Zone 1 (60-65% of maximum heart rate): For long, easy rides, to improve the combustion and storage of fats.
Zone 2 (65-75% of MHR): The basic base training zone. Longish rides of medium stress.
Zone 3 (75-82% of MHR): For development of aerobic capacity and endurance with moderate volume at very controlled intensity.
Zone 4 (82-89% of MHR): For simulating pace when tapering for a race.
Zone 5 (89-94% of MHR): For raising anaerobic threshold. Good sessions for 10- and 25-mile time-trials.
Zone 6 (94-100% of MHR): For high-intensity interval training to increase maximum power and speed
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By Harry Blackwood, Cycling Plus