COCO

A recent Cambridge study validated a simple non invasive and low risk process for measuring overall fitness level focused on V02MAX using simple monitoring and no testing ‘to the point of exhaustion’ or other capacity centric exercise activities. 11,000 participants over seven years.

https://www.cam.ac.uk/research/news/fitness-levels-can-be-accurately-predicted-using-wearable-devices-no-exercise-required