The Science of Fitness Tracker Calorie Burn

We’ve all guessed that our fitness band’s calorie count is probably off. But it’s shocking just how wrong many calorie burn estimations are.

You’ll learn: A more useful way to use calorie burn data, a better metric for measuring workout intensity, and why exercise isn’t great for weight loss.

Quick housekeeping:

• The 2% shop is live. In the words of the great American poet Odd Future, “Swag me out, b*tch.”
• I appeared on Mark Groves’ podcast. Here’s a link for those interested.
• I’ll be at MTN TOUGH’s TOUGHFEST in Bozeman, Montana, on Thursday. Come by and say hi, if you’re in the area.

On Monday, we covered fitness trackers and step counts. Read it here if you missed it (it was a holiday, after all). The comments on that post are fantastic.

Today we’re following up on that post. We’ll dive deeper into activity trackers and explore calorie counts. We’ll explain:

• How fitness trackers measure calorie burn
• Whether calorie burn counts are accurate
• A bit on why exercise isn’t great for weight loss—it’s a controversial theory worth exploring
• The problem with measuring exercise in calories

Let’s roll …

How fitness trackers measure calorie burn

It depends on the tracker, but most use a combination of data points.

First, they estimate your Basal Metabolic Rate (BMR), or the number of calories you burn doing nothing. This is why you enter your height, weight, and age when you get a new tracker.

Then they combine that BRM data with data from the band’s movement and heart rate sensors.

Those three metrics go into a formula that spits out calorie burn.

Are tracker calorie counts accurate?

In short

Nope.

The details

Many studies have shown that fitness trackers don’t measure calories accurately. But one from Stanford stands out and can help us understand this topic.

First, the Stanford scientists recruited 60 people from all different backgrounds. Then they purchased a bunch of units of seven different fitness bands—like Apple, FitBit, and Samsung. It was an expensive day at BestBuy.

Back in the lab, the researchers had the participants place up to two bands on each arm.

Next, they attached FDA-approved equipment for measuring heart rate and calorie burn to the participants. This equipment was:

1. A chest-worn ECG monitor for heart rate.
2. A device that is basically a gas mask. It accurately calculates calorie burn by analyzing the air you breathe in and comparing it to the air you breathe out (more on that here, for those interested).

Then they had the participants do a 40-ish-minute training routine. It included walking, running, and cycling at two different speeds each. Here’s the routine:

The scientists then compared the data from the medical devices to the data from the fitness bands.

• The result: “None of the devices provided estimates of energy expenditure that were within an acceptable range in any setting,” wrote the scientists.

The trackers overestimated calorie burn by anywhere from 27 to 93 percent.

Walking and running both had average error rates of around 31 percent. Cycling’s average error rate was 52 percent. The researchers noted that something about sitting while exercising seemed to make the trackers less accurate.

The problem with measuring exercise in calories

In short

There are better methods than calorie burn to measure the impact of your workouts.

The details

I can’t tell you the number of people who’ve contacted me to ask if I know of a fitness tracker that counts calories from rucking.