What are Heart Rate Zones
Heart rate zones are defined ranges of exercise intensity — expressed as a percentage of maximum heart rate — that correspond to different physiological effects, from active recovery and fat burning to anaerobic threshold and maximum effort.
Heart rate zones are ranges of exercise intensity, each corresponding to a percentage of your maximum heart rate (MHR) and producing distinct physiological training effects. Training across different zones allows you to target specific adaptations — fat oxidation, aerobic base building, VO2 max improvement, or pure speed and power development.
Most endurance coaches and sports scientists use a 5-zone model (some use a 3-zone or 7-zone model), with higher zones representing more intense effort and a greater demand on the cardiovascular and metabolic systems.
Estimating Your Maximum Heart Rate
The simplest formula: MHR = 220 − age
| Age | Estimated MHR |
|---|---|
| 20 | 200 bpm |
| 30 | 190 bpm |
| 40 | 180 bpm |
| 50 | 170 bpm |
| 60 | 160 bpm |
This is an estimate with significant individual variation. A true MHR can only be determined through a maximal exercise test or a very hard race effort. Many people have MHRs noticeably above or below their age-predicted value.
The 5 Heart Rate Zones
| Zone | % of MHR | Perceived Effort | Primary Effect |
|---|---|---|---|
| Zone 1 | 50–60% | Very light; can hold long conversation | Active recovery, blood flow |
| Zone 2 | 60–70% | Light; easy conversation | Aerobic base, fat oxidation |
| Zone 3 | 70–80% | Moderate; can speak in sentences | Aerobic endurance, cardiovascular efficiency |
| Zone 4 | 80–90% | Hard; few words at a time | Lactate threshold, race pace fitness |
| Zone 5 | 90–100% | Maximum; can't speak | VO2 max development, peak power |
Zone 2 Training: Why It Matters
Zone 2 has received significant attention in endurance and longevity research. Extended time in Zone 2 develops mitochondrial density in slow-twitch muscle fibers — improving the body's ability to produce energy aerobically and oxidize fat. Elite endurance athletes spend roughly 80% of their training volume in Zone 2. This zone is also often called the fat-burning heart rate zone, because fat provides the highest proportion of energy during moderate-intensity aerobic exercise.
For general fitness, Zone 2 represents the intensity foundation that makes all higher-intensity training more productive. Most recreational athletes spend too much time in Zone 3 (sometimes called the "grey zone" — too hard to be truly easy, too easy to be a meaningful stimulus) and too little in Zone 2.
Zone 4 and the Lactate Threshold
Zone 4 training targets the lactate threshold — the intensity at which lactate begins to accumulate in the blood faster than it can be cleared. Training at and around threshold pace improves the speed you can sustain before accumulating fatigue. Most competitive runners and cyclists do one or two threshold workouts per week alongside their Zone 2 base.
Zone 5 and HIIT
Zone 5 work corresponds to the intensity targeted in HIIT — efforts at 90–100% of maximum heart rate lasting 20 seconds to several minutes each. These intervals primarily drive VO2 max adaptations and are highly demanding; 2–3 sessions per week is appropriate for most athletes.
Tracking Heart Rate Zones
Chest straps (Polar, Garmin HRM) are the most accurate option for real-time heart rate during exercise. Optical wrist sensors (Apple Watch, Garmin, Polar, Whoop) are convenient but less accurate during high-intensity movement due to motion artifact.
Most modern GPS watches automatically calculate your heart rate zones based on either age-predicted MHR or from user-entered data. Garmin's health and fitness tracker lineup includes models with real-time zone tracking for running, cycling, and swimming.
Practical Training Application
A well-structured training week for aerobic fitness might look like:
- 3–4 runs/rides in Zone 2 (base volume)
- 1 threshold session (Zone 4)
- 1 HIIT session (Zone 5)
- 1–2 rest days
This distribution captures the full adaptation spectrum — from mitochondrial efficiency to VO2 max ceiling — while managing cumulative fatigue.