Heart Rate Zones Explained: Training by Heart Rate

Why Train With Heart Rate?

Heart rate is one of the most accessible and informative metrics available to endurance athletes. Unlike pace, which is affected by terrain, wind, and fatigue, heart rate reflects your body's internal effort. Training by heart rate zones ensures you are working at the right intensity for each session's purpose, building aerobic capacity on easy days and pushing lactate threshold on hard days.

Setting Your Zones

Heart rate zones are typically based on your maximum heart rate (HRmax) or your lactate threshold heart rate (LTHR). The LTHR method is generally more accurate because maximum heart rate varies significantly between individuals regardless of fitness.

To estimate your LTHR, perform a 30-minute time trial at maximum sustainable effort. Your average heart rate over the final 20 minutes approximates your lactate threshold. From there, zones are calculated as percentages of this value.

The Five-Zone Model

Zone 1 - Active Recovery (below 68% of LTHR): Very easy effort. Used for warmups, cooldowns, and recovery sessions. Promotes blood flow without adding meaningful training stress.

Zone 2 - Endurance (69-83% of LTHR): The foundation of endurance training. Comfortable, conversational pace. Develops aerobic efficiency, fat oxidation, and mitochondrial density. Most of your weekly training volume should be here.

Zone 3 - Tempo (84-94% of LTHR): Comfortably hard effort. You can speak in short sentences. Improves muscular endurance and lactate clearance. Useful but should be applied deliberately, as it creates more fatigue than Zone 2 without the peak stimulus of higher zones.

Zone 4 - Threshold (95-105% of LTHR): Hard effort at or near your lactate threshold. Sustainable for 20 to 60 minutes in trained athletes. This is where you push the ceiling of your aerobic capacity higher.

Zone 5 - VO2max and Above (above 105% of LTHR): Very hard effort for short intervals. Develops maximum oxygen uptake and anaerobic capacity. Intervals typically last 2 to 5 minutes with equal or longer recovery periods.

The 80/20 Principle

Research consistently supports the polarized training model: approximately 80 percent of your training time in Zones 1 and 2, and 20 percent in Zones 4 and 5. This distribution produces better long-term results than spending excessive time in the moderate Zone 3, sometimes called the gray zone.

Heart Rate Drift and Decoupling

During long sessions, heart rate gradually increases even at a constant pace, a phenomenon called cardiac drift. This is normal and caused by dehydration and thermal regulation. Excessive drift, more than 5 percent, may indicate inadequate hydration or that the session intensity is too high.

Heart Rate in Strilab

Strilab uses your heart rate to calculate the training stress of every session when there is no power or pace threshold to rely on, and it checks how your heart rate behaves inside each workout: drift across intervals and aerobic decoupling on long, steady efforts. Add your resting heart rate and threshold heart rate in your athlete settings and the numbers become yours instead of an estimate. See how it fits into running and triathlon training.