Why VO₂Max falls or does not rise

A smartwatch’s VO₂Max is an estimate calculated from information such as the relationship between speed and heart rate during exercise. Broadly, a lower number can reflect a change in your body or a change in estimation conditions. One reading cannot distinguish the two, so review how you feel and the calculation conditions in turn.

Reasons the smartwatch estimate may change

Hills, trails, soft surfaces, and traffic stops

Uphill or uneven terrain can raise heart rate at the same speed and produce a lower estimate. Polar’s technical document explains that without an altitude sensor, uphill routes tend to produce lower values and downhill routes higher values. Firstbeat’s method, used by Garmin and others, excludes segments involving soft ground, steep descents, and traffic stops, but cannot exclude every unsuitable condition. Some Garmin smartwatches also let you disable recording VO₂Max during trail runs.

Indoor exercise and treadmills

Apple Watch does not use indoor workouts for estimation. Google Health currently measures only outdoor running with GPS enabled, not treadmill exercise. Garmin Forerunner 265 also requires outdoor GPS records for running estimates. Polar’s technical document reports an indoor test with an uncalibrated foot pod in 47 people: estimates were 50±8 versus measured values of 55±7 mL/kg/min. That validation used an older algorithm.

Maximum heart rate settings

Many methods use the percentage of maximum heart rate at which you are running. An incorrect maximum heart rate setting can shift the estimate. Firstbeat’s technical document reports about 9% estimation error when age-predicted maximum heart rate is 15 bpm too low, about 7% when it is 15 bpm too high, and about 5% with actual maximum heart rate. Polar also emphasizes setting maximum and resting heart rate as accurately as possible. Age-based equations, such as 208 − 0.7 × age, have individual variation. If you already know a measured maximum heart rate, use it in your settings.

Changing devices

Reported errors vary in direction and size. The following studies involved different participants and measurement conditions; they are not direct comparisons of which model is better. Compared with laboratory measurements, Apple Watch underestimated by an average of 6.1–6.3 mL/kg/min: the 2025 study enrolled 30 people and analyzed 28; the 2026 study enrolled 40 and analyzed 35. In 35 endurance athletes, Garmin Forerunner 245 initially underestimated by 4.7 mL/kg/min on average, with greater underestimation in the fitter group. By contrast, a Forerunner 265 study reported values about 2.6 mL/kg/min above measured values after 12 weeks of training. In 19 healthy adults, Garmin fēnix 6 had a mean absolute percentage error (MAPE) of about 7.1% against a 30-second average of measured oxygen uptake. That study used a chest strap for heart rate, rather than wrist optical heart rate alone. Values before and after a device change cannot be compared directly.

Heat, altitude, and cold

Some Garmin models adjust estimates above 22°C or 800 m altitude. Heat acclimation features require GPS activities and weather data from a connected phone. Heat and altitude also change actual exercise capacity, so compare values under similar conditions. For cold conditions, Garmin recommends warming up indoors to stabilize wrist heart rate readings. However, a study testing ten devices in 45 healthy adults at indoor temperatures of 10°C, 23°C, and 36°C found no significant temperature effect on heart rate accuracy under those test conditions. Some devices had greater error when walking and stopping repeatedly. These were short indoor tests; the findings may not apply to outdoor or prolonged cold or heat exposure.

A longer session than usual

Even at a steady pace, the amount of blood pumped per heartbeat can begin to fall and heart rate rise after about 10–20 minutes: cardiovascular drift. In ten endurance-trained participants cycling for two hours at 22°C, stroke volume fell 15% and heart rate rose 10% between minutes 20 and 120 without fluid intake. Over the same period with fluid intake, stroke volume was maintained and heart rate rose 5%. In nine people exercising at 35°C, heart rate rose 12% from minute 15 to minute 45, associated with a reduction in VO₂Max. Later in prolonged exercise, a higher heart rate relative to speed may affect an estimate. Firstbeat says its method excludes segments affected by this prolonged rise in heart rate, but algorithms handle this differently.

Temporary changes in the body

Heat and dehydration

Heat itself can lower VO₂Max. In 13 men, values were 8% lower at 49°C than at 21°C and remained 7% lower after heat acclimation. Dehydration can add to the effect. A review reported consistently lower aerobic performance when dehydration exceeded 2% of body mass. A meta-analysis of time-trial exercise found smaller performance effects, so results depend on study conditions. This does not establish that dehydration is safe. Japan’s MHLW guidance recommends replacing water and sodium during exercise; replenish them regularly.

Poor sleep

A review of 69 papers on acute sleep loss in healthy adults found exercise performance was 7.6% lower on average after no more than six hours of sleep in 24 hours, compared with normal sleep of more than six hours. Morning tasks were less affected. Consistent reductions were observed after total sleep deprivation or earlier-than-usual waking. This combined different performance outcomes. It does not mean VO₂Max or a smartwatch estimate falls by the same percentage. Do not force yourself to exercise after poor sleep just to check the number.

After a cold, infection, or prolonged bed rest

A meta-analysis of nine studies found that people with symptoms at least three months after COVID-19 had peak oxygen uptake 4.9 mL/kg/min lower on average than those without persistent symptoms; certainty was low. Another analysis of 80 studies, mainly in young men undergoing strict bed rest for 1–90 days, reported a relationship equivalent to about a 0.4% daily reduction in body-mass-relative VO₂Max. These numbers should not be directly applied to an individual’s recovery or rest period.

Altitude

In eight endurance athletes tested in a hypobaric chamber, VO₂Max declined by 6.3% per 1,000 m between simulated altitudes of 300 and 2,800 m, with a reduction already evident at 800 m. A trip or training camp at altitude can temporarily lower the value.

Factors that change gradually

Body weight

mL/kg/min expresses oxygen uptake per minute divided by body weight. Even with the same oxygen uptake, gaining weight lowers the relative value and losing weight raises it. Update your smartwatch’s weight setting when your weight changes.

Age

A study following 810 community-dwelling adults without heart disease for a median of 7.9 years found that VO₂Max decline accelerated with age: about 3–6% per decade in the twenties and thirties and over 20% per decade from the seventies onward. These are group findings, not a fixed percentage to subtract from or use to correct your own value.

Smoking

In a study of 176 sedentary Indian men, smokers aged 20–29 had lower VO₂Max than nonsmokers of that age (38.9 versus 42.1 mL/kg/min). Among smokers in this age group, smoking more cigarettes and for longer was also correlated with lower values. This observational study alone cannot predict whose value will rise, or by how much, after quitting.

When to consult a doctor first

• You take medication that suppresses heart rate. In 15 healthy men and women, two weeks of the beta blocker propranolol reduced peak heart rate from 186 to 134 bpm and peak oxygen uptake from 42.3 to 39.1 mL/kg/min compared with placebo. Effects differ by medication and conditions of use. MHLW guidance states that heart rate cannot be used as an exercise intensity guide with beta blockers, some calcium channel blockers, alpha blockers, and certain other medications. Apple also notes that conditions or medication suppressing heart rate can lead to overestimation. The heart rate explanations on this page do not apply directly to people taking such medication.
• Breathlessness or palpitations during usual activities have worsened, or you have chest pain. Tell your doctor about the symptoms before focusing on the number.
• The value keeps falling for weeks without an apparent reason. A smartwatch value is a guide, not a diagnosis. Seek medical advice if you are concerned.

What if I train but the value does not rise?

If the factors above do not apply and you are exercising consistently, the amount or intensity of exercise may be relevant. See a comparison of ways to improve fitness for research methods and results, and HIIT for high-intensity methods. Repeat measurements using the same device on a similar route and in similar conditions to make comparisons easier.