Secondary Logo

Journal Logo

Features

REVISITING HEART RATE TARGET ZONES THROUGH THE LENS OF WEARABLE TECHNOLOGY

Scheid, Jennifer L. Ph.D.; O’Donnell, Emma Ph.D.

Author Information
ACSM's Health & Fitness Journal: 5/6 2019 - Volume 23 - Issue 3 - p 21-26
doi: 10.1249/FIT.0000000000000477
  • Free

Abstract

INTRODUCTION

Figure
Figure
Figure
Figure

Exercise prescription is fundamental in creating exercise programs that are specific to fitness goals and help clients to achieve optimal fitness outcomes. The FITT-VP principle outlines guidelines for considering frequency, intensity, time, type, volume, and progression when designing exercise programs (1). If exercise targets are given for a specific intensity, then a client will need a way to accurately monitor intensity during exercise. During cardiovascular workouts, monitoring intensity can range from tools as simple as monitoring ratings of perceived exertion (RPE), or using a talk test, to measuring heart rate directly using palpation, heart rate monitors, or electrocardiograms (ECG) (1). The underlying principle of using heart rate to monitor exercise intensity lies in the well-documented direct linear relationship between exercise intensity and heart rate, such that with increasing exercise intensity, there is a linear increase in heart rate (2).

Heart rate monitors are readily available to help clients monitor changes in exercise intensity. Heart rate monitors are easy to use and for the most part are inexpensive. Some heart rate monitors include chest straps that measure heart rate in a similar manner that an ECG measures heart rate. These chest straps transmit information to watches, exercise equipment, or smart phones, known as receivers. More recently, a number of activity monitors and smart watches that can be worn on the wrist without the need for a chest strap have become available. These devices have optical sensors that detect blood flow at the wrist, and from this information, heart rate is determined. In the current article, we address the specifics of how to use wearable technology to accurately assess heart rate, including the use of heart rate during exercise prescription intensity “zones.”

Figure
Figure

Some heart rate monitors include chest straps that measure heart rate in a similar manner that an electrocardiogram (ECG) measures heart rate. These chest straps transmit information to watches, exercise equipment, or smart phones, known as receivers. More recently, a number of activity monitors and smart watches that can be worn on the wrist without the need for a chest strap have become available. These devices have optical sensors that detect blood flow at the wrist, and from this information, heart rate is determined.

THE BASICS OF MONITORING EXERCISE INTENSITY USING TECHNOLOGY

Current technology offers clients many ways to measure their physical activity throughout the week: steps, stairs, calories, and minutes of exercise, in addition to minutes of planned exercise. Although these metrics give one an idea of the volume of physical activity one accomplished during the day, heart rate monitoring gives clients a way to retrospectively review the intensity of a specific exercise workout; that is, after a workout, a client can look at their fitness technology and see how hard they worked. If a client is using heart rate in this capacity (retrospectively), then the client should also learn how to evaluate exercise intensity subjectively during a workout, for example, by using the RPE scale to assess “how hard” they perceived they were working. This permits the client to be able to retrospectively compare the objective measures (e.g., heart rate) provided by the biometric feedback wearable device with the subjective measures (e.g., RPE) of exercise intensity.

Heart rate monitors also can allow clients to know their heart rate intensities during a workout and achieve specific target heart rates. This method is slightly different because a client may have heart rate intensity goals, that is, heart rate intensities determined prospectively as part of an exercise prescription, and the heart rate monitor can help the client achieve these goals. This difference between prospective exercise goals and retrospective review of a workout is important to understand, because different technologies can be better suited for retrospective review of a workout, prospective goals during a workout, or both. As the technology to measure heart rate is improving, retrospective and prospective information from heart rate monitor is becoming more accessible during a workout and after a workout.

Heart rate chest straps have been around since the early 1980s (3). They take some practice to get on efficiently (especially for women), because they have to be fairly snug and close to the heart (see Figures 1 and 2 for examples of heart rate chest strap placements). However, once the chest straps are secure, they can accurately monitor heart rate and are comparable with heart rate recordings made using a three-lead ECG (3). As such, chest straps tend to be the heart rate monitor of choice when accuracy is important. For example, electrode-containing chest straps are recommended if a client is using heart rate to accurately monitor exercise intensity during a workout or to follow prospective goals set by an exercise professional. Optical wrist-based heart rate monitoring is also deemed to be relatively accurate during aerobic activities (i.e., when using treadmills, indoor bikes, or ellipticals) (4,5). However, due to differences in the technology between chest straps (telemetric) versus wrist-based monitors (optical), the accuracy of the optical wrist-based heart rate monitor can vary widely (6), resulting in underestimation or overestimation of heart rate by as much as 30 beats per minute (7). Higher rates of error can occur due to darker skin tones, higher body mass index, and the type of activity (4). Activities that include excessive or erratic wrist or arm movement can cause errors in heart rate monitoring of wrist-based heart rate monitors (e.g., boxing, aerobics, and circuit or weight training). In addition, if the wrist monitor is not snug (or is too tight), then measurement errors also can occur. Despite these limitations, wrist-based monitors provide an “in the ball park” measure of heart rate. As such, wrist-based monitors still have utility to retrospectively estimate overall exercise intensity patterns (i.e., in a week) to see if a client is meeting their exercise goals.

Figure 1
Figure 1:
This male athlete is wearing the heart rate chest strap correctly, just below his chest muscles.
Figure 2
Figure 2:
This female athlete is wearing the heart rate chest strap correctly, just below her sports bra. Some females even prefer to tuck the chest strap just under their sports bra, which also is appropriate.

Heart rate chest straps have been around since the early 1980s. They take some practice to get on efficiently (especially for women), because they have to be fairly snug and close to the heart. However, once the chest straps are secure, they are able to accurately monitor heart rate and are comparable with heart rate recordings made using a three-lead ECG.

Optical wrist-based heart rate monitoring is also deemed to be relatively accurate during aerobic activities. However, due to differences in the technology between chest straps (telemetric) versus wrist-based monitors (optical), the accuracy of the optical wrist-based heart rate monitor can vary widely, resulting in underestimation or overestimation of heart rate by as much as 30 beats per minute.

OPTIMIZING TECHNOLOGY BY UNDERSTANDING MAXIMUM HEART RATE

Maximal heart rate is commonly used to develop exercise prescriptions, set target heart rate zones for workouts, and monitor workouts using those target heart rate zones. Although maximum heart rate may be measured during a maximal aerobic capacity exercise test (also known as a V·O2max test), this is not always feasible. In addition, submaximal exercise tests or field tests could be used by a fitness professional to estimate maximum heart rate (1). However, because of the simplicity and convenience, many health and fitness professionals, including personal trainers, group fitness instructors, and exercise physiologists, use an estimate of age-predicted maximal heart rate (APMHR). The most common equation to estimate heart rate is APMHR = 220 − age (1). However, a handful of other equations have been developed in larger populations and with larger age ranges (1). It is important to remember that this calculated maximum heart rate is an estimate of actual maximum heart rate and that actual maximum heart rate could be higher or lower by 12 to 15 beats per minute (8).

Most wearable technology uses this method of estimating heart rate (APMHR = 220 − age) as a default setting (9), whereby the device requires the user to input an age or birthday, and then uses age to calculate APMHR. It doesn't matter if maximum heart rate is measure or predicted; target heart rate is calculated the same way: target heart rate = maximum heart ratemeasured or predicted × % of desired intensity.

Once maximal heart rate is measured, estimated, or calculated, heart rate monitoring devices can then be used to effectively and efficiently monitor intensity. Although APMHR is typically the default setting in most wearable fitness technologies, many devices allow the user to input a measured maximum heart rate. For example, most Polar™ devices allow the user to enter a measured maximum heart rate for more accurate measurement of exercise sessions (10). In addition, the Fitbit app that manages most Fitbit devices allows users to go into their account settings (under heart rate zones) and enter a custom max heart rate, for example, a measured maximum heart rate (9). Similarly, the Garmin Connect™ app that manages most Garmin devices allows users to go into settings, user profile, and heart rate zones to enter a maximum heart rate (11). If possible, once an exercise professional measures maximum heart rate, they should work with their client to make sure that this measurement is entered into the wearable technology.

Although this method of calculating target heart rate from APMHR is common practice, some limitations are noted. Beyond the potential for inaccurate calculations of exercise training intensity, estimations of maximum heart rate also do not account for medications, such as beta-blockers, which alter heart rate at rest and during exercise (1). Exercise heart rate can also be impacted in clients who suffer from diabetes mellitus or cardiovascular disease (1). Thus, after consideration of the client’s general health, age, fitness level, and habitual physical activity, target heart rate should be calculated by a health or exercise professional (1). In addition, heart rate training zones may be calculated in accordance with input from a medical provider, such as a primary care physician or cardiologist. Target heart rate is normally calculated as a range, that is, a training zone. Many devices have options to set a custom workout zone (9,11), and these zones can be created with the help of an exercise professional.

OPTIMIZING TECHNOLOGY BY UNDERSTANDING EXERCISE INTENSITY

Technology that measures heart rate can give the user a variety of information: (a) actual heart rate in beats per minute, (b) percentage of maximal heart rate (typically a percentage of estimated maximal heart rate unless maximal heart rate had been measured and entered into the equipment), or (c) the specific “zone” or intensity achieved. Unfortunately, the specific “zone” or intensity achieved based on percentage of maximal heart rate can vary by device and may be different than exercise intensities defined by the American College of Sports Medicine (see Table 1). For example, the American College of Sports Medicine defines moderate exercise as 64% to 76% of maximal heart rate (1). This is similar to the lower end of the cardio zone defined by Fitbit (70% to 84% of maximal heart rate), moderate zone defined by Polar™ (70% to 80% of maximal heart rate), and zone 3 defined by Garmin (70% to 80% of maximal heart rate). Although these differences are minor, it is important to understand that these target heart rate zones are not consistent between devices and are not necessarily aligned with the American College of Sport Medicine’s cardiovascular intensity classifications. An exercise professional such as an ACSM Certified Exercise Physiologist® (ACSM-EP) or ACSM Certified Clinical Exercise Physiologist® (ACSM-CEP) can be beneficial to help a client properly interpret their heart rate zone based on ACSM recommendations. As noted previously, many devices have options to set a custom workout zone (9,11), and these zones can be optimized with the help of an exercise professional.

TABLE 1
TABLE 1:
Percentage of Maximal Heart Rate in Each Target Workout Zone for Companies That Make Popular Fitness Products

Athletes and fitness enthusiasts may choose to monitor exercise intensity in multiple ways during a workout. For example, watts and/or speed can be used in conjunction with heart rate for a more detailed record of training. The higher end devices have the ability to measure (or integrate information from other technology) cycling speed/power or running speed. Although watts and/or speed are direct indicators of performance, heart rate can be impacted by factors such as heat or dehydration. For example, target heart rate will be higher at the same absolute workload in the heat compared with a cooler environment. Dehydration will also cause heart rate to be higher for a given workload compared with exercising in a hydrated state (15). Overtraining can also cause heart rate to be elevated both at rest and during exercise compared with training in a well-recovered state (16). Thus, using target heart rate in combination with other performance metrics can help clients maximize performance, while also helping to decrease the risk of heat illness, dehydration, or overtraining (1).

OPTIMIZING TECHNOLOGY BY USING HEART RATE RESERVE

Heart rate reserve, also known as the Karvonen method, is the most accurate way for a health and fitness professional to establish a target heart rate for a client (1). When using percentage of maximum heart rate to calculate exercise prescription, there is a larger chance of underestimating or overestimating the ideal heart rate to reach fitness goals. In comparison with using maximum heart rate alone to calculate target intensity during a workout, heart rate reserve uses maximum heart rate (measured or estimated) and resting heart rate to calculate target intensity. To calculate target heart rate using heart rate reserve, the equation is as follows: target heart rate = (maximum heart ratemeasured or predicted − resting heart rate) × % of desired intensity + resting heart rate.

Although the majority of new technologies do not use heart rate reserve to calculate exercise intensity, measurement of heart rate during exercise when used in combination with an exercise prescription will still help a client meet their prescribed exercise intensity. The American College of Sports Medicine (1) defines % of heart rate reserve as follows:

  • very light, <30
  • light, 30–39
  • moderate, 40–59
  • vigorous, 60–89
  • near maximal to maximal, ≥90

However, some wearable technologies do have the ability to set zones as a percentage of heart rate reserves (11), and many technologies have the ability to enter custom workout zones based on absolute heart rate values (9,11). These workout zones based on percentage of heart rate reserve can also be created and entered into wearable technology as absolute heart rate values with the help of an exercise professional.

BEST PRACTICES FOR USING WEARABLE TECHNOLOGY THAT MEASURES HEART RATE

  1. Make sure the device has as much information as allowed.
    •  Age. Most devices will have age since this is a major component of estimating heart rate max (or estimating heart rate reserve).
    •  Your birthday (and the current date). A device can use birthday and current date to calculate age, if age is not directly entered.
    •  Resting heart rate. Some devices may ask for resting heart rate. This is important because it is a component of calculating heart rate reserve. Make sure you follow the directions to measure resting heart rate accurately.
    •  Actual maximum heart rate during exercise. A lot of wearable technology depends on estimating maximum heart rate using prediction equations, but some devices may have the wearer complete a fitness test to measure maximum heart rate. If possible (and if the exerciser is cleared by a fitness professional and/or physician to participate in maximal exercise), complete this test and make sure the device has this information.
    •  Custom heart rate zones. Custom heart rate zones can be programmed in some devices. This allows exercise professionals to prescribe heart rate zones as a way to monitor exercise intensity during a workout.
  2. Make sure the device fits properly. Both chest straps and wrist-based monitors need to be snug. Make sure to follow all the directions of your specific device. The better the fit, the better the accuracy of the device, especially with wrist-based heart rate.
  3. Understand the limitations of your device. Does your device use maximum heart rate or heart rate reserve to determine work zones? How does the device determine maximum heart rate?
  4. Know what the work zones mean for your device. For example, the American College of Sports Medicine defines moderate exercise as 64% to 76% of maximal heart rate (1). This is similar to the lower end of the cardio zone defined by Fitbit (70% to 84% of maximal heart rate), moderate zone defined by Polar™ (70% to 80% of maximal heart rate), and zone 3 defined by Garmin (70% to 80% of maximal heart rate).

CONCLUSIONS

Although chest straps are more accurate than optical wrist-based monitoring, heart rate monitoring using chest straps or wrist-based monitoring can be a great way to track exercise intensity both during a workout and retrospectively after a workout. Tracking exercise intensity is an important component of the FITT-VP principal and an important part of the exercise prescription because it helps a client go “easy” and “hard” or “harder” as necessary. Without tracking intensity, clients may end up exercising in an inappropriate work zone and not see the improvements that they hoped for. With heart rate monitoring technology becoming more readily available and used by many individuals, this is an excellent opportunity for fitness professionals to leverage their exercise training knowledge and combine it with the technology that their clients already use to help optimize exercise prescription.

BRIDGING THE GAP

Monitoring exercise intensity is important so that a client knows if he/she is meeting his/her exercise goals. Heart rate monitoring using new technologies can be helpful and a fitness professional can help optimize heart rate parameters in wearable technology. One such parameter includes heart rate reserve (maximum heart rate minus resting heart rate) rather than a percentage of predicted maximal heart rate, which is recommended by the American College of Sports Medicine to monitor exercise intensity during aerobic exercise. Further, monitoring heart rate may be achieved using different wearable devices that use very different technologies. Client preference or budget may dictate technology choices. Regardless, when used in conjunction with exercise prescription from a fitness professional, such devices can effectively support client exercise training intensity monitoring.

References

1. American College of Sports Medicine. ACSM’s Guidelines for Exercise Testing and Prescription. 10th ed. Philadelphia (PA): Lippincott Williams & Wilkins; 2018.
2. Kenny WL, Wilmore JH, Costill DL. Physiology of Sport and Exercise. 6th ed. Champaign (IL): Human Kinetics; 2015.
3. Laukkanen R, Virtanen P. Heart rate monitors: state of the art. J Sports Sci. 1998;16(4):3–7.
4. Shcherbina A, Mattsson CM, Waggott D, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. J Pers Med. 2017;7:3. doi:10.3390/jpm7020003.
5. Thiebaud R, Funk MD, Patton JC, et al. Validity of wrist-worn consumer products to measure heart rate and energy expenditure. Digit Health. 2018;4:1–7.
6. Gillinov S, Etiwy M, Wang R, et al. Variable accuracy of wearable heart rate monitors during aerobic exercise. Med Sci Sports Exerc. 2017;49(8):1697–703.
7. Benedetto S, Caldato C, Bazzan E, Greenwood DC, Pensabene V, Actis P. Assessment of the Fitbit Charge 2 for monitoring heart rate. PLoS One. 2018;13(2):0192691.
8. Bayles MP, Swank AM. ACSM’s Exercise Testing and Prescription. 1st ed. Philadelphia (PA): Wolters Kluwer; 2018.
9. Fitbit Website [Internet]. San Francisico (CA): Fitbit. [cited 2018 August 5]. Available from: https://blog.fitbit.com/heart-rate-zones/.
10. Polar™ Website [Internet]. Bethpage (NY): Polar. [cited 2018 August 5]. Available from: https://support.polar.com/us-en/support/Maximum_Heart_Rate__HRmax.
11. Garmin Website [Internet]. Olathe (KS): Garmin. [cited 2018 August 5]. Available from: https://www8.garmin.com/manuals/webhelp/vivoactive/EN-US/GUID-A6B45717-C27D-42F4-A8B2-4CB88F1A3B83.html.
12. Centers for Disease Control and Prevention Website [Internet]. Atlanta (GA): Centers for Disease Control and Prevention. [cited 2018 August 5]. Available from: https://www.cdc.gov/physicalactivity/basics/measuring/heartrate.htm.
    13. Polar™ Website [Internet]. Bethpage (NY): Polar. [cited 2018 August 5]. Available from: https://support.polar.com/us-en/support/tips/Polar_Sport_Zones.
      14. Garmin Website [Internet]. Olathe (KS): Garmin. [cited 2018 August 5]. Available from: https://www8.garmin.com/manuals/webhelp/vivosmarthr/EN-US/GUID-A8716C0B-B267-4C42-B45F-B9C7928BCA19.html.
        15. Saltin B. Aerobic and anaerobic work capacity after dehydration. J Appl Physiol. 1964;19(6):1114–8.
        16. Achten J, Jeukendrup AE. Heart rate monitoring: applications and limitations. Sports Med. 2003;33(7):517–38.
        Keywords:

        Exercise; Heart Rate; Heart Rate Monitors; Intensity; Technology

        Copyright © 2019 by American College of Sports Medicine.