Is your run smartwatch data lying to you?

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A deep dive into the tech and artificial intelligence that goes on your wrist

Smartwatches. Nearly every runner has one – a small computer loaded on your wrist that tracks data about your running. We’re going to talk about what a smartwatch can and can’t do for you you, so you can avoid the common pitfalls and make the most of your training.

What is a running smartwatch?

We know you’ve seen them and probably own one, but we’re going to make sure we’re all on the same page before really kicking off. A running smartwatch is a digital watch that captures data about your body while running and uses artificial intelligence (AI) to give you additional insights into your running.

The most popular ones are Garmin and Apple. And they don’t even need to be watches – Polar’s Whoop and Oura are smart rings (similar functions, smaller package). These smartwatches (or rings, but we’ll just say ‘watches’) connect with your phone to your favorite running app to deliver the insights, charts and performance stats that we all get way too hooked on. 

What do smartwatches measure?

Smartwatches give you tons of insights, but let’s dial it back a level and to the data a smartwatch actually measures:

  • Heart rate (current rate, variability and average resting HR)
  • GPS location
  • Time

From these three simple pieces of data, your watch does a whole bunch of fancy calculations. Open your smartwatch app, and you’ll see stats for:

  • Speed/distance of your last run
  • Quality of your sleep last night
  • VO2 max 

VO2 max is the maximum rate of oxygen your body can consume – the higher the number, the larger your body’s engine.

Most runners don’t know that VO2 max is calculated using AI and HR data – after all, that little device isn’t actually tracking the oxygen going into your lungs, is it? The faster you run (a stat that’s already being calculated using the time and your GPS location), the faster your HR  as a way to pump more oxygen. It’s this increase your watch uses to calculate your VO2 max.

Scroll through the features and you’ll also see recommendations about how long you need to recover from training load stress, which zone you’re training in, and so on.

What is a running smartwatch good for?

All this data on your wrist is pretty amazing! And for the most part, the data and insights are pretty good. Have a glance at your watch in the morning and it’ll tell you whether you’ve had a good or bad night’s sleep, and for the most part, it’s pretty spot-on.

30 years ago, you would have had to go into a lab to get data such like VO2 max and you’d need a track and a friend with a stopwatch to get accurate running split timings. Having this data available has opened up a world of insights to the average runner.

Smartwatch problem 1: accuracy

As much as technology has improved, the HR calculation on a smartwatch isn’t as good as a chest strap. There, we said it. Sorry for the bad news.

Your smartwatch uses an optical sensor to measure HR, meaning that LEDs and a light sensor are working together to determine how your blood is flowing by how much light goes through your tissues (high blood volume = less light). If your skin is too sweaty or you applied too much sunscreen, you may not get accurate results.

On the other hand, a chest strap uses electrodes to sense the electrical current of your heart beats, much more like a doctor using a stethoscope on your chest. And the strap is, of course, actually on your chest. It’s a much more accurate measurement.

Since most of the insights are based on the HR measurement, it’s easy to see how a small inaccuracy can ripple throughout all of your data. Daren’s VO2 max is 10 points lower in the lab compared to what his watch says.

Smartwatch problem 2: long-term fitness improvements

Even if accurate, VO2 max is a measurement of how you perform at a single time on a given day. If you’re tired, you’ll have a lower VO2 max than if you’re not. It’s a complex measurement and smartwatch insights aren’t designed to measure structural changes to your body’s system.

If you do a lot of aerobic training – which is great for your health and mitochondria development – your VO2 max will look like it’s going down. A famous case is Derek Clayton, who broke the marathon world record at 2:09. His VO2 max was 69, which is pretty low. If you make yourself an efficient running machine, you’ll need to use less oxygen.

If Darek Clayton had worn a smartwatch in the ‘60s and saw his VO2 max going down, it would have been wildly inaccurate for him to think he was becoming less fit. Because of course, the opposite was true.

Smartwatch problem 3: average data assumptions

Let’s talk training zones (Suggest link to 3 or 5 zone article) and recovery. As runners, we use training zones to guide our training intensity. And many runners also rely on the recovery recommendations on their smartwatch. But both of these numbers fall into the trap of making average data assumptions.

Most people get their watch, put in their date of birth and weight and they’re off on their run. But do you really think your health information is the same as every other person on the planet who was born on the same day and weighs the same as you? Of course not.

Mike’s HR is amazing (that’s what you get from 50 years of triathlons!) and so his smartwatch is always telling him he’s under training and that he should go out on a hard run.

And after a hard run, it’ll tell him he only needs 12 hours to recover, whereas he knows it’s actually 36-48 hours. A runner who doesn’t understand their own body and blindly follows the recommendations is going to be in for a world of hurt.

Smartwatch problem 4: where’s the coach?

Has your smartwatch ever asked you: what are you trying to accomplish right now? No, of course it hasn’t.

If you’re working with a coach, that’s the first thing they’ll ask. Because how you train for an Ironman is completely different than how you train for a 5k. Even if your watch is telling you that you’re doing aerobic or anaerobic training, you have to know what do to with that information and how it applies to your training routine.

A smartwatch is fairly objective. It’s running an algorithm based on your settings (see problem 3), but a coach adds a goal and subjectivity. A coach makes changes to your training based on what they see. A watch can’t. A watch can’t see your pace and say, “Oh, actually, you need to get into the weight room and do some strength work to give you more stability.” And if your goal is to become a better runner, you’ll need that.

The takeaway about smartwatches

Smartwatches give us a lot of data and insights, but they’re really just a guide. The journey for an athlete is about understanding your own body and digging deeper into what great performance looks like for them. 

By coming over to our blog today, you’ve done the first step in learning more about what a smartwatch can and can’t do for you – and that’s awesome! We’re all about breaking down the hype and trends in running so that you can integrate them into a program that works for you. We’re not going to give you anything that’s just ‘follow blindly’ advice – we want you to understand the research and how it works. And we have loads of experience to help you.

So, if improving and understanding stuff is your thing, you’re in the right place. Sign up for our newsletter, share us with friends and the full podcast is below.

What You Will Learn & Highlights

  • [2:48] What exactly the insights are for each platform/watch
  • [6:12] Why you still need a coach along with your smartwatch
  • [7:12] Why most smartwatches don’t give you an objective to work towards
  • [8:55] Take your smartwatch data with a grain of salt
  • [9:53] Recovery times are not optimised for everyone and Daren’s sum up
  • [12:50] Is Vo2 a necessary statistic to run fast?
  • [14:11] How do smartwatch insights help you run faster?
  • What we love about it and don’t like
  • Comparing Vo2max to Victoria’s Secret bras and measuring male genitals… yes… this one is fun
  • and more!



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