How Accurate is my Smartwatch?
Most runners these days have some version of a smartwatch. These watches are amazing technological innovations and can track are heartrate, mileage, elevation, as well as plenty more. They also can tell us how we slept, if we are ready to train, and approximate our V02 Max (an aerobic capacity measurement). Or can they?
No matter what your preferred brand is, your watch gathers a lot of data, and gives you a lot of information. But what is worth paying attention to, and when. Let’s start with some of the basics:
1. Heartrate. Many watches nowadays have an optic sensor on the wrist that measures heartrate. This technology has improved significantly in recent years, but is still not as accurate as some other methods. Wearing the watch in a slightly different position will impact the reading as will the tightness of the wrist strap. If you want more accurate heartrate data, I would recommend getting a chest strap. Brands have also started releasing arm bands which can also be decent options, especially for smaller runners. That said, the arm bands also use an optical sensor, but they can typically be placed in a way that allows for more consistent readings. Overall, the heartrate from the watch is decent, but getting a chest strap that pairs with your watch will give you significantly more accurate data.
2. Distance/Elevation. Most watches are outfitted with some GPS measurement. This is a great way to keep track of mileage, or follow routes. You can also log elevation gain/loss and save routes for future routes. This functions by sending out a signal at predetermined intervals (this can often be adjusted) to track progress and distance covered. Depending on the watch, many people on the same run may end up with varied distances. You can see it on your group runs when everyone can have slightly different mileage while running the same route. Another reason that running around at the end of a run to get a round number may be even more pointless. On the whole, we can keep solid track of our mileage from week-to-week and determine how much we can ramp up our mileage in the future.
These two features give us valuable information that we can use to determine future training, measure efforts, and track improvement. They are some of the more ‘basic’ features for watches at this point. Let’s get into some of the other ‘data’ that your watch gives you and if it’s helpful. There are many things, but I’m going to focus on 3 main ones that I hear frequent questions about:
1. V02 max. V02 max is essentially a measurement of aerobic capacity, or endurance fitness. The higher the number, the fitter you are. To most accurately measure V02 max, you need to have some measure of breathing rate, and also the Oxygen and Carbon Dioxide ratio of inhalation/exhalation. To put it bluntly, there is no way your watch can measure that, even if you wore it on your tongue. There are other tests that are used to give approximations of V02 max, and the watch notes that it is an estimated V02 max. That said, it’s important to note it is very approximate and factors such as air temp and humidity as well as running surface will likely impact the estimated number.
2. Training Scores. Many watches give you some form of a training readiness or training stress score. These purport to tell you how well your training is working or how hard a particular session was. They use equations based on data such as heart rate and speed. That said, it’s really hard to know what the equation actually is. From Garmin: “[Training Readiness] insight is achieved with the help of a multi-layered analysis that considers combinations of activity and lifestyle data.” Super fancy, but hard to say what all the scores are actually based on. And I’m not just picking on Garmin, as Polar has an equally vague description of their score: “Training Load gives you an estimate on how your training affects your body and helps you compare the load of different workouts. It is based on wrist-based acceleration and energy expenditure.” One challenge is that we don’t know what all the factors are that go into the score. The other is that it’s a score based on data that is imperfect and can’t accurately account for environmental factors such as heat, humidity, hydration levels, non-exercise stress, etc. Training scores can be part of the information we have but we should also learn to listen to our bodies.
3. Sleep Scores. If you are someone who wears your watch to sleep, many will give you some version of a sleep score. Similar to the challenges with the training score, we don’t really know what goes into this score. It is based on a proprietary equation that factors in the data from your night of sleep. The problem is the data isn’t very accurate. Most of these devices don’t take frequent enough measurements to determine sleep quality or duration. There is no way a watch can measure if you are in REM sleep or deep sleep. It is estimating based on past data and infrequent measurements. I would ditch the watch while you sleep and measure either resting HR or HRV when you first wake up (more on that in a different column)
Overall, new watches provide us with tons of information that we can use to shape our training and try to determine our performance. They are consistently getting better in how they measure and track data. Heartrate data and distance can be great in informing training structure and progression. On the other hand, or wrist, the scores and estimated fitness numbers that the watch gives us should be looked at with discernment. It’s unclear what they’re based on and therefore hard to tell what they actually tell us. It’s important to use quality data and also develop subjective feel as a runner.
Martin Benes is a DPMR Board Member and endurance coach
