Your watch makes an educated guess. Here is what a real test measures, and why the difference matters for your plan.
Most smartwatches now show a VO2 max. It is a useful nudge, but it is an estimate built from your heart rate and pace, and in our clinic it is often wrong by five points or more in either direction.
What the test actually measures
On the bike you wear a light mask that samples every breath. We measure how much oxygen you take in and how much carbon dioxide you breathe out while the resistance rises every minute. The highest oxygen uptake you reach, per kilo of body weight, is your VO2 max.
Because we see every breath, we also see where your body switches fuels and where your breathing starts to strain. Those two points are what your training zones are built on.
Why five points matter
Moving from the bottom quarter of fitness for your age to the next quarter up is linked to a larger fall in risk than most medicines offer. A five-point error can put you in the wrong group, and give you the wrong plan.
So we test it properly, once a year, and let your watch do what it is good at: telling you whether this week was harder than last.
