Why did my Apple Watch VO₂max drop?

Can one week of runs lower the number?

It can. Cardio Fitness is Apple's estimate of your VO₂max. VO₂max is your aerobic capacity. It is the most oxygen your body can use when you go all-out. You will see it written as mL/kg/min, and this page calls one of those units a point.

The estimate moves even when your fitness does not. Apple followed 221 people for about 390 days on average^[1]. A typical person's estimates spread by 1.2 points around their own average. One person in ten saw a spread of 2.6 points or more^[1].

Here is what that looks like with real numbers. Say your readings usually land near 42. A week of readings at 40 or 41 can come from that ordinary spread. For one person in ten, a reading near 39 can come from it too.

Across months the estimate holds together better. Apple compared each person's last estimate with one from more than 28 days before. The two agreed with a score of 0.86 on a scale where 1.0 is a perfect match^[1]. So the trend over months tells you more than any single week.

Which of your runs could have pulled it down?

Look at the walks and runs from the days before the drop. Apple's paper names the conditions that can pull an estimate low^[1].

• Heat. Extreme heat raises your heart rate.
• Not drinking enough. Dehydration raises your heart rate too.
• Caffeine. Apple lists it with the things that raise heart rate.
• High altitude. A recent move to high altitude is on the same list.
• A heavy load. A heavy backpack or a child you carry makes you work harder.
• Soft ground. Sand makes each step take more work.

The watch cannot see any of these. It sees a higher heart rate at your usual pace and reads it as lower fitness. The authors of a 2026 study of the Series 10 made the same point about terrain, slope and heat^[4].

Hills matter too. Apple Watch makes its estimate on fairly flat ground. Apple sets the limit at a slope under 5% up or down^[1]. A hilly route gives the watch fewer stretches it can use.

Apple also says pushing a stroller can lower the accuracy of an estimate or keep one from being made^[1].

If the drop came in the week after a race, running after a race covers what heart rate does in those days.

Did your training change?

A change in how you train can move the number with no change in your fitness.

Fewer outdoor workouts. Apple makes the estimate from Outdoor Walk, Outdoor Run and Hiking workouts^[2]. Indoor workouts are left out, and that includes workouts on gym equipment^[2]. A month on the treadmill gives the watch no new workouts to read. Apple says more outdoor walks, runs and hikes make an estimate more likely and more accurate^[1].

Walks in place of runs. The watch can also make an estimate during an everyday outdoor walk with no workout running. In Apple's own test those walking estimates came out close to the workout estimates^[1]. Apple also says harder efforts during workouts make an estimate more accurate^[1]. So a stretch of easy walks gives the watch fewer of the harder efforts that help it.

A loose watch. Apple suggests a tighter band for workouts, because the heart rate sensor should stay close to your skin^[3]. In the cold, blood flow in the skin of your wrist can drop too low for the sensor to get a reading^[3].

Your Health details. Apple's paper says a wrong age, sex or weight in the Health app can keep your estimates off^[1]. VO₂max is a figure per kilogram of body weight, so your weight is part of the number. The estimate also needs a sense of your peak heart rate, even though you never have to reach it^[1].

The guide to Apple Watch VO₂max accuracy covers how far the estimate lands from a lab test.

What does a real drop after a break look like?

A real loss of fitness after time off is a slide over weeks. A 1984 study followed 7 trained endurance athletes who stopped training^[5]. Their VO₂max fell 7% in the first 21 days. It kept falling and settled 16% below where it started by day 56. After 84 days it was still higher than in 8 people who had never trained^[5].

A 2022 study found the same early drop. Two weeks without training lowered VO₂max in 15 trained young men^[6].

A two-part review from 2000 split breaks into two kinds. In breaks shorter than 4 weeks, highly trained athletes lost VO₂max and blood volume quickly. People who had only started training lost less in that time^[7]. In breaks longer than 4 weeks, athletes lost more but stayed above people who had never trained. People who had trained only recently lost their recent gains^[8].

Fitness comes back with training. A 2024 case study followed one 53-year-old triathlete through 12 weeks off and 12 weeks back^[11]. His VO₂max for running fell 10.9% during the break. Twelve weeks of structured training brought it back to where it started. That is one person, so another runner may take more or less time.

Now put the sizes side by side. A 7% loss on a VO₂max of 45 is about 3 points. One person in ten sees a 2.6-point spread in their readings with no change in fitness at all^[1]. So one low reading cannot tell a three-week break from a hot week.

The shape over several weeks can. A break shows up as readings that keep sliding across comparable outdoor runs. A hot or hilly week can show up as a few low readings that come back once the weather cools or the route flattens. The table puts the two side by side.

How do you hold your fitness through a busy stretch?

Keep some hard running in the week and cut the volume. A 2000 review found that losses can be avoided or limited when training intensity stays the same and the number of sessions drops only a little. The total volume can drop a lot^[8].

Two studies show it. In a 1981 study, 12 young adults trained 6 days a week for 10 weeks^[9]. Then they trained either 4 or 2 days a week for 15 more weeks. Their sessions kept the same effort and the same length. VO₂max held at its trained level in both groups^[9].

In a 1990 study, 10 well-trained runners cut their weekly distance by 70% for 3 weeks^[10]. They ran 5 days a week in place of 6 and kept some intervals and races in the week. Their VO₂max and their 5 km race times did not change^[10].

Both studies kept the effort the same and cut how much training there was. A short busy stretch with that shape can leave your fitness close to where it was.

What should you do after a drop?

1. Check the conditions. Look at the heat, the hills and any load on the walks and runs before the drop.
2. Compare like with like. Read the number after flat outdoor runs in similar weather.
3. Use the longer view. The Health app shows your average for a day, a week, a month, six months or a year^[2]. The six month view shows the trend.
4. Check your Health details. Make sure your age, sex and weight are right^[1].
5. Rebuild after a break with easy running. Add easy minutes back week by week, at the pace easy run pace describes.
6. Give it weeks. The guide on VO₂max not improving shows how long a gain takes to show.

Easy is the pace you can hold a conversation at, and it builds your aerobic base. The Zone 2 you know is Diastra's Easy band. Type your age below to see your own bands.

The top band shares its name with the number on your watch. That VO₂max band is for hard intervals, and the guide on how to improve VO₂max shows how a week puts the two together.

Where does Diastra fit?

Diastra does not use the Cardio Fitness number. It reads every workout your Apple Watch saved in Apple Health, treadmill runs included. It finds your max heart rate in those workouts and draws Easy, Threshold and VO₂max from it. It credits only the minutes your heart held in a band. Over the weeks Diastra shows whether your training is working. Getting fitter. Holding steady. Slipping.