I let Fitbit Air and Whoop automatically track the same 8km run

Fitbit Air and Whoop are supposed to work in the background, so I decided not to help either of them. I wore one on each ankle for a run and let both figure it out for themselves, while a Garmin Forerunner on my wrist logged 8.10km in 47 minutes.

I left both trackers alone

Fitbit Air and Whoop are designed to blend into the background. You wear them, get on with your day and check the results later.

That convenience formed a big part of my earlier Fitbit Air versus Whoop comparison. This time, I wanted to see what happens when neither tracker receives any help. Will they properly track my run?

I wore Fitbit Air on one ankle and Whoop on the other because I wanted to use both as secondary background trackers while keeping my Garmin Forerunner on my wrist. I did not start a workout from either phone app, so both devices had to recognise the run automatically.

Garmin measured 8.10km in 47 minutes and 17 seconds, with an average heart rate of 139 bpm and a maximum of 161 bpm. An easy run. The total elapsed time was 49 minutes and 15 seconds because I stopped briefly mid-session.

Whoop recorded far too much

Whoop recognised the activity as walking (which I then changed to running in the app). But it placed the start time at 3:06pm and the finish at 5:53pm. That gave me a running workout lasting 2 hours, 47 minutes and 59 seconds.

Now, I was actually walking before and after the run. So Whoop appears to have pulled much of that surrounding movement into a single activity. The result was 11,938 steps, 1,169 calories and an activity strain of 15.8.

Its average heart rate came in at only 114 bpm, well below Garmin’s 139 bpm. Whoop also recorded a maximum of 180 bpm, compared with 161 bpm on the Garmin.

The heart-rate graph makes the problem obvious. It includes long low-intensity sections, repeated drops and several sharp changes that do not resemble one continuous 8km run.

Whoop detected that something was happening, but it failed to understand when the run actually started and ended. That undermined almost every number attached to the activity.

Fitbit Air missed the opening section

Fitbit Air produced a much cleaner activity, but it only recorded only 6.11km in 39 minutes and 34 seconds. Its detected session started at 4:26pm, around ten minutes after I began the Garmin activity.

The timing suggests Fitbit Air missed roughly the first 2km, then followed the rest of the run through to the finish. The brief stop appears as a clear dip in the heart-rate graph, but the recorded session continues afterwards.

Fitbit’s average heart rate of 142 bpm was only 3 bpm above Garmin, which is a solid result given the missed part at the beginning when my heart rate was low. The maximum heart rate was 159bpm, so 2bpm lower than the Garmin. The Fitbit also counted 6,580 steps and estimated 513 calories.

There was no route map in the app, so I cannot confirm that Fitbit Air used the phone’s GPS. The 6.11km figure may have come from movement and stride estimates instead.

The bigger issue is the missing start. Google Health does not give you an editing option. So it cannot recreate the first ten minutes and turn the partial record into a complete 8.1km activity.

Background tracking still needs checking

So what’s the takeaway?

Fitbit Air performed better once it recognised the run. Its heart-rate average and max was very close to Garmin and its finish time looked accurate.

Whoop captured far too much, while Fitbit captured too little. Neither produced a record I could accept without checking it against the Garmin.

This was an ankle test, so it should not be treated as a verdict on normal wrist performance. But for devices built around passive tracking, the basic lesson is hard to ignore. Wearing them and forgetting about them still means checking their work afterwards.

For a deeper look at each device, read my full Fitbit Air review and Whoop review.