After you run and feel that you enjoyed your favorite running sport, your smart watch may sometimes tell you otherwise. They show that your fitness index has dropped, that you have burned too few calories, that your recovery rate is very low, and may even require you to stop exercising for 72 hours.
Dr. Hunter Bennett, a lecturer in exercise science at the University of Adelaide in Australia, explains this paradox in an article on The Conversation website, explaining that the reason is that smart watches and other fitness tracking devices are not always accurate.
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For about a decade, wearable devices – such as smart watches and fitness bands – have become one of the most prominent health and sports trends for millions of people around the world. However, studies reveal that its performance is far from perfect.
- In 2021, German experts found, after testing nine smart watches and fitness bracelets, that they were not accurate in swimming, running, and cycling measurements most of the time.
- In 2024, the US Food and Drug Administration issued a warning about the risks of relying on watches that claim to measure blood sugar levels directly.
Hence, Bennett warns against treating clock data as final facts, and calls on us to understand the limitations of these devices before relying on them.
Common metrics that smartwatches may fool you about
According to Dr. Bennett, smart watches do not measure most basic indicators directly as users imagine, but rather rely on estimated models and algorithms. Many of the metrics you see on the screen are just approximations in the following cases.
1- Calories burned
Calorie tracking is one of the most common features of smart watches, but its accuracy is not perfect.
- Wearable devices may underestimate or overestimate energy consumption by up to 20%.
- Errors increase with some activities such as strength training, cycling, and high-intensity interval training.
The danger of this is that many people determine their food intake based on these numbers.
- If the clock overestimates calories, the person may think that he needs more food than he actually needs, and thus gain weight.
- If you underestimate it, this may cause him to eat less than necessary, and his performance and health will be negatively affected.
2- Number of steps
Step count is a simple indicator of general physical activity, but it is not always accurate. Wearable devices rely on arm movement to count steps, which leads to:
- Reduce the number of steps by approximately 10% under normal exercise conditions.
- Increased error during activities such as pushing a stroller, carrying objects, lifting weights, or walking with limited arm movement.
However, step counts are still useful for tracking the general direction of activity. Bennett advises treating the step count reading on a watch as a guideline rather than an accurate laboratory measurement.
3- Heart rate
Smartwatches use optical sensors that measure the change in blood flow across the wrist to calculate heart rate.
- Readings are reasonable for rest or low-intensity exercise.
- Accuracy decreases as exercise intensity increases.
Factors such as arm movement, sweating, skin tone, and how tightly the watch is attached to the wrist affect the measurement. These factors make accuracy vary from one person to another, which represents a problem for those who adjust the intensity of their training based on the heartbeat, as simple errors may lead them to train at a higher or lower intensity than required.
4- Sleep tracking
Most smartwatches provide sleep reports that divide the night into light, deep, and REM sleep.
The gold standard for measuring sleep is still a laboratory test that records brain activity. In comparison, smart watches rely only on movement and heart rate.
- It can determine your bedtime and wake-up time with reasonable accuracy.
- But it is less accurate in determining the different stages of sleep.
So even if the watch shows you that you didn’t get enough deep sleep one night, that may not really reflect what happened in your brain during sleep, Bennett points out.
5- Indicators of recovery
The accuracy of measuring heart rate during exercise is affected by many factors: movement, sweating, blood flow changes, and the sensor not being in contact with the skin all the time.
This makes heart rate data during intense exercise or strength training less reliable. Most smartwatches use heart rate variability and sleep quality metrics to build recovery indicators.
Heart rate variability reflects how the body responds to stress, and is accurately measured in laboratories using electrocardiography machines. As for smart watches, they are among the wrist sensors most susceptible to measurement errors.
The result is that the recovery index may depend on two relatively inaccurate measures, giving a picture that does not reflect your true condition. The clock may tell you that you have not recovered, so you postpone exercise even though you feel fine and ready to train.
6- Maximum oxygen consumption
Many wearable devices display a number called VO2max, as an indicator of your peak physical fitness.
The most accurate way to measure this indicator is laboratory tests in which a person wears a mask that analyzes the amount of oxygen inhaled and exhaled, and determines the volume of oxygen used to produce energy.
The smart watch cannot measure this directly, but rather estimates it through movement and heart rate data.
- It tends to overestimate VO2max in less active people.
- It tends to be undervalued in fitter athletes.
So the VO2max number on your watch may not reflect your true fitness level, Bennett emphasizes.
How do you benefit from your smart watch without it deceiving you?
Smartwatch faults do not mean that they are useless. But it’s a reminder that you should know the limits of what you’re measuring, and what you shouldn’t base your health decisions on alone.
Limits of what the watch offers
- Watch data is not a substitute for medical examinations or laboratory tests.
- What it offers is more of a rough map of your health and fitness trajectory over time.
- It becomes misleading if you get caught up in the daily fluctuations of numbers instead of following the general trend.
Dr. Bennett advises that you give more weight to three simple signals: how you feel during your workout, your performance from session to session, and how quickly you recover between sessions. These subjective scores often give you a truer and more useful picture than screen numbers.
A safe use guide for smart watches
Health analyzes published by Forbes magazine indicate that wearable devices:
- Provides guidance information for general health follow-up and self-monitoring.
- They are not medical devices approved for diagnosis or to modify treatment without medical supervision.
Thus, you can look at your smart watch as a motivational tool that helps you move more, and a way to notice major trends in your activity, sleep, and recovery, but it is not a final ruling that decides when to exercise, when to sleep, and when to go to the doctor.
