Received a low cardio fitness notification?

Apple Watch estimates VO₂Max from information including heart rate during outdoor walking, running, and hiking, and records it as Cardio Fitness in the iPhone Health app. An estimate classified as low for your age and sex may trigger a notification. Here is what that notification does and does not mean, based on Apple’s guidance and research.

What triggers the notification?

Apple’s support page, published in September 2026, says a notification can appear when cardio fitness is low for your age and sex, with additional notifications if it remains low. Classification applies to people aged 20 and older.

The support page does not specify the numerical low threshold. Apple’s May 2021 technical document describes the following:

• Notifications are enabled by the user after reading the feature explanation in Health and entering information including age, sex, and medication.
• They are issued when estimated VO₂Max is consistently and confidently low. Apple describes this as a level suggesting risks to long-term health or current functional limitations in daily life.

What is FRIEND?

The data came from eight US testing laboratories that contributed to FRIEND from January 2014 to February 2015. The analysis used 7,783 maximal treadmill tests with respiratory gas analysis in men and women aged 20–79 without cardiovascular disease. It calculated percentiles—the position from the lower end—by sex and ten-year age group. Median values were 48.0 for men and 37.6 for women in their twenties, and 24.4 for men and 18.3 for women in their seventies, in mL/kg/min. Values were about 10% lower per decade across age groups. This was not a long-term follow-up of the same individuals.

The Japanese MHLW tables use different data

Table 1 in Japan’s Physical Activity and Exercise Guide for Health Promotion 2023 gives estimated means synthesized from 23 studies of Japanese people, totaling 54,611 men and 24,100 women. FRIEND uses US laboratory data; the values below are its medians. Populations, measurement methods, and statistical measures differ, so the same age groups need not have matching values.

FRIEND medians are higher in the twenties; estimated Japanese means are higher in the seventies. A smartwatch’s low classification can therefore differ from your position in the MHLW tables. See Japanese age-group VO₂Max values for averages and reference values, or use the comparison tool.

MHLW Table 2 presents reference values expected to be associated with lower risks of developing lifestyle-related diseases and related mortality. Both tables are guides: being below a value does not establish a disease.

Smartwatch estimates have errors

Two studies compared Apple Watch estimates with laboratory measurements using maximal treadmill exercise and respiratory gas analysis:

• A study enrolled 30 healthy adults, tested them after five to ten days of wear, and analyzed the 28 who met maximal-test criteria. Estimates were 6.1 mL/kg/min below measured values on average, with a mean absolute percentage error of about 13%. The 95% limits of agreement for smartwatch minus measured values were −18.3 to 6.1 mL/kg/min (Lambe et al., 2025).
• An Apple Watch Series 10 study enrolled 40 healthy adults and analyzed 35, with a mean age of 22.7. Estimates were 6.3 mL/kg/min lower on average; the 95% limits of agreement for smartwatch minus measured values were −18.2 to 5.7 mL/kg/min. The FRIEND percentile band matched the measured result in five participants (14%); 12 (34%) were in an adjacent band (Lambe et al., 2026).

The 95% limits of agreement are calculated from variation in study differences. They are neither the observed minimum and maximum nor guaranteed bounds for an individual’s error. Both studies mainly involved young, healthy, fit people. The same average differences have not been established in people with low fitness, older adults, or actual notification recipients. You cannot correct your display by adding the average difference.

Apple’s technical document reports a mean difference of 1.4 mL/kg/min with a standard deviation of 4.7 mL/kg/min in a validation sample of 221 people. However, its reference was not a direct maximal exercise measurement: it extrapolated the relationship between heart rate and oxygen uptake during submaximal exercise to age-predicted maximum heart rate. Because the reference method differs, these findings cannot be directly compared with the external studies.

It is plausible that estimation error could produce a different notification classification than direct measurement. However, these two studies did not test the accuracy of low cardio fitness notifications. Do not diagnose yourself from a notification or ignore it because of a study-average difference. Check settings, measurement conditions, and symptoms.

What to check after a notification

Health app settings

• Are age, sex, weight, and height correct? These inputs are used in estimation. Apple’s technical document says incorrect age, sex, or weight can cause systematic bias. Update your weight when it changes.
• Medication affecting heart rate. If medication that suppresses heart rate, such as beta blockers or calcium channel blockers, is not recorded, VO₂Max can be overestimated. Apple’s validation reported a reduction in error from 11.8±4.0 to 1.6±3.1 mL/kg/min with correct medication entries. The document also says that when medication does not consistently lower maximum heart rate, such as occasional low doses, not recording it may be more accurate. If unsure how to enter medication information, ask the prescribing doctor or a pharmacist.

Conditions when the estimate was made

Apple’s technical document lists conditions that may lower the estimate or reduce accuracy:

• Carrying a heavy backpack or carrying a child while walking
• Surfaces that require extra effort, such as sand
• Pushing a stroller or using a walking aid, which may reduce the likelihood or accuracy of estimates
• Dehydration, extreme heat, or a recent move to altitude, where a higher heart rate can lower the estimate
• Pregnancy, when physiological changes may reduce accuracy

For hills, indoor exercise, maximum heart rate settings, prolonged exercise, and factors affecting other models too, see why VO₂Max falls or does not rise.

Conditions that can produce an estimate

According to the 2021 technical document, estimates may be produced during outdoor walking, running, or hiking on relatively flat ground with a grade below 5%, sufficient GPS and heart rate signals, and a heart rate elevated by roughly 30% of the reserve between resting and maximum heart rate. Apple says repeated outdoor workouts, some exercise intensity, and all-day wear beyond exercise can improve accuracy.

This is not a reason to push harder just to measure. Rest when unwell. Stop immediately if chest pain, dizziness, or similar symptoms occur during exercise (starting safely).

Look at trends

Review several records from walks or runs under similar conditions instead of one reading. Health lets you switch between daily, weekly, monthly, six-month, and yearly averages. If past estimates can be recalculated more accurately, an option to update data appears on the Cardio Fitness screen.

Low fitness and health

Lower cardiorespiratory fitness assessed through exercise testing is associated with higher mortality and heart disease risk. A review of 33 cohort studies in healthy adults adjusted for age and sex using a scale standardized to 50-year-old men. On that scale, the low-fitness group (below 7.9 METs) had an all-cause mortality relative risk of 1.70 compared with the high-fitness group (at least 10.9 METs; Kodama et al., 2009). These categories cannot be applied directly to an individual’s raw smartwatch or measured value. A review of 34 cohort studies reported an all-cause mortality relative risk of 0.88 per 1-MET higher fitness (Han et al., 2022).

Both syntheses concern observational associations and fitness assessed by exercise tests, not smartwatch estimates. A single notification does not determine an individual’s future. See VO₂Max and health for how to interpret the research.

When to consult a doctor first

• Breathlessness or palpitations during usual activities have worsened, or you have chest pain. Tell your doctor about symptoms before focusing on the number.
• You have heart or lung disease, a condition affecting heart rate, or a cardiac device. Apple’s technical document says a blunted heart rate response to exercise, which can occur with heart failure or chronic obstructive pulmonary disease, may cause overestimation. Arrhythmias, pacemakers, pain, and conditions that make walking difficult can also reduce accuracy. Prioritize your clinician’s assessment over the smartwatch classification.
• You have questions before beginning exercise. See starting safely for a review of medical conditions, medication, and recent activity.

If you want to improve

Apple’s support page says most people can increase VO₂Max by increasing the intensity or frequency of aerobic exercise. A synthesis of controlled trials in healthy adults aged 18–45 found a tendency toward larger training gains among those with lower starting VO₂Max: 28 trials, 723 participants, mean age 25 (Milanović et al., 2015).

If you have been inactive, check your health, medical conditions, and medication, start with short periods of light exercise, and increase gradually as appropriate (moderate continuous exercise). See a comparison of ways to improve VO₂Max for research findings by method, and viewing VO₂Max on a smartwatch and phone for device instructions.