IJCS | Volume 32, Nº4, July/August 2019

402 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article Figure 4 - Monitoring of heart rate in professional soccer players in a maximal incremental treadmill exercise test (November 2018 – end of season) and in specific soccer-related activities (training and official matches) in January, February and March 2019 (Bottino, unpublished data); HR: heart rate; CPET: cardiopulmonary exercise test. However, it was only in 2003, during the 8 th annual congress of the European College of Sports Science held in Salzburg, Austria, that HR monitoring became one of the most important variables on internal load control, 57 and the concepts of acute and chronic loads regarding stimuli and consequent adaptations emerged (Figure 5). It is important to mention that, among all objective variables of exercise intensity, HR is, so far, the only variable that enables a real-time measurement and analysis. The other variables (hormones, metabolites, thermography etc.) require an interruption or ending of activity, training session or game for data collection and analysis. 58 Consequently, a new generation of GPS (and one of the leading brand in the area), is at final testing stage to incorporate HR measurement as one of its components. 59 This will allow an even closer integration of information with HR as a variable of training/game total load. In field conditions, the use of apps that analyze HR alone may face some operational limitations. The signal coverage range is still small (approximately 60 m), which is very limited considering the official soccer field size. In addition, storage capacity is limited in the absence of available Bluetooth connection. This is a common situation when the team goes to the dressing rooms and become practically isolated in a Faraday shield due to the metallic structures of the beams and cladding of the facilities. Such interruption in signal transmission results in a nearly 15 minutes of recording loss, which could affect the information about the athletes’ level of recovery after exercise. This special limitation regarding Bluetooth communication does not occur when the signal is transmitted from the chest strap to the GPS, since the distance between these components is less than one meter. From the GPS unit, the signal is transmitted to external antennas in the field and then to satellites. Besides, GPS devices have enough storage capacity and, in case of signal loss during the match, the HR data can be recovered for further analysis. Potential clinical applications Considering that some of the HR monitors have a large sample rate and large capacity of data storage, it