Heart rate zones
Heart rate zones divide exercise intensity into bands, usually five, based on a percentage of your maximum heart rate, heart rate reserve or threshold heart rate. Training by zone helps keep easy days easy and hard days at the intended effort.
Three ways to set them
Percentage of maximum heart rate is the simplest. Heart rate reserve (the Karvonen method) uses the gap between your maximum and resting heart rate: target = (max − rest) × percentage + rest. Zones based on threshold heart rate, found with a 20–30 minute test, are the most closely tied to how you perform.
Their limits
Heart rate lags behind effort, so it is a poor guide for short intervals. It also drifts upward over long sessions, in heat and when you're tired or dehydrated. Use heart rate for steady efforts, and power, pace or perceived effort for intervals.