Coggan's seven-level power model is calculated from FTP; common five-zone systems may use heart rate; and three-zone research models are usually separated by two physiological thresholds. The labels are not interchangeable: severe-intensity work called Zone 3 in a three-zone model can overlap work labelled Zone 5 in another system. Zone boundaries simplify a physiological continuum and should be interpreted with the rider's test method and observed response.
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