# Intermonitor Variability of Garmin Vivofit(r) Jr. Wristband
## 1 Introduction  
Physical activity (PA) is widely acknowledged as an essential and fundamental component for achieving and maintaining optimal health and well-being, as numerous scientific investigations have conclusively shown. The benefits of regular PA extend beyond the physical realm and encompass mental health, and research has consistently demonstrated this [1]. PA, in general, is considered to be the most effective and beneficial when performed at moderate to vigorous intensity, consistently maintained over a prolonged period of time, without interruptions or pauses [2]. In light of the multiple health, wellness, and personal development benefits resulting from the practice of moderate-to-vigorous-intensity PA over an extended duration, a vast market of wearable, mobile, and technological devices have emerged, specifically designed and intended to monitor daily PA [3, 4]. Arguably, it could be argued that among the instruments and devices used to measure and monitor PA, those that stand out for their precision and accuracy are accelerometers, which enable the direct and real-time recording of PA across a wide population over an extended period [5, 6]. However, generating data and results from accelerometers generally involves highly complex, elaborate processing and treatment of information, aswell as a significant financial investment, which limits and complicates their applicability, particularly for conducting research studies in the field of PA, health, and well-being [7,8]. Therefore, among the various consumer wearable activity devices available, activity wristbands have proven to be the most valued and utilized type of device by school-aged children [9,10]. For this reason, the attention of the scientific community in recent years has focused on validating and verifying wearable, mobile, and technological devices designed and intended to monitor PA, such as PA wristbands, which are characterized by being more comfortable, practical, and accessible, as well as more economical and affordable for conducting research in the area of PA [11,12]. Studies focusing on the validation of PA wristbands to measure PA primarily aim to evaluate and examine their accuracy, validity, and variability. In this regard, there are several interesting points to highlight. On one hand, there are many brands of physical activity wristbands, the most analyzed in research in recent years for validation in school populations have been the Xiaomi Mi Band, Fitbit Charge or Ace, and Garmin Vivofit [9,10,13], where it has been observed that they are valid for estimating step volume [14,15]. Thus, there is evidence that these models are valid for estimating steps; however, there is little evidence of whether they are suitable for measuring the level of physical activity [16]. On the other hand, a very important aspect is the age at which this validity is demonstrated. Among the different models available on the market, only the Garmin Vivofit Jr. The PA wristband has been validated as a reliable and effective device for monitoring steps taken and minutes dedicated to moderate-to-vigorous-intensity physical activity (MVPA) in children aged 4 to 10 years [17] and has been used and applied in different research studies [18, 19, 20]. Regarding this specific model, previous studies in children aged 9 years and older indicate its validity for step measurement and portray it as an encouraging device for measuring physical activity at those ages [16,21]. Despite numerous studies utilizing the Garmin Vivofit(r) Jr. wristband to investigate PA, no research has specifically examined the reliability and variability among various units of the device during PA. The absence of detailed research on the coherence and consistency of data collected by multiple Garmin Vivofit(r) Jr. devices during PA constitutes a significant gap in the scientific literature on this topic.