients, chest straps monitors showed the highest agreement with ECG (r = 0.99), whereas wrist-worn devices demonstrated lower concordance (r = 0.52-0.80) [18]. As far as the different devices' technology is concerned, there are two main types, electrocardiography (chest straps and vests) and photoplethysmography (PPG) (smartwatches, armbands, etc. ), raising the importance of inter-device comparisons [13; 17]. Electrocardiography is based on detecting the heart's electrical impulses, which travel through the muscles and are recorded on the skin by electrodes located on the chest strap [17]. In PPG, green LED light beams are used (paired with light-sensitive photodiodes) at a given brightness and frequency to detect the volume of blood flow in the arteries, registering each heartbeat with the blood flow volume increase [17; 20]. However different aspects can affect the accuracy of the sensors. For example, effort intensity, device movement, and position can affect the accuracy of chest straps. When it comes to PPG devices, factors such as gender, body mass index, and skin tone can also affect precision [21; 22; 23], hence the need to compare different HR devices and technologies to verify measurement accuracy. Focusing on armband studies, Polar OH1 presented high reliability in endurance activities but lower values in sports contexts involving arm movements [24].