Your smartwatch might be deceiving you about your fitness progress. Step counts, heart rate readings, and recovery scores could all be significantly inaccurate, according to insights from Hunter Bennett, a Lecturer in Exercise Science at Adelaide University, originally published by The Conversation.
The Problem with Wearable Fitness Technology
Wearable fitness technology, including smartwatches, has dominated fitness trends for nearly a decade, with millions globally relying on them daily. These devices influence how people perceive health and exercise by offering data on calories burned, fitness levels, recovery status, and readiness to train. However, most of these metrics are not directly measured; instead, they are estimates, making them less reliable than users might assume.
Calories Burned: A Flawed Estimate
Calorie tracking is a popular feature, but its accuracy is questionable. Wearable devices can under- or overestimate energy expenditure by over 20%, with errors varying across activities like strength training, cycling, and high-intensity interval training. This inaccuracy can lead to dietary missteps: overestimation might cause overeating and weight gain, while underestimation could result in under-eating, harming exercise performance.
Step Counts: Not Always Precise
Step counts serve as a general measure of physical activity, but smartwatches often under-count steps by about 10% under normal conditions. Accuracy decreases during activities that limit arm movement, such as pushing a pram or carrying weights, since these devices rely on arm swings to register steps. While still useful for tracking overall activity, step counts should be viewed as a guide rather than an exact figure.
Heart Rate: Variable Accuracy
Smartwatches estimate heart rate using sensors that monitor blood flow in the wrist. This method is reasonably accurate at rest or low intensities but becomes less reliable as exercise intensity increases. Factors like arm movement, sweat, skin tone, and how tightly the watch is worn can affect readings, leading to variability between individuals. For those using heart rate zones to guide training, even small errors can result in exercising at incorrect intensities.
Sleep Tracking: Limited Precision
Most smartwatches provide sleep scores and categorize sleep into light, deep, and REM stages. While they can detect sleep versus wakefulness fairly well using movement and heart rate, they are much less accurate at identifying specific sleep stages compared to the gold standard of polysomnography, which records brain activity in a lab. Thus, a report of "poor deep sleep" may not reflect reality.
Recovery Scores: Based on Inaccurate Measures
Recovery or readiness scores are typically derived from heart rate variability and sleep quality. In labs, heart rate variability is measured with an electrocardiogram, but smartwatches estimate it via wrist sensors, which are prone to errors. Since these scores rely on two inaccurate metrics, they may not meaningfully indicate recovery, potentially causing users to skip training unnecessarily.
VO₂max: Over- and Underestimations
VO₂max, a measure of maximal fitness based on oxygen usage, is estimated by smartwatches using heart rate and movement data rather than direct oxygen analysis. These devices tend to overestimate VO₂max in less active individuals and underestimate it in fitter ones, meaning the displayed number may not accurately reflect true fitness levels.
Practical Advice for Users
Despite these inaccuracies, smartwatch data is not entirely worthless. It can help track general trends over time, but users should avoid focusing on daily fluctuations or specific numbers. Paying attention to personal feelings, performance, and recovery cues often provides more valuable insights than device readings. By understanding these limitations, individuals can make more informed decisions about their health and exercise routines.
