The easiest aerobic effort zone using a very comfortable, conversational pace. Heart rate is typically 50-60% of maximum. This zone is perfect for recovery runs, warm-ups, and building aerobic base with minimal stress. You should be able to breathe only through your nose at this intensity.
Example: Your easy recovery run the day after a hard workout should be in Zone 1.
Training at mild effort, around 60-70% of maximum heart rate. The pace should feel slightly uncomfortable but sustainable, allowing easy conversation. This is the foundation zone for building aerobic base and should comprise the majority of your training volume.
Example: Most of your weekly mileage, including long runs, should be in Zone 2.
Moderate, sustainable effort at 70-80% maximum heart rate. This "tempo" zone improves running economy and builds sustainable endurance. Conversation becomes difficult but you can still speak in short sentences. This zone bridges easy running and hard efforts.
Example: Tempo runs and marathon pace efforts typically fall in Zone 3.
Hard but sustainable effort at 80-90% maximum heart rate, at or near lactate threshold. Training here improves your threshold pace and ability to clear lactate. Conversation is very difficult, only a few words possible. These efforts are challenging but controlled.
Example: Threshold runs and the hard portions of interval workouts are typically Zone 4.
Maximum effort zone at 90-100% maximum heart rate. This is sprinting or racing all-out. Unsustainable for more than a few minutes, often resulting in deteriorating form if extended too long. Used for short, intense intervals to improve VO2 max and top-end speed.
Example: All-out 400m repeats or the final sprint of a 5K race are Zone 5 efforts.