Watches and tests

Why did my VO2 max drop?

By GyroscopeUpdated November 14, 20266 sources

Check the inputs first. Your watch learns only from certain outdoor workouts and divides by the weight in your profile. A real loss takes time off training, and trained people lost about 7 percent in their first three weeks off.

A drop of two or three points can feel like a month of training wasted. Often it isn’t anything you did. A watch estimate moves when its inputs move, and a review of 14 wearable studies put a change that’s meaningful for health at 1.75 to 3.5 points, which is smaller than the error of a single watch.

We’d ignore any drop under 3 points, check your profile weight and workout mix for anything bigger, and retest the same way before deciding you’ve lost fitness.

The weight in your profile

VO2 max is the oxygen you use per kilogram, so the watch divides by the weight you gave it. It’s the same engine in a heavier car. Update a 180-pound profile to 190 pounds and a 42 becomes about 39.8 overnight, with your heart unchanged. Apple uses age, sex, weight and height, and Garmin asks for a full profile. Try it in the weight tool below.

The Dallas men showed the same thing in real life. Over 30 years their oxygen use fell only 11 percent in liters a minute, but per kilogram it fell 28 percent, from 43 to 31, because they’d gone from 77 to 100 kg (170 to 220 lb).

The workouts it saw

• Apple Watch only learns from Outdoor Walk, Outdoor Run and Hiking workouts. Swap your outdoor runs for a treadmill or the gym, and it’s left with your walks.
• Garmin learns from outdoor runs of 10 minutes or more above 70 percent of your max heart rate. It lets you turn off VO2 max recording for trail and ultra runs if those pull the number around.
• A changed max heart rate in your profile changes how hard every run looks to the watch.

A real drop

If you stopped training, the loss is real and fast at first. Seven trained endurance athletes stopped completely, and their VO2 max fell 7 percent in 21 days. It settled about 16 percent lower by day 56, and even then they were still fitter than people who had never trained.

Aging is slow by comparison. In healthy adults followed for about eight years, VO2 max fell 3 to 6 percent a decade in the 20s and 30s. That’s well under 1 percent a year, too small to explain a drop you noticed in a month.

What to do

Retest the way you tested before, ideally with the same field test on the same course (see how can I test my VO2 max at home?). If you took time off, how fast do you lose VO2 max? has the full curve, and it comes back with training.