IJCS | Volume 32, Nº4, July/August 2019

399 Figure 2 - Theoretical model of heart rate response after exercise, modulated by cardiac vagal reactivation during 30 seconds of digital palpation. We can see the widening of the time between the systoles (RR intervals at electrocardiography) at the end, and at 15 seconds and 30 seconds after exercise. Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article accuracy and reliability. Also, not rarely, the sensor was removed from the ear lobe during exercise because of a wider movement of the arms for example, affecting HR monitoring. In the early 1970’s, an exercise physiologist and future swimming coach, Dr. Robert Treffene, developed a handheld monitor with wires and electrodes that could be used by the coach outside the pool to check the HR of swimmers as soon as they stopped at the pool wall. 35 At the end of the same decade, Professor Seppo Säynäjäkangas invented a HR monitor for Finland’s national cross-country ski team to use while training. In 1977, he founded the Polar Electro Oy ® , a pioneer company in this field, based in Kempele, Finland. Only six years later, the first HRmonitor became commercially available, the Polar Sport Tester PE 2000, 35 that could be used by athletes and practitioners who wanted to obtain a more accurate HRmeasurement to adjust their training load. 36 Due to modern technology, there are several brands and models of HRmonitors available nowadays, with more sophisticated features, including higher memory capacity, stopwatches, time and pace alarms, estimator of energy expenditure, GPS, among others. Mobile apps In the study by Lee et al., 37 the authors reported that HR monitors could be expensive and, therefore, not accessible to patients. In addition, HR monitors worn with elastic bands could cause discomfort and limit a wider use of these devices. Therefore, considering the increasing use of smartphones in Brazil and in the world, mobile apps for HR measurement seem to be very interesting. The technology required to detect cardiac chronotropy encompasses from photoplethysmography (PPG) via smartphone cameras 38 to accessories, including smartphone cases containing ECG sensors. 39 Therefore, mobile apps (and often associated watches and monitors) are capable not only to measure the HR but also to identify arrhythmias, such as atrial fibrillation. 40 However, Coppetti et al. 41 pointed out possible differences between HR measured by contact and non- contact PPG. The authors showed that non-contact PPG- based apps can show relatively high margin of errors (7 to 8 bpm), compared with errors varying from 2 to 4 of contact PPG-based apps. Heart rate monitors What is a HR monitor? HR monitor is a device that allows the measurement and display of real-time HR data and, depending on the model, storage of data for posterior analysis. HR monitors have been widely used not only to evaluate aerobic performance but also to monitor the intensity of predominantly aerobic exercises. The monitors contain an elastic band with contact sensors (electrodes) that detect cardiac electrical activity (systoles) and send it to a receptor via radio waves, 42 or optical sensors (PPG). Today, there aremanymodels and brands of HRmonitors available in themarket, providing different features, from those with time of exercise and HR measurement on