Heart rate zones explained: what each zone trains

Heart rate zones are a framework for translating training intensity into physiological adaptation targets. The five-zone model used in most endurance literature isn't arbitrary: each zone corresponds to specific metabolic and cardiovascular adaptations.

Understanding zones helps you train deliberately rather than by feel.

The five zones#

A common formulation (Coggan, Friel, polarised training literature):

Zone 1: Recovery (50–60% HRmax). Very easy effort; you can speak in full sentences and could comfortably read aloud. Used for warm-up, cool-down, and recovery days.

Zone 2: Aerobic base (60–70% HRmax). Conversational pace; full sentences but couldn't comfortably sing. Lactate stays <2 mmol/L. Builds mitochondrial volume, capillary density, fat oxidation capacity.

Zone 3: Tempo (70–80% HRmax). Strong but sustainable; speech becomes choppy. Lactate begins to rise (~2–3 mmol/L). The "grey zone", felt productive but accumulates more fatigue than zone 2 for similar fitness benefit.

Zone 4: Threshold (80–90% HRmax). Hard; can speak in short phrases. Lactate ~3–4 mmol/L (rises sharply). The functional threshold zone, sustainable for ~1 hour in trained athletes.

Zone 5: VO2 max / supra-threshold (90–100% HRmax). Very hard; can't sustain conversation. Lactate accumulating rapidly. Limited to intervals; the zone where VO2 max gains happen.

How to find your zones#

The "MAF formula" (220 - age = HRmax) is roughly accurate for population averages but can be off by 10–20 bpm for individuals. This is the source of widespread frustration with HR-based training.

Better methods:

Lab testing. Gas exchange (CPET) with ventilatory thresholds, the precise method. £150–£300 privately in the UK.

Field testing. 20-minute time trial at maximum sustainable pace. Your average HR is roughly your threshold (zone 4 upper). Subtract appropriate percentages for lower zones. Several published protocols exist (Joe Friel's, Steve House's).

Lactate testing. Finger-prick lactate during incremental exercise. Available at some sports labs; gives the most direct mapping to zones.

Pace or power-based zones. Where available (running pace, cycling power), pace and power zones are more reliable than HR zones for hard efforts because HR drifts and lags.

Why zone 2 plus high intensity beats endless zone 3#

This is the "polarised training" insight that has reshaped endurance training:

• Zone 2 builds the aerobic substrate at low cost: minimal recovery needed
• Zone 4/5 drives the high-end adaptations
• Zone 3 ("the grey zone") produces aerobic-like fatigue without aerobic-base benefit

Descriptive studies of elite endurance athletes report roughly 80% of training sessions at low intensity (zone 1–2) and about 20% dominated by high-intensity work (zone 4–5), with little time in zone 3 (Seiler 2010). Recreational athletes often invert this: too much zone 3, not enough easy or hard work.

The "polarised" or "80/20" distribution is supported by both observational data on elite athletes and intervention trials.

Common mistakes#

"My easy days are too hard." Zone 2 should feel embarrassingly easy when you're starting. If you can't have a full conversation, slow down.

"My hard days are too easy." Zone 4–5 intervals should feel genuinely hard. If you can sustain the pace forever, it isn't an interval.

"I'm always in the grey zone." The most common amateur pattern. Too hard to be properly easy, too easy to be properly hard. Move both endpoints further apart.

"My wrist HR is wrong." Optical wrist HR is unreliable during interval work, weight training, and certain motions. Chest strap is the consumer-accessible gold standard.

Maximum heart rate#

The 220 - age formula is a population average with substantial individual variability. Your actual HRmax can be 15+ bpm off the formula.

The only reliable way to know your HRmax is to actually approach it during testing or hard intervals. For most adults, a maximum heart rate reached during the last 30 seconds of a maximal effort is a usable reference.

HRmax declines with age but varies hugely between individuals. Some trained athletes maintain near-youthful HRmax into their 50s; some untrained adults have lower HRmax in their 30s. Don't take the formula too seriously.

Wearable zone displays#

Most wearables (Garmin, Apple, Whoop, Polar) display zones during workouts. These typically use a HRmax-based estimate. If your actual HRmax is different from the device's assumption, the zone displays will be off.

For Garmin and Polar, you can manually set HRmax in the settings. For Apple Watch, the workout zone display defaults to formula-based.

If you want reliable zone tracking, set your HRmax based on actual data, not formula.

Application to other modalities#

The zone framework applies most cleanly to endurance modalities (running, cycling, rowing). For:

• Strength training: HR-based zones don't apply usefully, different physiology
• HIIT classes: can be useful if HR is monitored
• Yoga / mobility: zones generally not relevant
• Swimming: HR is harder to capture accurately; pace zones may be more useful

What we'd actually say#

The five-zone framework is useful for endurance training. Knowing roughly where your zones sit, through field testing, lab testing, or careful interpretation of wearable data, enables deliberate training that matches intent to physiology.

For most recreational athletes, the practical takeaway is:

• Most training should be in zone 1–2 (easier than feels productive)
• Hard sessions should be in zone 4–5 (harder than the grey zone)
• Limit zone 3: it accumulates fatigue without delivering proportional adaptation

The formula-based zone estimates are rough. If you're serious about training, getting your zones accurate via testing is worthwhile.

When to talk to a clinician#

• Chest pain or unusual breathlessness during exercise
• Inability to reach near-maximal HR (chronotropic incompetence)
• Symptoms of overtraining despite reasonable load
• Cardiac arrhythmia detected on wearable or symptomatic
• Significant difference between formula-predicted and actual HRmax (could be physiological or could be cardiac)