Why Your Smartwatch Fitness Tracker May Be Inaccurate

After a run, you look at your smartwatch. Your fitness score is lower, it says you burned very few calories, and your recovery score is extremely low. It even recommends avoiding exercise for the next 72 hours.

The frustrating part is that the run itself felt brilliant.

Why, then, is your watch reporting something so different?

The answer is that smartwatches and other fitness trackers do not always provide accurate readings.

Smartwatches can influence your exercise habits

Wearable fitness devices, including smartwatches, have ranked among the leading fitness trends for almost 10 years. Every day, millions of people worldwide use them.

They can affect the way users view their health and exercise. Among other things, they offer figures on calories burned, fitness level, post-exercise recovery and whether you are ready to train again.

Yet a smartwatch does not directly measure most of these figures. Many of the commonly displayed metrics are estimates, meaning they may be less precise than you expect.

1. Calories burned

Calorie monitoring is among the most widely used smartwatch functions, although its precision is far from ideal.

Wearable devices may either underestimate or overestimate energy expenditure - commonly shown as calories burned - by more than 20%. The margin of error also differs by activity. Strength training, cycling and high-intensity interval training, for instance, may produce even greater inaccuracies.

This is important because many people use these figures to decide how much food to eat.

If your watch overstates the calories you have burned, for example, you may believe you need more food than you actually do, potentially leading to weight gain. On the other hand, an underestimate could cause you to eat too little, which may impair exercise performance.

2. Step counts

Although step counts provide a useful indication of overall physical activity, wearables do not record every step perfectly.

In typical exercise settings, smartwatches may undercount steps by roughly 10%. Activities including pushing a pram, carrying weights or walking with little arm movement are also likely to reduce accuracy, because smartwatches use arm motion to detect steps.

For most users, this is not a serious concern, and step totals remain helpful for monitoring broad activity levels. However, they should be treated as an indication rather than an exact measurement.

3. Heart rate

Smartwatches calculate heart rate through sensors that detect changes in blood flow in the veins at your wrist.

This approach is reasonably accurate while resting or exercising at low intensity, but its accuracy declines as exercise becomes more intense.

Arm motion, perspiration, skin tone and how snugly the watch is worn may all affect the heart-rate figure it produces. As a result, accuracy differs from one person to another.

This may cause difficulties for anyone using heart-rate zones to direct their training, since even minor errors can result in exercising at an inappropriate intensity.

4. Sleep tracking

Nearly every smartwatch available provides a "sleep score" and divides the night into light, deep and REM sleep stages.

Polysomnography is the gold-standard method for assessing sleep. This laboratory test records brain activity, whereas smartwatches estimate sleep from movement and heart rate.

Consequently, they can identify whether you are asleep or awake fairly well, but are substantially less accurate when distinguishing between sleep stages.

Therefore, a report of "poor deep sleep" from your watch may not be correct.

5. Recovery scores

Most smartwatches monitor heart-rate variability, combining it with your sleep score to produce a "readiness" or "recovery" score.

Heart-rate variability indicates how your body reacts to stress. In laboratory settings, it is assessed using an electrocardiogram. Smartwatches, however, estimate it with wrist-based sensors that are considerably more susceptible to measurement error.

Most recovery measures are therefore built on two imprecise readings: heart-rate variability and sleep quality. The resulting metric may not provide a meaningful picture of your recovery.

If your watch tells you that you have not recovered, you may consequently miss a training session - despite feeling well and actually being ready to exercise.

6. VO₂max

Most devices provide an estimate of VO₂max, a measure of maximal fitness. It refers to the greatest amount of oxygen your body can use during exercise.

The most accurate way to assess VO₂max requires wearing a mask that analyses the oxygen you inhale and exhale, establishing how much oxygen is being used to generate energy.

Your watch cannot directly measure oxygen use, however. Instead, it calculates an estimate from your heart rate and movement.

Smartwatches also generally overestimate VO₂max for less active people while underestimating it for fitter individuals.

The number displayed on your watch may therefore not represent your actual fitness.

What should you do?

Although smartwatch data is vulnerable to errors, it is not entirely without value. These devices can still help you monitor general patterns over time, but daily changes and individual figures should not receive too much attention.

It is equally important to consider how you feel, how you perform and how you recover. These factors are likely to offer more insight than your smartwatch readings.

Hunter Bennett, Lecturer in Exercise Science, Adelaide University

This article is republished from The Conversation under a Creative Commons licence. Read the original article.