Does your wrist affect optical heart rate accuracy?

Most athletes trust the number on their wrist. A heart rate reading during a tempo run, a zone calculation between intervals, a recovery figure from the previous day’s workout: all feed decisions about training volume, fitness levels, and race preparation. The question is whether all wrists produce an equally accurate reading.

Optical heart rate monitors embedded in every modern smartwatch, including those from Garmin, Apple, Fitbit, and WHOOP, are based on reflected light technology. Skin tone and wrist size are both plausible sources of interference. The evidence on each is less settled than the headlines suggest.

How optical heart rate sensors work

All wrist-worn heart rate monitors use photoplethysmography (PPG). A light-emitting diode, typically green, shines into the skin. A photodetector measures how much light is reflected. Blood absorbs light more strongly than surrounding tissue, so each heartbeat produces a detectable change in reflection. Green light is used because it provides a strong signal-to-noise ratio even in the presence of motion. Its limitation is absorption by melanin, the pigment that determines skin colour. Higher melanin content absorbs more green light before it reaches the photodetector, thereby reducing signal quality.

What the research actually shows about skin tone

The most extensive analysis is Koerber et al. (2023), published in the Journal of Racial and Ethnic Health Disparities. Across 10 studies, 469 participants, and 26 devices, four studies showed no skin-tone effect on accuracy, four found decreased accuracy for participants with darker skin, and two produced mixed results. That split does not support confident universal conclusions in either direction.

A widely cited 2020 paper by Bent et al. found no significant differences in heart rate accuracy across skin tones using the Apple Watch 4, Fitbit Charge 2, and Garmin Vivosmart 3. The protocol covered seated rest, paced breathing, and walking to 50 per cent of maximum heart rate. The sample at the darkest end of the Fitzpatrick scale was small, and exercise intensity was low throughout, which limits the extent to which the findings generalise.

More recent research points in a different direction during harder efforts. Hung et al. (2025) found no skin-tone differences at rest using the Fitbit Charge 5, but as exercise intensity rose, accuracy diverged. Mean error for darker skin tones at greater than 60 per cent of heart rate reserve reached 11.7 bpm. A separate 2025 study by Icenhower et al. at Wake Forest University, testing the Garmin Forerunner 45, found no significant skin-tone effect overall, but did find that accuracy dropped during periods of rapidly changing heart rate, regardless of skin tone.

Taken together, the current evidence suggests skin-tone-related differences are most likely to emerge during moderate-to-vigorous exercise rather than at rest.

The measurement tool is also the problem

The Fitzpatrick Skin Type scale, used in virtually all of these studies, was developed in 1975 to determine PUVA phototherapy doses. The scale measures reported sun sensitivity, not melanin concentration, and within each of its six categories, actual melanin content varies substantially. That within-category variance weakens any finding that relies on Fitzpatrick groupings.

Mulholland, MacDonald, and Aguiar (2025), published in the European Journal of Applied Physiology, is among the first studies to assess accuracy using objective measurement of melanin content via colourimetry rather than Fitzpatrick self-report. It represents the direction the field needs to move.

What manufacturers have said and not said

Manufacturer transparency on this is inconsistent. Garmin’s support documentation acknowledges that optical HR tracking can affect battery life under some conditions but has not published a detailed methodology for melanin compensation or the internal validation data underlying it. Fitbit has referenced thousands of hours of internal testing without providing any demographic breakdown. Apple has stated that skin perfusion and tattoos can affect optical sensor performance, but does not mention skin tone directly.

Newer devices from Apple, Garmin, and Fitbit combine red and infrared with green wavelengths. Red and infrared penetrate deeper into tissue than green light and are less strongly absorbed by melanin, improving signal quality across different skin tones. Whether those hardware changes have closed the accuracy gap observed in validation studies has not yet been established by independent data.

The wrist size question: less evidence, no signal

Wrist circumference as a variable affecting PPG accuracy is under-researched. The physics are plausible: a narrower wrist places the sensor over different subcutaneous anatomy, including greater proximity to muscle and bone, and varying vascular density relative to the radial artery.

A 2025 validation study in JMIR Cardio compared the Polar Verity Sense upper-arm sensor and Polar Vantage V2 wrist watch against a Polar H10 ECG chest strap across nine activities in 16 participants. The Verity Sense produced a mean absolute error of 1.43 bpm. The Vantage V2 produced a mean absolute error of 6.41 bpm with considerably higher variability across activities. What it confirms is that sensor placement alone produces meaningful differences in PPG accuracy.

The claim that smaller wrists affect optical HR accuracy is biologically reasonable, practically intuitive, and supported by anecdote. Controlled research to quantify it does not yet exist.

What this means for training decisions

At rest, the available evidence does not support a meaningful difference in optical HR accuracy by skin tone. During moderate-to-vigorous exercise, the picture changes on some devices. For some runners, consistent underestimation could lead to training harder than the displayed zone suggests, affecting training load calculations, VO2 max estimates, Recovery Time, and Training Readiness.

Device selection matters more than general claims about wearable sensors. The published research covers specific models: Fitbit Charge 5 and Garmin Forerunner 45 feature more skin-tone data than WHOOP, Oura, or Apple Watch. For serious training, the chest strap remains the benchmark. The Polar H10 measures heart electrical activity and is not affected by melanin or wrist anatomy.

The regulatory gap nobody is closing

Consumer wearables are not classified as medical devices and are not subject to the validation requirements that apply to clinical pulse oximeters or heart rate monitors. No regulatory body currently requires manufacturers to validate optical HR accuracy across the full Fitzpatrick spectrum before a product reaches market. Pulse oximeters used in hospitals were shown to overestimate blood oxygen saturation in patients with darker skin, contributing to delayed care during the COVID-19 pandemic. Consumer wearable companies have not faced equivalent scrutiny.

The bottom line

The physics is established, and several controlled studies have found that skin tone can influence the accuracy of optical HR during exercise. Whether it affects any individual athlete depends on the device, the exercise intensity, and their own skin tone.

The practical test for any runner is a direct comparison between the chest strap and wrist during a hard workout. That comparison is the only way to know whether the effect is present in your own data.

This article is part of the site’s female athlete tech coverage. For tested product recommendations across wearables, recovery trackers, and heart rate monitors, see the best wearables for female endurance athletes.

---

Explore the full resource library

This site covers endurance sport technology across a range of dedicated reference sections. Each one collects the most relevant articles, tests, and analysis on its topic in one place.

Brand and product guides

• Amazfit — the full Amazfit range from Balance to Cheetah to T-Rex, accuracy tests, HYROX partnership, and Zepp Health analysis
• Apple Watch for Sport — athlete-first coverage of Apple Watch across running, cycling, and triathlon
• COROS — watches, features, and firmware across the full COROS range
• Garmin — the company, the platform and the full range, and the starting point for choosing across every Garmin product line
• Garmin Edge — bike computers from entry-level navigation to flagship endurance and mountain biking
• Garmin Fenix — every model, feature, and firmware development for Garmin's flagship outdoor watch
• Garmin Forerunner — the full Forerunner line covered from entry level to triathlon flagship
• Garmin Instinct — rugged GPS watches for endurance and adventure athletes
• Garmin Features Explained — how Garmin's metrics work, from Training Load and Body Battery to Race Predictor and HRV Status
• Polar — watches, sensors, Polar Flow and training science across the full Polar range
• Suunto — Race, Vertical, Run and the SuuntoPlus ecosystem
• Strava — features, privacy, segments, and how Strava fits into a serious training setup
• Wahoo — KICKR trainers, ELEMNT bike computers, and the Wahoo ecosystem
• WHOOP — strain, recovery, sleep and the full WHOOP ecosystem

Sport and topic guides

• Running Watches — how to choose by discipline: road racing, trail, track, beginner, and multisport
• Triathlon and Multisport Technology — watches, sensors, and race-day tools for swimmers, cyclists, and runners
• HYROX — training science, race analysis, and technology for the functional fitness race format
• parkrun — technology, training, and performance for the weekly 5K
• Hiking Technology — navigation, safety, and trail tech for walkers and hikers
• Heart Rate Monitoring — optical sensors, chest straps, accuracy comparisons, and how to set training zones
• GPS Accuracy — how satellite systems perform across brands, terrains, and conditions
• Recovery Trackers — WHOOP, Oura, and the science of readiness scoring
• Female Athlete Tech — wearables, physiology, and performance for female endurance athletes, covering cycle-synced training, RED-S, HR accuracy, and VO2max
• Sports Science — peer-reviewed research on HRV, VO2max, lactate threshold, running power, wearable accuracy, and supplementation
• Testing Methodology — how this site tests GPS accuracy, heart rate, battery life, and other performance claims

Content series

• Release Radar — confirmed launches, leaks, and rumours across Garmin, Apple, COROS, Polar, Suunto, and Wahoo
• Deep Dive Feature Files — weekly firmware feature updates across all brands (bug fixes excluded)
• Fix Files — weekly firmware bug fix tracking across all brands

Last Updated on 24 July 2026 by the5krunner