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Wearable Heart Rate Trackers: Which Works Best?
Len Kravitz, PhD

When it comes to heart rate monitoring, wearable fitness devices and wrist-worn heart rate monitors now flourish in a market of fitness enthusiasts who have an insatiable desire to include technology in all aspects of their daily life. Approximately 15% of consumers, or one person in six, currently uses some type of wearable technology such as a fitness band or smartwatch (Piwek et al., 2016). Piwek and colleagues cite industry research that indicates 19 million activity monitoring devices will be sold in 2018, attaining a predicted sales of 110 million dollars. Unlike the earlier generation of precise heart rate monitors, which relied on chest straps and attached electrodes, the newer health and fitness trackers use optical sensing technology capable of measuring changes in an organ or the entire body. The potential use for these wearable trackers is growing for monitoring health analytics, such as heart rate, body temperature, oxygen saturation, physical activity, electrical activity of skin, and sweat (Piwek). Yet, a simple question, recently addressed in a study by Gillinov 2017 and colleagues, is how accurate are the heart rate monitors on this new line of consumer health wearables. The accuracy of these devices is paramount for exercise enthusiasts and athletes tracking personal progress, as well as for physicians tracking heart rate for specific patient health interventions..

Study Reviewed: Gillinov, S., Etiwy, M., Wang, R. et al. (2017). Variable accuracy of wearable heart rate monitors during aerobic exercise. Medicine & Science in Sports & Exercise, 49(8), 1697-1703.

Purpose: The main objective of this study was to evaluate the precision of five popularly used optical-based heart rate monitoring devices. Four of these monitors are worn on the wrist; Apple Watch, Fitbit Blaze, Garman Forerunner 235, TomTom Spark Cardio and the Scosche Rhythm+ is worn on the forearm..

Participants: Fifty aerobically active men and women (average age=38yrs old; BMI=25 kg/m2) volunteered to participate in this study. The participants were screened to ensure they could safely complete an 18-minute exercise protocol on a stationary bicycle, treadmill and elliptical trainer..

Methods: All participants wore standard ECG leads (Leads I, II and III), a Polar H7 chest strap monitor, and a Scosche Rhythm+ on the forearm. Then, each participant was randomly assigned two different wrist-worn heart rate monitors (one on each wrist). Thus, each type of wrist-worn monitor was assessed by 25 of the 50 participants. The accuracy of all devices was compared to the ECG, which stands for electrocardiograph. The ECG is a visual display of the heart's electrical activity and is considered the standard (e.g., most correct) heart rate measurement instrument. As the heart beats, the electrical signals travel through the body fluids to the skin and onto the ECG electrodes, which can measure heart rate and signal changes (e.g., irregular heartbeats) of the heart.

Exercise Protocol:
Each participant performed a 4.5-min light, moderate and vigorous exercise sequence randomly assigned to four exercise modalities: treadmill, stationary bicycle, elliptical trainer with arms levers and elliptical trainer without arm levers. There was a 2-min rest period after completion of each modality-testing period. Thus, each participant completed 18-min of exercise in a 24-min testing trial. The researchers explained that multiple exercise devices, with three different intensities on each device, were utilized because their previous research had determined these new optical devices performed irregularly on various exercise equipment at mixed intensities..

Statistical Results: The researchers used a specialized correlational approach to assess the accuracy of all of the devices to the ECG. If a device's correlation to the ECG was >.80 it was considered acceptably accurate. Results for each device on each modality are shown in Table 1. The Polar chest strap had the highest agreement with the ECG, averaging a .99 on each modality. When comparing the averages of all of the optical device correlation scores, the Apple Watch (ave =.87) performed best followed by TomTom, Garman, Scosche Rhythm+ and Fitbit (averages = .74. .67, .61, .56 respectively).

Regardless of the exercise modality, the first-rate accuracy of the Polar chest strap monitor reflects decades of research and development in refining this heart rate monitoring device. The chest strap is used because of its proximity to the heart. Although more exact in measuring heart rate, the chest strap monitors are somewhat inconvenient for many consumer enthusiasts, yet they are regularly used by athletes and in research laboratory conditions. The newer heart rate monitors tested in this study use an optical technology known as PPG for photoplethysmography (Gillinov et al, 2017).

These devices optically measure the volumetric changes of an organ, such as the heart. In essence, they measure changes in the size of blood vessels under the skin. They are then programmed to interpret and calculate a person's heart rate. With this technology, a PPG signal can be literally taken from anywhere on the body such as the earlobe, wrist or even a finger. However, as Gillinov and colleagues discuss, currently there are several potential sources of error with these optical based monitors such as physical movement, misalignment between the skin and the sensor, variation in skin color/tone, and poor tissue perfusion. As can be noted from this study, the optically based wearables are less accurate as compared to the Polar chest strap, and they vary quite a bit with the type of aerobic activity being performed. These findings indicate that fitness pros need to educate clients that the use of these devices at this time may not be best for the precise assessment, management and implementation of exercise programs for health and fitness.

It should be noted that this study, dated 2017, reflects the most recent monitors available by the respected manufactures at the time of testing. Clearly, since optical technology is in its early development, it may be assumed that the manufacturers of these products are currently making advances with well-conducted research.

Best Practices Bottom Line
Optically based wrist-worn heart rate monitors vary in accuracy and differ on the type of exercise modality. In this study, the Apple Watch provided the best agreement with the ECG. However, for fitness pros testing clients and seeking the best possible heart rate measurement, the results of this study indicate the Polar chest strap is recommended. Additionally, until new research suggests otherwise, estimating maximum heart rate may be best be attained with either of these formulas: 206.9 - (0.67 x age) or 208 - (0.7 x age).

Table 2. Question and Answers on Heart Rate and Wearable Exercise Technology
1. Why do people buy these wearable fitness trackers?
The majority of users seek wearable technology to eventually become the all-inclusive platform for heart rate measurement during exercise, positive habit formation, improvement in sleep, management of stress, increased productivity and improved physical fitness (Piwek et al. 2016).

2. What formula is best for estimating maximum heart rate?
Although the 220-age estimated maximum heart rate calculation is easy to compute, it was never intended for the general population, and has quite a bit of error (approximately ± 12 beats per minute). Foster and Porcari (2014), in the American Council on Exercise Personal Trainer Manual (5th Edition), recommend using two other formulas for estimating maximum heart, which have an error of about ±7 beats per minute.
Gelish formula: 206.9 - (0.67 x age)
Tanaka formula: 208 - (0.7 x age)

3. How good are the wearable devices in affecting behavior change?
At this time, there are few randomized controlled studies focusing on the impact of wearable technology on health behavior change (Piwek et all, 2016). These monitors do include goal setting, self-monitoring, and feedback content that closely matches recommendations for positive behavior change. Therefore, the future of this technology has broad applications for use in fitness, clinical, public health and rehabilitation settings.

@bio:Len Kravitz, PhD, CSCS, is the program coordinator of exercise science at the University of New Mexico, where he received the Outstanding Teacher of the Year and Presidential Award of Distinction. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Len was awarded the 2016 CanFitPro Specialty Presenter Award.

Additional References:
Foster C. and Porcari, J.P. (2014). Cardiorespiratory training: Programming and progressions. ACE Personal Trainer Manual (5th Edition). Edited by Bryant, C.X. and Merrill, S., American Council on Exercise
Piwek, L., Ellis, D.A., Andrews, S., Joinson, A. (2016). The rise of consumer health wearables: Promises and barriers. PLoS Med 13(2): e1001953.