IJCS | Volume 32, Nº4, July/August 2019

401 Almeida et al. HR monitors and apps in exercise Int J Cardiovasc Sci. 2019;32(4):396-407 Review Article in the first five seconds after a very high-intensity exercise, different from what is physiological and precisely identified by an objective measurement of RR intervals at ECG. 48 Horton et al. 49 showed that the Polar M600 is validated for HR measurements in aerobic exercises, such as cycling, walking, jogging and running, but only during steady-state exercise, i.e., with relatively constant intensity. However, HR responses to circuit resistance exercise were, again, underestimated. Other models such as the Apple Watch 1 and the FitBit Charge HR were also shown to be able to measure HR properly, 50,51 whereas the Garmin Forerunner 225 overestimated HR measurements during low-to-moderate walking and jogging on a treadmill, but obtained HR measurements similar to reference values during high-intensity exercise. 51 The authors suggested that this could be explained by the fact that the Garmin Forerunner 225 had been developed for running, and not for walking exercise. It is also possible that technical difficulties are even higher in other types of aerobic exercises, such as swimming and rowing. The findings described in this section should be interpreted with caution, and at least in part, considered in the context of clinical practice. For some measurements, although some statistical differences were observed between the HR monitor and ECG, they may not influence the reliability of monitors, since absolute differences were not greater than 2 bpm. In addition, the device settings should be not be ignored to avoid measurement errors. 52 Therefore, in general, HRmonitors may be considered sufficiently accurate to be used during physical exercise for most individuals and most daily life conditions. Main applications in exercise Common situations in exercise There are many situations where a real-time recording of the HR favors the control of the exercise intensity proposed. This is somewhat commonly seen at gyms, physical activity centers or even in outdoor settings. For example, Eddolls et al. 53 suggested high-intensity interval training (known as HIIT) to children and adolescents, based on an intensity higher than 90% of maximum HR for improvement of health indicators. Although this exercise modality includes recovery intervals, these intervals are not sufficient to perform HR measurement, be it by palpation or auscultation. In this context, the HR monitor provides an immediate feedback that facilitates the maintenance of exercise intensity within pre-established ranges. In resistance exercises, the use of HR monitors is less frequent. However, Latella et al. 54 suggested that the establishment of recovery time between sets based on HR responses may be one of the most effective strategies to optimize the results. During circuit resistance training, HR tends to be maintained at higher levels due to its shorter and more active intervals. Alcaraz et al., 55 though, did not find differences in performance (repetition maximum and muscle power) between high-intensity resistance circuit training and traditional strength training. HR values registered (Polar S625X monitor) at the end of each exercise bout were similar between the two types of exercises, although decrease of HR was greater in intervals between the sessions of traditional training. Barbosa-Netto et al. 56 observed that, although the magnitude of HR responses depends on the intensity of exercise, HR kinetics was similar throughout a 10 repetitions set, with rapid acceleration in the first 10 seconds and attenuation in the last five seconds, followed by rapid recovery at the end of the last repetition. In these studies, HR monitor allowed the identification of responses that were easily obtained and interpreted. Field conditions and team monitoring The systematization of HR monitoring in trainings and games started in 1990 with soccer players. This strategy has been very effective in establishing training loads and compare them to physiological responses obtained during official matches and competitions. Also, it has helped the monitoring of the intensity of the training sessions (Figure 4). HR measurements were carried out using chest straps and wrist-based monitors during physical trainings, whereas in technical and tactical training and games, the signal was transmitted by telemetry, since according to the rules of the sport modality, players are not allow to wear wrist watches, rings, earrings and bracelets. The obvious reason for that was to allow the players to be as free as possible to move, and to be focused on the main goal of the training. Members of the coaching staff started tomonitor players' HR in order to make adjustments in the intensity of the training and performance of matches. Computers and tablets have become crucial instruments of biofeedback, comparably to chest straps.