A watch might tell you that your fitness age went up, or that your estimated VO2 max went down. That can feel personal when all you did was take your usual walk. Before changing your routine, look at what the device is actually estimating.
Your watch is estimating a number, not grading you. It can be interesting information, but it is not the same as a clinical assessment.
What VO2 max describes
VO2 max refers to the maximum amount of oxygen your body can use during intense exercise. It is one way to describe cardiorespiratory fitness. In a laboratory, professionals can use specialized equipment during an exercise test; a watch instead calculates an estimate using the signals and information available to that model.
A watch may use heart rate, movement, activity details and information you entered about yourself. The exact calculation and eligible workouts depend on the product. One manufacturer, for example, describes estimates from certain outdoor walks, runs and hikes. Another device may use a different approach.
Where fitness age fits
Some products translate several measurements into a “fitness age” or a comparison with a reference group. The score is a product-specific way of presenting an estimate. It is not a measurement of the age of your organs, and two brands may give different results from the same day of activity.
Check what your app says goes into its score and when it last updated. A missing workout, changed profile detail or different measurement method may make comparisons less straightforward. If you change watches, a sudden difference may reflect the device as well as any change in you.
Why one reading can be off
A wrist sensor has to estimate rather than directly measure oxygen use. How it fits, the kind of activity recorded and the accuracy of the information in your profile can affect the data it receives. Certain health conditions or medicines that influence heart rate may matter for some algorithms, so ask your clinician how to interpret the result in your situation.
In a small study of one watch model, researchers found differences between watch estimates and laboratory measurements. That doesn’t tell you exactly how far your own watch is off, or establish the accuracy of every model. It is a reason to treat an individual number with some flexibility.
Some estimates are shown relative to body weight. If the weight stored in your profile is out of date, ask how your particular device uses that information before interpreting a change. Keep your details accurate when you choose to use the feature, but don’t treat a change in the score as proof of a change in health.
Use trends with care
If looking at the estimate motivates you, compare readings from the same device and similar activities over time. Read the manufacturer’s guidance about eligible workouts and update your profile when appropriate. There is no need to repeat a hard workout to make the score move.
If the score makes you anxious or pushes you to exercise through pain, hide the notification or take a break from it. A watch can’t decide what an appropriate exercise level is for you. If you experience chest pain, unusual breathlessness, dizziness or other concerning symptoms with activity, stop and seek appropriate medical advice; urgent or severe symptoms need emergency care.
Notice what you can do
You might care more about an easier walk to the store, getting up from a chair comfortably or enjoying a regular activity than a changing score. Those observations can be helpful to discuss with your care team alongside any measurements.
Our strength and function guide has more ways to notice progress. Keep the watch if it helps, and remember that your health is larger than a single estimate.