Fitness Bands and Activity Trackers

What they measure and what they guess

• Measured directly: movement from an accelerometer, and often a pulse signal from light on the skin.
• Estimated: steps, distance, active minutes, heart rate and calories burned.
• Calculated by the company: daily activity or "readiness" scores, which have no standard meaning.

Steps and activity time

A systematic review of 65 articles found the Fitbit Charge family had a mean absolute percentage error under 25% for step counts across 20 studies. [1] A broader review of 67 studies of Fitbit devices found they were likely to meet acceptable step accuracy about half the time, tending to underestimate steps in controlled testing and overestimate them in free-living settings. [2] The same review found accuracy depended on walking speed and where the device was worn. [2]

In older community-dwelling adults (7 studies, 290 people, average age 70), step counts from consumer trackers correlated highly with reference devices, though slower walking and impaired walking reduced agreement. [3] Evidence label: promising but limited; results are for the models studied, not all bands.

Calories burned

Across brands, calorie estimates had error above 30% in one review. [1] A meta-analysis of 60 studies found accuracy varied with activity type, and that adding heart rate to motion sensing reduced error. [4] Treat calories burned as a rough guess. Evidence label: no good evidence of accuracy.

Do trackers improve health?

Accuracy is not benefit. A meta-analysis of 12 randomized trials (1,693 healthy adults) found trackers were associated with modest short-term increases in physical activity, with no clear benefit for activity or weight reduction in subgroup analyses and unknown long-term effect. [5] A meta-analysis of 19 trials reported moderate effects on body weight and waist circumference when wearables were part of an activity intervention. [6] Evidence label: mixed.

Who should be careful

• People who feel pressure from daily targets, or who check compulsively, may do better with a break; see When Taking a Break From Tracking May Help.
• Anyone with chest pain, fainting, breathlessness or a heart condition should not use a band's numbers to decide how hard to exercise; see Fitness Tracking: When Professional Assessment Matters.
• A band reading is not a diagnosis. People with diabetes should not use a band to estimate blood glucose, since the FDA has not authorized any smartwatch or ring to do so on its own. [7]

How to judge a claim

Ask which exact model was tested, who wore it, what the reference was, and whether the walking speed and placement match your own use. See How to Evaluate a Biohack, and the overview Wearables and Health Trackers.