Heart rate training zones divide exercise intensity into bands based on percentage of maximum heart rate (MHR). A common formula estimates MHR as 220 minus age — though this is population-level and can vary by ±10–15 bpm individually. Each zone targets different physiological adaptations. Training across multiple zones provides the most complete cardiovascular fitness.
The five main zones
| Zone | % of MHR | Feel | Primary benefit |
|---|---|---|---|
| Zone 1 (Very light) | 50–60% | Easy conversation, barely elevated | Recovery, base fitness |
| Zone 2 (Light / aerobic base) | 60–70% | Comfortable, can speak in sentences | Fat burning, aerobic base, mitochondrial density |
| Zone 3 (Moderate) | 70–80% | Harder to hold conversation | Cardiovascular efficiency |
| Zone 4 (Hard / threshold) | 80–90% | Difficult to speak, can sustain for 20–40 min | Lactate threshold, speed |
| Zone 5 (Maximum) | 90–100% | Cannot sustain > 1–2 min | VO₂ max, peak power |
Why Zone 2 gets so much attention
Zone 2 training — comfortable, conversational intensity, typically 60–70% of MHR — has attracted considerable interest from longevity researchers and elite endurance coaches. At this intensity, the body preferentially uses fat as fuel and the stress on the cardiovascular system is low enough to allow high training volume without accumulated fatigue.
Research on mitochondrial biogenesis shows that Zone 2 is the most efficient stimulus for increasing mitochondrial density in muscle cells — a marker of metabolic health strongly associated with longevity. For general health, 150–180 minutes of Zone 2 per week is a target supported by the available literature.
How to calculate your zones
Start with an estimated MHR: 220 minus your age. Apply the zone percentages. For a 40-year-old, MHR ≈ 180 bpm. Zone 2 would be 108–126 bpm. Verify this with a talk test — if you can hold a conversation comfortably, you're likely in Zone 2; if sentences become fragmented, you've moved into Zone 3.
More accurate MHR can be determined with a maximal effort test (e.g. a 3 km time trial, checking peak heart rate) or a field test with a heart rate monitor. Consumer wrist-based optical heart rate monitors are less accurate during high-intensity exercise; chest strap monitors (Polar, Garmin) are more reliable.
Heart rate in health monitoring
Beyond exercise, resting heart rate (RHR) is a useful health marker. A lower RHR reflects greater cardiovascular fitness and parasympathetic tone. Tracking RHR over time — available from most wearables — can detect early signs of illness (a sudden 10+ bpm rise often precedes symptoms by 1–2 days), overtraining, or improvements in fitness.
"Fitness is not built in the high-intensity minutes. It is built in the months of consistent, well-targeted effort across all zones."
Omar Khalil, Health Educator, Looms