JMIR Publications

ABSTRACT

Background:

Wearable fitness trackers are valuable devices that can record and enhance physical activities of users. Recently, photoplethysmography (PPG) devices using optical heart rate sensors to display real-time heart rates have become popular, and they help in monitoring and controlling exercise intensity. Although studies have highlighted the practical benefits of using optical heart rate monitors, the accuracy of readouts these commercial devices generate has not been widely studied, especially among different age groups and for East Asian population with Fitzpatrick skin type Ⅲ or Ⅳ.

Objective:

This study aimed to examine the accuracy of two wearable fitness trackers with PPG to monitor real-time heart rates during moderate exercises for young and older adults.

Methods:

A total of 20 young adults and 20 older adults were recruited to this study. All participants were asked to undergo a series of sedentary and moderate physical activities by using indoor aerobic exercise equipment. In this study, the Polar H7 chest strap heart rate monitor was used as the criterion measure in 2 fitness trackers, namely Xiaomi Mi Band 2 and Garmin Vivosmart HR+. The real-time second-by-second heart rate data obtained from both the devices were recorded using the broadcast heart rate mode. To critically analyze the results, multiple statistical parameters, including the mean absolute percentage error (MAPE), Lin’s concordance correlation coefficient (CCC), intra-class correlation coefficient, Pearson’s product correlation coefficient, and Bland–Altman coefficient were determined to examine the performances of the devices.

Results:

Both test devices exhibited acceptable overall accuracy as heart rate sensors based on several statistical tests. Notably, the MAPE values were below 10% (the designated threshold) in both devices (GarminYoung = 3.77%; GarminSenior = 4.73%; XiaomiYoung = 7.69%; XiaomiSenior = 6.04%). The reliability test of CCC scores for Garmin was found to be 0.92 (Young) and 0.80 (Senior), whereas the CCC scores for Xiaomi were 0.76 (Young) and 0.73 (Senior). However, our results obtained using the Bland–Altman analysis indicated that both of the test optical devices underestimated the average heart rate. Most importantly, this study documented some unexpected extreme readings reported by these devices when used on certain participants.

Conclusions:

This study revealed that commonly used optical heart rate sensors, such as the ones reported in the current study, generally produce accurate heart rate readings regardless of the age of the users. However, users should be cautious to rely entirely on these readings to indicate exercise intensities, as these devices have a tendency to produce erroneous extreme readings which might misinterpretation of actual real-time exercise intensity. Future studies should, therefore, emphasize the occurrence rate of such errors, as this will likely benefit the development of improved models of heart rate sensors.