[jonathansizz]

For cycling, I do a long warmup then ride as hard as I can (uninterrupted) for 30 minutes and take the average HR for the last 20 of the 30 minutes. There are online calculators you can plug this value into to give you the 5 zones.*

I do a test every 3 months, as the zones can shift as you get fitter.

Running might use a different protocol, but the same principle applies.

* These days I base my training on a 3-zone system: low intensity, below aerobic threshold; medium intensity, between aerobic and anaerobic (lactate) threshold, and high intensity, above lactate threshold. This is the system preferred by researchers, as it's based on real physiological markers.

Translating the 3 zone system into the 5 zone: low intensity equates to zones 1-2, medium is zone 3 and zone 4 up to your (lactate) threshold value, and high intensity is everything above this, i.e. the top part of zone 4 and all of zone 5.

So 'vigorous' per the Mayo Clinic would be medium intensity on the 3 zone system.