Racing through New York City, you’re feeling great, fast, and accomplished. You take a quick glance at your watch, ready to see the satisfying stats of your pace dropping, heart rate rising, and mileage accruing. Instead, you see nothing– your watch isn’t tracking, is frozen, or displays a wildly inaccurate measurement. And the feeling of your effort going unrecorded sinks in. You think: Wait, did this whole run ever happen if my watch didn’t track it? Is my watch broken or do I need to figure out a software update?
We can rely on fitness trackers or wearables for proof, validation, and planning. They are meant to record and monitor your daily physical activity and health metrics. Your heart rate, exercise, caloric expenditure, VO₂max, and sleep are among the list of the many metrics recorded depending on the device. What you do with the information and how you interpret your data can influence your health and training.
The most popular consumer wearables include smart jewelry (rings, wrist bands), wrist watch, arm bands, clip-ons, and chest straps. Each device utilizes different algorithms to estimate their metrics, but they aren’t required to validate their methodology or disclose what the algorithm includes. This makes the research behind each device’s metrics tough to validate. In this blog, we will explore the accuracy and inaccuracies of consumer wearables for activity levels and how to best utilize the data in tandem with your workouts.
How Do Wearables Track Your Health?
Photoplethysmogram (PPG) Sensors
Consumer wearables use photoplethysmogram (PPG) sensors, which emit a green LED light onto the skin to measure blood pressure and flow changes. The sensors emit light onto your skin, and your blood absorbs some light while the rest is reflected back. The sensor measures the amount of light absorbed and reflected and utilizes those measurements to determine your heart rate, heart rate variability, oxygen saturation, blood pressure, and VO2 max.
However, the sensor’s accuracy can be affected by anything blocking the sensor, excessive movement, and skin tone. Hair, sweat, lotion, creams, and repellant can cover the sensor and prevent light from being absorbed or repelled into your skin, affecting the accuracy of your readings. Excessive movement can occur if the wearable is fitted too loosely or if you’re doing an intense workout where sweat is heavy causing the wearable to slip. Additionally, studies have found the accuracy of heart rate readings is reduced in darker-skinned individuals.
To maintain as much accuracy of the sensors as possible during activity, it is recommended to:
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- Wipe and dry your wearable to remove any buildup, especially if you are a heavy sweater or regularly use lotions/creams/repellants.
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- Make sure the wearable is not too loose or too tight, which can restrict blood flow and impact the data. The best fit is where the wearable feels secure but comfortable with a little bit of space.
Heart Rate Tracking
When tracking resting heart rate, wearables perform relatively well. Studies show a mean bias of approximately ±3–5 bpm and a mean absolute error of less than 10%, making it a reliable tool to use when at rest. Resting heart rate is an important vital sign, with a normal range for healthy adults typically between 60 and 100 beats per minute (bpm). A chronically elevated resting heart rate has been associated with an increased risk of all-cause mortality and the development of cardiovascular disease.
During exercise, the activity intensity, activity type, and amount of arm movement involved affects heart rate accuracy. Rapid changes in heart rate during vigorous exercise can be particularly challenging for wrist-worn devices such as the Apple Watch, Whoop, or Fitbit to detect with precision. And strength training, in particular, has shown the greatest variability in heart rate tracking compared to activities like running or circuit training. Systematic reviews have found that wearable accuracy can vary by device, with some underestimating and others overestimating heart rate by as much as ~12%. This margin of error can be substantial, especially if you’re aiming to keep your heart rate within a specific aerobic training zone.
Wearable placement also plays a significant role in data accuracy. Among commercially available devices, chest strap monitors like the Polar H10 consistently show the highest agreement with the gold-standard electrocardiogram (ECG) during physical activity. Arm-worn wearables, like the Polar Verity Sense, generally outperform wrist-worn devices and can provide comparable accuracy to chest straps during moderate- to high-intensity exercise.
In contrast, wrist-worn devices tend to display larger errors during activity, with accuracy declining as exercise intensity increases. This is largely due to increased skin movement, reduced sensor stability, and greater distance from the heart. Research suggests that the Apple Watch is currently the most promising wrist-worn option for in-exercise heart rate monitoring, though it may still overestimate heart rate at higher intensities.
Overall, if your goal is to get the most accurate heart rate monitoring during exercise and to analyze your heart rate data in greater depth, a chest strap monitor is the best option.
Heart Rate Variability
Heart rate variability (HRV) is the difference in time between each beat-to-beat interval. For example, if your heart rate is 60 beats per minute, the time between each heartbeat changes. As your HRV increases, the more adaptable and ready your body is. If your HRV lowers, the more stressed or fatigued your system is.
HRV is highly sensitive to each individual so it is more important to track changes in trends as opposed to achieving a singular HRV value. However, devices measure HRV differently. For example, Oura ring takes an average throughout the whole night of sleep, whereas Whoop Bands take a 5 minute measurement in “deep sleep” followed by “dynamic averages” throughout the sleep cycle. Oura Ring’s methodology has been validated, but Whoop’s methodology is flawed because HRV varies in different sleep stages and changes during different parts of our circadian rhythm. A recent study comparing multiple devices found the Oura Generation 3.0 and 4.0 had the strongest accuracy and lowest errors similar to the ECG and chest strap, whereas the Whoop had moderate agreement.
The Apple Watch also measures daily HRV, but it samples HRV during different times each day. This makes the information inaccurate and quite useless to interpret change over time or trends. Because HRV is a sensitive metric that can change because of anything, like caffeine, alcohol, exercise, emotions, work stress, sleep, etc, the Apple Watch’s inconsistent sampling method makes it highly inaccurate to track trends for training or recovery purposes.
Having accurate trending data provides applications for self-awareness– to optimize training, adjust intensity, or take recovery days when HRV is low. If you’re wondering what HRV is and what to do with it, check out our blog about it.
Recovery or Readiness Scores
You’ve likely seen your watch or wearable provide a daily score meant to represent training load, preparedness, or recovery. Metrics such as Recovery (Whoop) or Readiness (Oura) are generated from proprietary algorithms that combine multiple data points in a largely “black-box” manner. Importantly, these scores and the algorithms behind them are not yet validated in the scientific literature and have not been shown to reliably predict performance.
What may be more useful is examining individual data points alongside your subjective experience of how you’re feeling. When multiple variables are trending in the same direction or showing meaningful changes from your normal baseline, it may be worth adjusting your training or taking a closer look at recovery and overall load.
Energy Expenditure
Energy expenditure is the least accurate metric measured by wearables. No consumer wearable has been validated to be equivalent to the gold standard methods. According to a 2020 systematic review, arm-worn and wrist-worn devices had mean absolute errors between 29-80%, with most devices tending to underestimate rather than overestimate. Another 2020 study determined that Garmin devices underestimated calorie expenditure 69% of the time, Apple Watches overestimated it 58% of the time, Polar devices overestimated it 69% of the time, and Fitbit devices both underestimated (48%) and overestimated (39%) calorie burn. If your device sometimes underestimates or sometimes overestimates, it’s not helpful until you know what is happening; but even then, the number provided will most likely be inaccurate.
If you’ve been using your wearable to track calories and it’s been supporting your goals, there’s no need to stop, so continue using it as a helpful tool. However, if you feel it hasn’t been effective, it may be more beneficial to determine your resting metabolic rate and instead focus on tracking your caloric intake alongside changes in body weight.
Your resting metabolic rate accounts for approximately 60–80% of your total daily energy expenditure and represents the calories your body burns at rest to sustain essential functions such as breathing, circulation, and cellular activity. At Perfect Stride, we offer resting metabolic rate testing using our VO₂ Master and provide a comprehensive report to help guide your weight management goals.
Estimating VO₂max
VO₂max is the maximum amount of oxygen your body uses during highly intense exercise. The higher your VO₂max, the more oxygen your body can use during exercise, and the more energy produced. This is a measurement on your aerobic system’s capacity and cardiorespiratory fitness. The gold standard of VO₂max testing is a clinical cardiopulmonary exercise test (CPET). CPET also provides accurate heart rate zones, lactate threshold, and ventilatory thresholds to individualize and optimize your training.
Wearables can estimate VO₂max through algorithms that include your anthropometrics (age, height, weight, sex), heart rate and heart rate variability response during exercise, and physical activity metrics (distance, pace, step counts, and energy expenditure). However, your wearable cannot directly track how much oxygen you’re breathing which leads to inaccuracies in the estimated value.
Specific brands have different accuracy profiles for VO₂max estimation. Garmin devices were the most superior brand and had the most consistent accuracy across athletes. The Garmin fenix 6 and Forerunner 245 had acceptable accuracy and high agreement to the actual value. But training level was a factor since the accuracy was being better in moderately trained athletes versus underestimated by ~6.3mL/kg/min in highly trained athletes. Comparatively, the Apple Watch generally had poor correlation due to potential over and underestimations across individuals.
For VO₂max estimates by wearables, prioritize trends over the individual number. Because devices and individual biometrics vary, the direction of your data is more meaningful than the specific number. It will tend to improve as you exercise more consistently. If it is decreasing over time, it is a sign that your body is losing its fitness.
At Perfect Stride, we offer VO₂max testing to determine your true VO₂max value. In the clinic, we screen and guide you through a graded exercise test to assess your cardiorespiratory fitness. This testing also provides personalized heart rate training zones and valuable insight into your actual exertion levels across different intensities, performance within each energy system, and how efficiently your body utilizes fats and carbohydrates for fuel.ery.
Summary
Wearable technology has become increasingly popular as people look to monitor their health, track fitness, and manage chronic conditions. These non-invasive devices offer a convenient and accessible way to monitor personal health, behaviors, and lifestyle factors over time. Their small size and ease of use make them well-suited for long-term, continuous tracking and for providing a range of health-related estimates.
However, the accuracy and reliability of wearables vary widely depending on the device and the specific metric being measured. For this reason, it’s best to use wearable data alongside your own subjective experience (how you feel day to day) and to focus on overall trends rather than relying on exact numbers or scores.
Whether you want to track your caloric expenditure through a resting metabolic rate (RMR) test or accurately assess your VO₂max, Perfect Stride offers comprehensive testing and detailed reporting to help you reach your goals.
For more information, contact Perfect Stride Physical Therapy today:
Website – https://perfectstridept.com/
Email – info@perfectstridept.com
Location: 32 Union Square East, Suite 215, New York, NY, 10003
Phone: (917) 494-4284
References
Hutchinson A. How Meditation Can Combat Mental Fatigue and Supercharge Your Endurance. Outside Online. Published online October 9, 2024. https://www.outsideonline.com/health/training-performance/mindfulness-endurance-performance/
Koerber D, Khan S, Shamsheri T, Kirubarajan A, Mehta S. Accuracy of Heart Rate Measurement with Wrist-Worn Wearable Devices in Various Skin Tones: a Systematic Review. J Racial Ethn Health Disparities. 2023;10(6):2676-2684. doi:10.1007/s40615-022-01446-9
Zhang Y, Weaver RG, Armstrong B, Burkart S, Zhang S, Beets MW. Validity of Wrist-Worn photoplethysmography devices to measure heart rate: A systematic review and meta-analysis. J Sports Sci. 2020;38(17):2021-2034. doi:10.1080/02640414.2020.1767348
Yu H, Kotlyar M, Dufresne S, Thuras P, Pakhomov S. Feasibility of Using an Armband Optical Heart Rate Sensor in Naturalistic Environment. Pac Symp Biocomput. 2023;28:43-54.
Schweizer T, Gilgen-Ammann R. Wrist-Worn and Arm-Worn Wearables for Monitoring Heart Rate During Sedentary and Light-to-Vigorous Physical Activities: Device Validation Study. JMIR Cardio 2025;9:e67110. DOI: 10.2196/67110
Dial MB, Hollander ME, Vatne EA, Emerson AM, Edwards NA, Hagen JA. Validation of nocturnal resting heart rate and heart rate variability in consumer wearables. Physiol Rep. 2025;13(16):e70527. doi:10.14814/phy2.70527
Allen S, Tierney P. Whoop vs Oura Ring: Real-life data and comparison. Sportsmith. https://www.sportsmith.co/articles/whoop-vs-oura-ring-real-life-data-analysis-and-comparisons
O’Driscoll R, Turicchi J, Beaulieu K, et al. How well do activity monitors estimate energy expenditure? A systematic review and meta-analysis of the validity of current technologies. Br J Sports Med. 2020;54(6):332-340. doi:10.1136/bjsports-2018-099643
Jamieson, A., Chico, T.J.A., Jones, S. et al. A guide to consumer-grade wearables in cardiovascular clinical care and population health for non-experts. npj Cardiovasc Health 2, 44 (2025). https://doi.org/10.1038/s44325-025-00082-6
Carrier B, Marten Chaves S, Navalta JW. Validation of Aerobic Capacity (VO2max) and Pulse Oximetry in Wearable Technology. Sensors (Basel). 2025;25(1):275. Published 2025 Jan 6. doi:10.3390/s25010275
Engel FA, Masur L, Sperlich B, Düking P. Validity of V̇O2max estimates from the forerunner 245 smartwatch in highly vs. moderately trained endurance athletes. Eur J Appl Physiol. Published online August 6, 2025. doi:10.1007/s00421-025-05923-x
Caserman P, Yum S, Göbel S, Reif A, Matura S. Assessing the Accuracy of Smartwatch-Based Estimation of Maximum Oxygen Uptake Using the Apple Watch Series 7: Validation Study. JMIR Biomed Eng 2024;9:e59459. DOI: 10.2196/59459
