Research Challenges Consumer Fitness Data
Smartwatches frequently miscalculate calories burned during exercise, often leading users to overestimate their metabolic output. A study from Florida International University indicates that these devices commonly error by 15 to 25 percent, meaning an individual might miscount hundreds of calories over the course of a week. This discrepancy creates a significant risk for those attempting to maintain a strict caloric deficit.
Lead investigator Jason Kostrna notes that these numbers are not precise indicators of physical exertion. Instead of tracking exact energy expenditure, most commercial trackers rely on photoplethysmography to measure blood volume fluctuations near the skin surface. This method frequently lacks the necessary precision to accurately gauge total metabolic cost during intense movement.
Testing Methodology and Device Performance
Researchers tested four major wearable devices on 58 participants. The team evaluated the Samsung Galaxy Watch 5, the Garmin Forerunner 955, the Apple Watch Series 8, and the Fitbit Sense 2. Participants cycled on a recumbent ergometer while researchers compared the watch data against a laboratory-grade analyzer to determine the margin of error for each device.
Results showed that the Samsung Galaxy Watch 5 and the Garmin Forerunner 955 consistently overestimated energy expenditure. While the Apple Watch Series 8 showed less bias, the Fitbit Sense 2 produced inconsistent data, with 13 percent of its results categorized as implausible. Still, the researchers found no significant variance in accuracy based on the skin tone of the participants.
Impact of Body Composition on Accuracy
Body fat percentage directly affects the reliability of these measurements. Participants with a higher percentage of body fat received less accurate readings than those with lower levels. It remains unclear whether this error stems from the sensor technology or the proprietary algorithms used to calculate energy burn from blood flow data.
These findings have specific implications for athletes and clinicians who depend on precise data. For the casual user, these devices remain useful for tracking personal trends over time rather than exact calorie counts. Experts suggest that if the data creates unnecessary stress, users should rely on personal physical sensation and consistency rather than fixed numerical targets provided by a screen. The research appeared in the journal PLOS One.