IJCS | Volume 32, Nº4, July/August 2019

398 Figure 1 - Hypothetical example of heart rate variation in response to prolonged, constant-load aerobic exercise and dehydration. Even at constant exercise intensity, heart rate varies throughout the first two minutes of exercise, and the steady state is affected by dehydration, with an increase in heart rate. Adapted from Chagas et al. 22 and Araújo. 19 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article Epidemiological data have shown that a slight decrease in HR after exercise represents an increased risk of mortality. 30,31 However, althoughHR recovery tends to be faster inmen andwomenwith higher maximal VO 2 , there is a weak association between these two physiological variables, where regression model explains no more that 11% of HR variation. 32 The association between initial and final transient periods of HR recovery is not strong either. 33 On the other hand, analysis of post-exercise HR recovery may contribute to the identification of athletes of different sports with favorable autonomous nervous system adaptive changes. 34 History of HR measurement in exercise Palpating arteries or auscultating the heart The simplest and most original way to measure HR is to palpate the arteries, and the most commonly used for this purpose are the radial, the common carotid and the superficial temporal arteries. The time of palpation may vary from 6 to 60 seconds, but it is important to highlight that the shorter the time, the greater the error, since the number of heartbeats is multiplied by a factor to obtain the number of heartbeats in one minute. For example, if the time of palpation was 6 seconds, the number of heartbeats is multiplied by 10 and the multiplier represents the margin of error. 35 On the other hand, a longer measurement time, or closer to 60 seconds, has lower margin of error but ismore susceptible to autonomic modulation and other disturbances or artifacts, especially during or after exercise (Figure 2). 4 Monitors Obviously, it would be necessary something more efficient operationally, without stopping the run or other exercise to obtain a real-time measurement of HR, without mathematical calculations. The first devices had old technology optical sensors; the first versions were composed of wires connected to the panel of treadmills or similar devices, with sensors fixed under pressure on the digital pulp surface or the ear lobe, which were extremely sensitive to body motion during exercise. Thus, HR measures obtained by this method had low