INTRODUCTION In swimming, the standard method of monitoring heart rate (HR) in practice is the palpation of the pulse for 10 seconds at the end of a training set. However, this method has an error of at least 6 bpm. A more accurate ECG-based method is to measure the HR with a chest strap. But also this method is error-prone as the strap slips often during swimming or wall push-offs (when no swim suit is worn). Technological developments in recent years have led to the launch of an optical HR sensor from Polar, the Verity Sense (VS). This sensor may overcome the practical limitations of chest straps due to the measurement on the head (temple). In this study, we evaluated the agreement between the VS and Polar H10 chest strap sensors during swimming. METHODS 60 competitive swimmers and triathletes (29f, 31m, median 17.9 yrs) were equipped with VS and H10 sensors during a regular training session. The VS and H10 were worn as recommended by Polar, at the right temple and at the chest covered by a swim suit. Agreement between HR data series (stored with 1 HR value per second and manually synchronised for optimal overlap) was analysed at the individual level and for the complete data set using Bland-Altman analysis. In addition, qualitative analyses were performed by visual inspection of individual BlandAltman plots and HR time series to identify potential sources of measurement error. RESULTS The analysed data sets had a median length of 95 min (Q1,3 81, 102 min). In the overall statistical model, the bias of the VS was -0.6 bpm (95% CI -1.1; -0.1 bpm) compared to the H10. Here, the 95% limits of agreement ranged from -13.5 bpm (95% CI -14.1; -12.9 bpm) to 12.3 bpm (95% CI 11.7; 12.9 bpm). The standard deviation of differences (SDdiff) was 6.6 bpm, with an intra-individual variability of 6.2 bpm and a between-subject variability of 2.2 bpm. Visual analysis indicated that larger errors tend to be present at low to moderate HR values, e.g. typically during rest periods or at the start and end of exercise sets. DISCUSSION Our analyses show an acceptable agreement between the VS and H10 during regular swimming. The difference (bias) of around -1 bpm in combination with an SDdiff of =7 bpm is acceptable from a practical point of view for the VS, considering the problems described in the introduction and the error of the previous measurement standard (pulse palpation). However, there is also a substantial variability in results between swimmers that have to keep in mind. Qualitative analyses further indicated that a notable proportion of the observed errors occurred during specific exercise or rest periods. Therefore, users (coaches, athletes, scientists) must be aware that although the VS can measure with an acceptable accuracy, the measurement error may be unacceptably high in some athletes or during certain training periods, requiring habituation and an evaluation of the physiological plausibility of the measured values.
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