A smartwatch gives an estimate; a test with a metabolic analyzer gives a measurement. Watches calculate VO2 max from your heart rate, pace and personal details, and for many people the number lands in the right range, but it can be off by enough to put you in the wrong training zones.
A direct test measures the oxygen you actually breathe in and out while you exercise to your limit. At our clinic we use the VO2 Master analyzer for this.
How a watch gets its number
Your watch compares how fast you are moving with how hard your heart is working, then applies a formula based on your age, sex, weight and estimated maximum heart rate. It never measures oxygen.
Why the estimate can be wrong
- Wrist heart rate errors. Optical sensors can misread during intervals, in the cold, or with a loose strap.
- A guessed maximum heart rate. Age-based formulas can be well off for an individual.
- Conditions. Heat, hills, wind, trails and treadmill running all distort the pace-to-heart-rate relationship.
- Your sport. Estimates are built mainly on running and cycling. Hockey, strength training and team sports give the watch little to work with.
- Medication or illness that changes your heart rate.
What a watch is good for
Tracking your own trend. If the same watch, worn the same way on similar routes, shows your number rising over months, your fitness is very likely improving. Treat the direction as more reliable than the exact value.
When a measured test is worth it
- You want heart rate training zones based on your own physiology
- You are training for an event and want a true starting point
- Your watch number does not match how you feel or perform
- You want to check whether a training block worked
If you have a heart condition or have been told to avoid hard exercise, speak with your doctor before any maximal test.
See what happens during a VO2 max test? and what is a good VO2 max for my age? More detail is on the VO2 max testing page.
Want a measured number instead of an estimate?