The heart rate monitors on Apple Watches show varied accuracy during intense workouts, with several studies, user reports, and expert analyses providing insight into their performance.
Research published in 2017 examined the validity and reliability of the Apple Watch heart rate sensor during treadmill exercise at walking, jogging, and running intensities. The study found that the Apple Watch offers very good accuracy during walking but that its accuracy declines as exercise intensity increases to jogging and running. Specifically, correlation with a criterion device (a Polar heart rate monitor) was very good at walking speeds but reduced at higher intensities. The typical error and mean bias increased with exercise intensity, indicating less precise heart rate measurements. Additionally, the Apple Watch tended to record fewer heart rate data points at higher intensities, especially during the initial minutes of exercise, presumably due to motion artifacts and blood flow changes at the wrist. Inter-device reliability (consistency between different Apple Watch units) was very good, but intra-device reliability (repeatability of the same unit) decreased with intensity. The study recommended caution when interpreting Apple Watch heart rate data collected during jogging and running, primarily under 3 minutes in duration, as accuracy and data capture can be inconsistent during intense exercise initiation.
User reports from Apple community forums frequently highlight issues with the Apple Watch heart rate monitor's accuracy during intense workouts. Some users notice their heart rate readings are wildly inaccurate during runs or high-intensity interval training, with values spiking incorrectly or being delayed in 'latching on' to the correct rate. These inaccuracies are reported particularly in the first few minutes of a workout. Users describe experiences where the watch shows unusually high heart rates that do not match their perceived exertion or ability to hold a conversation during exercise, leading to questions about data reliability. Despite wearing the device correctly, ensuring a snug fit, and exercising under moderate temperatures, errors in readings persist for some individuals. Some users have switched to chest strap monitors, citing superior accuracy for intense exercise, as chest straps tend to detect heart rate more precisely than wrist-based optical sensors during high movement activity.
Factors influencing wrist heart rate monitor accuracy in general, including the Apple Watch, include wrist placement and fit, motion artifacts, skin perfusion (blood flow), sweat, and intensity of the activity. Rapid arm movements or vigorous exercise can create noise that complicates accurate photoplethysmography (PPG) readings, the technology wrist-based monitors use to measure heart rate by detecting blood flow changes. At the onset of intense activity, blood flow to the wrist may reduce temporarily, affecting sensor signal quality. The Apple Watch algorithms filter data to remove implausible values, which may cause initial data gaps or delayed responsiveness at workout start. Tightness of the band is recommended to be snug but comfortable; too loose or too tight an arrangement can reduce measurement fidelity. Skin tone can also be a factor, as differing melanin levels affect light absorption, potentially impacting readings on darker skin tones.
While the Apple Watch can deliver generally reliable heart rate data for steady-state or low to moderate intensity exercise, its performance is less robust during high-intensity or very dynamic movements. Reports suggest the heart rate sensor becomes more stable and accurate after several minutes into the workout, as the algorithms adjust and adapt to the user's physiology and movement pattern. However, intermittent drops in measurement and occasional spikes can still occur during activities involving irregular arm movement or sweat accumulation.
Comparatively, chest strap heart rate monitors are widely regarded as the gold standard for exercise heart rate measurement due to their use of electrical signals (ECG) rather than optical sensors. They tend to maintain accuracy regardless of movement intensity or sweat, making them preferable for high-intensity workouts or when precise heart rate data is critical. Optical wrist sensors like those on the Apple Watch prioritize convenience and everyday wearability but trade off some accuracy during the most strenuous or erratic activities.
Apple has provided guidelines to improve heart rate measurement accuracy, such as calibrating the watch for improved workout and activity tracking, wearing the watch slightly higher on the wrist during intense workouts, and ensuring the latest watchOS updates are installed. Despite these optimizations, the nature of optical heart rate sensing at the wrist inherently limits precision under some intense or variable exercise conditions, which users and experts acknowledge.
In summary, Apple Watch heart rate monitors are generally accurate for walking and low to moderate intensity workouts but become less reliable during intense exercise, especially in the first few minutes. Data irregularities due to motion artifacts and physiological factors like blood flow to the wrist contribute to occasional inaccuracies. While the device can provide useful heart rate information for most fitness enthusiasts, users requiring the highest accuracy during intense workouts often prefer chest strap monitors. Apple's ongoing software improvements and user calibration can enhance accuracy but cannot fully overcome the physical and technological limitations of wrist-based optical sensors during vigorous activities.