Zone Drift and Fitness Changes
Your heart rate zones are not static — they shift as your fitness improves or declines. As you build aerobic fitness, your lactate threshold heart rate increases, which means Zone 2 expands upward and higher zones shift accordingly. An athlete who starts training with a lactate threshold of 155 bpm might see it rise to 165 bpm after three months of consistent training. If they don't update their zones, they'll be training too easily — what used to be Zone 3 is now comfortable Zone 2. Experts recommend retesting your zones every 8–12 weeks, or whenever you notice that your 'tempo' efforts feel easier than expected. For power zones (cycling), FTP testing every 6–8 weeks is standard practice. The Zone Builder lets you quickly recalculate and export updated zones to your Garmin device, so you're always training with current targets. Another phenomenon to be aware of is cardiac drift during long sessions — heart rate gradually rises even at constant pace due to dehydration, core temperature increase, and cardiac fatigue. This is normal and doesn't mean your zones are wrong; it means your Zone 2 pace may be slower in hour 3 than in hour 1 of a long run.
Heart Rate Training Zones Explained
Training zones divide your heart rate range into bands of intensity, each targeting different physiological adaptations. Using zones correctly helps you balance training stress and recovery to maximise fitness gains.
The Karvonen Method
When you provide a resting heart rate, this calculator uses the Karvonen (heart rate reserve) method, which is more personalised than simple max-HR percentages:
Target HR = ((MaxHR − RestHR) × intensity%) + RestHR
Entering Zones in Garmin Connect
Garmin devices don't import zone files — you need to enter BPM ranges manually. In Garmin Connect web, go to Training → Heart Rate Zones, select your device, and enter the BPM values for each zone. The same page controls your watch's zone display during activities.