Trackers have been shaping our health habits for at least a decade, but many of the metrics they show are estimates. Often with margins of error that change the decisions of those who read them
How many calories we burned, how we slept, whether we are ready to train: once upon a time these were questions that were answered based on sensation or accumulated experience. Today there are numbers, updated in real time on the wrist, precise to the decimal. For almost a decade, fitness trackers and smartwatches have been among the most followed tech trendsto the point that “these devices shape the way people think about health and exercise” writes on The Conversation Hunter Bennettresearcher in exercise science at the University of Adelaide (Australia). The problem, however, is that that precision is largely apparent: “The smartwatch does not directly measure most of these parameters. Many of the common metrics are just estimates.”
Deficient calories
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Calories are the most obvious case. “Calorie counting is one of the most popular features of smartwatches. However, the accuracy leaves a lot to be desired,” Bennett writes. Technological devices can overestimate or underestimate energy expenditure by more than 20%, with errors that amplify during weight training, cycling and high intensity interval training. “But if the clock overestimates the calories burned, we may think we have to eat more than we really need, resulting in weight gain. On the contrary, if it underestimates them, we risk eating too little, compromising performance.”
For the steps the error is more contained: smartwatches, in general, can approximate by about 10% by default under normal conditions but activities such as pushing a stroller or walking with little arm swing make the counting even less precise, since the detection occurs from the movement of the wrist. Heart rate, on the other hand, seems to be the most immediate measurement, but the optical sensors work reasonably well at rest and lose precision as effort increases. “Arm movement, sweat, skin tone and the position of the watch can all affect the measurement,” Bennett points out, with variations varying from person to person.
monitor your sleep
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Sleep is a separate chapter. Almost every latest generation smartwatch produces a “sleep score” and divides the night into light, deep and REM sleep phases. The gold standard of reference is poly-somnography, a laboratory test that records brain activity. “But smartwatches estimate sleep using movement and heart rate,” Bennett writes. “This means that they can detect moments of wakefulness and sleep quite well, but are much less precise in identifying the phases. So, even if the watch says you had poor deep sleep, that may not be the case.” The recovery score cumulates the previous inaccuracies: it is based on heart rate variability, which in the laboratory is measured with an electrocardiogram, combined with the sleep score. “But smartwatches estimate it using sensors on the wrist, which are much more subject to measurement errors,” the researcher points out. “This means that most recovery metrics are based on two inaccurate measurements, resulting in a metric that may not reflect in real recovery significantly. If the clock says he hasn’t recovered enough, we might skip a workout even when we feel good.”
VO2MAX TRUE DILEMMA
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The last parameter is VO₂max, the maximum amount of oxygen that the body can use during effort. Measuring it requires a respiratory mask connected to gas analysis instruments. The smartwatch estimates it from heart rate and movement, with a systematic distortion: “Watches tend to overestimate VO₂max in less active people and underestimate it in more trained ones” comments Bennett. “And the number on the display may not reflect true fitness.” What to do? Stop believing in our technological devices? At all: the data, although imprecise, remains useful for monitoring some parameters and following general trends over time, as long as you do not rely on daily fluctuations. The trick is to not stop listening to your body while looking at the display. “Paying attention to how we feel, how we perform, and how we recover,” Bennett concludes, “probably provides more information than the smartwatch says.”
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