The total global market for wearables (electronic devices worn on the body) is estimated to be worth between 91 and 118 billion US dollars in 2026, whilst the smartwatches and sports watches segment accounts for approximately 43 to 51 billion US dollars of this (Global Market Insights/Market Research Future, among others). So there’s a lot of money to be made from a bit of tech on your wrist – not just from the initial purchase, but subsequently through subscription models. This leads to grandiose promises and ever-increasing amounts of data. But what data do you really need? Which fitness metrics are reliable and which are merely rough estimates? We explain the most important metrics of modern sports watches and other wearables.
Heart rate – the most important piece of fitness data
In cycling, heart rate has become somewhat less important due to the widespread use of power metres. However, in many sports it remains the most important tool for managing training intensity zones. In particular, it helps prevent you from starting long sessions with too high an intensity. Wrist-based measurement works differently – and less accurately – than a chest strap. The chest strap measures the heartbeat directly on the skin via the heart’s electrical impulses (based on the ECG principle). Wrist-based measurement, on the other hand, is an optical method. Light-emitting diodes (LEDs) on the back of the watch emit light through the skin’s surface into the underlying tissue. Photodiodes measure how much light is absorbed. Put simply, with every heartbeat, a small pressure wave is generated in the blood. The optical sensors detect this with a delay. The measurement is highly prone to error. The main problems are: the watch is not fitted securely enough; sweat affects the measurement; and hair, skin and movement can also distort the data.
Conclusion: Always use a chest strap for measurement
HRV – an important training metric
Heart rate variability (HRV) describes the fluctuations in the time intervals between individual heartbeats. These values indicate how the body copes with physical exertion. In sports science, this metric has become increasingly important for managing training loads. It acts as a warning sign when athletes are weakened by illness, overtraining or other forms of stress, and there is a risk of training overload. A fundamental prerequisite for a meaningful HRV analysis is reliable and frequent measurement. When this is the case, the value serves as a very useful indicator that even professional athletes take into account.
Conclusion: An important indicator that warns of illness or overexertion
ECG – now included in the fitness data
A full ECG at the doctor’s cannot be replaced by a smartwatch, but can these watches detect heart problems? The German Society of Cardiology has commented on the ECG function of the Apple Watch 4. The medical experts classified the smartwatch as a valuable monitoring tool. One of the most common cardiac arrhythmias is atrial fibrillation, and early detection is crucial. “Preliminary studies do indeed show that there is a 95 per cent correlation between atrial fibrillation detected by a smartwatch and clinically documented atrial fibrillation,” they explained in a statement.
Conclusion: whilst these watches cannot replace visits to the doctor, they can complement or encourage them.
Oxygen saturation – limited benefit
A pulse oximeter measures the oxygen level in the blood and provides feedback on oxygen supply. The measurement is based on the fact that haemoglobin changes colour depending on how much oxygen it is carrying. In a medical context, this is usually carried out using a clip-on sensor on the finger, with a light source on one side of the finger and a photodetector on the other. Due to their design, smartwatches are unable to perform medical-grade measurements. Wristwatches can only measure the light reflected by bone and tissue. Scientific studies therefore show that the measurement is highly prone to error, and there are significant differences between individual manufacturers and products. According to one study, the watches tend to artificially inflate the reading, particularly in cases of low oxygen levels.
Conclusion: Not a reliable figure
VO₂max – fitness data without measurement
The VO₂max value displayed on sports watches is also only calculated, not measured. This value indicates the maximum amount of oxygen the body can take in and utilise during exercise. It is one of the most important performance indicators in endurance sports. In cycling, VO2max is determined in a fitness test (measuring respiratory gases during maximum exertion) and is the most important figure for talent identification. Furthermore, a high value is also an indicator of good health and is highly relevant in preventive medicine. Studies have found significant differences between various watches. Generally speaking, dedicated sports watches performed better than smartwatches such as the Apple Watch. Studies show that watches tend to underestimate VO₂max in very fit athletes, whilst they often overestimate the values for untrained individuals. What watches are good at is tracking trends in performance. This can help to see whether the training over the past few weeks is having an effect.
Conclusion: The exact figure cannot be determined – however, the trend can be observed
Recovery values – the esoteric side of fitness data
Recovery or stress scores are often marketed as objective indicators of physiological state, stress levels and recovery. However, the scientific evidence behind these scores is limited and inconsistent. Studies show that the calculation methods – in particular the integration of physiological data such as heart rate variability (HRV), resting heart rate and activity levels – are neither transparent nor consistent. This raises doubts about their scientific validity. Overall, studies question whether these scores provide reliable and actionable information about stress, recovery or resilience. At best, the use of wearables can improve awareness of one’s own physiological states. They are merely a tool for developing body awareness. It makes more sense to monitor directly measured values such as HRV.
Conclusion: Recreation values are a mathematical black box – their usefulness is questionable
Sleep: keeping you awake because of your fitness data?
Getting enough sleep is important for wellbeing and sporting performance – all wearables advertise sleep tracking and analysis. Studies in the past have revealed significant discrepancies in the sleep data recorded by different manufacturers. That alone shows just how questionable the significance of these figures is. There is another reason why the data should not be over-interpreted: US researchers from Chicago have reported in the *Journal of Clinical Sleep Medicine* that the sleep data from wearables can, in some cases, lead to uncertainty amongst users. For some patients, an excessive focus on the data actually caused sleep problems.
Conclusion: The figure is very imprecise and its usefulness is questionable
Calories burnt
Calorie expenditure, too, is merely an estimate based on models that vary depending on the manufacturer. Heart rate, activity data, age, weight, height and gender are used as the main criteria. Studies have shown that there are extreme discrepancies between different products. According to a recent study by Florida International University, the watches were off by 15 to 25 per cent. The margin of error increased particularly with a high body fat percentage, and cycling proved to be a challenge. Every athlete understands that you burn more energy on a local loop in the rain and cold than in mild temperatures and sunshine, even if the distance and heart rate are the same. Studies have shown that the readings sometimes deviated by 20 to 30 per cent – which highlights just how problematic they are. Only with a power meter can you obtain reliable readings.
Conclusion: The value displayed is inaccurate and therefore questionable
A side note on power metres
Many of the figures displayed by sports watches or other wearables are estimates and are therefore of dubious relevance. Fortunately, in cycling there is a device that provides data that is measured rather than estimated: a power metre with dual-sided measurement (crank/pedal) enables optimal training and race management. The actual power output provides data that is more valuable than any other metric. Power also forms the basis for other metrics, which can then be calculated reliably. A good example is calorie expenditure. The calculation is based on physical work (measured in kilojoules, kJ) and human efficiency. Using the measured watt values over a specific period, calorie expenditure can be calculated much more accurately than a smartwatch can estimate. This is particularly important for training blocks or stage races.
Interesting studies on the subject:
https://www.frontiersin.org/journals/sports-and-active-living/articles/10.3389/fspor.2024.1465515/full
https://pubmed.ncbi.nlm.nih.gov/38557808/
https://pubmed.ncbi.nlm.nih.gov/36016077/
https://www.degruyterbrill.com/document/doi/10.1515/teb-2025-0001/html
https://www.nature.com/articles/s41746-020-0226-6
https://www.thieme-connect.com/products/ejournals/html/10.1055/a-2376-6332#info
https://pubmed.ncbi.nlm.nih.gov/40116771/
https://onlinelibrary.wiley.com/doi/full/10.1111/sms.14271
Kristian Bauer
Editor