Cardio Training Zones

Heart rate zones divide the cardiovascular exercise spectrum into physiologically distinct training intensities, each triggering specific adaptations. Training randomly across all intensities produces mediocre results in all areas. Zone-specific training concentrates stimulus where adaptation is needed, producing faster and more targeted improvements.

The concept is backed by decades of research and used by Olympic-level endurance coaches. The polarised training model — developed from studying elite Nordic skiers, cyclists, and runners — consistently shows that athletes who spend 80% of their volume at low intensity (zones 1–2) and 20% at high intensity (zones 4–5) outperform those who train moderately hard all the time.

Training Approach	Intensity Distribution	Outcome
Random / intuitive	Mostly moderate (zone 3)	Moderate fitness gains; fatigue accumulation
Always hard	Zones 4–5 dominant	Fast short-term gains, overtraining risk
Polarised (80/20)	80% zones 1–2, 20% zones 4–5	Best long-term adaptation, lower injury risk
Pyramidal	70% zones 1–2, 20% zone 3, 10% zones 4–5	Strong aerobic base with moderate intensity work

Tip: Most recreational athletes make the mistake of training in zone 3 — too hard to recover easily, too easy to drive high-end adaptations. This "grey zone" produces mediocre returns for significant fatigue.

All heart rate zones are calculated as a percentage of your maximum heart rate (MHR). The accuracy of your zones depends entirely on how accurately you know your MHR.

The classic formula 220 minus age is widely used but notoriously inaccurate — standard deviation is approximately ±12 BPM, meaning it can be wrong by 20+ BPM for many individuals. Better options exist.

Method	Accuracy	Effort Required	Procedure
220 − age formula	Low (±12 BPM SD)	None	Calculation only
208 − (0.7 × age) (Tanaka formula)	Moderate (better for older adults)	None	Calculation only
Ramp test (5-min all-out)	High	Maximum effort	Warm up 10 min, run/cycle hard uphill for 3–5 min, note peak HR
Lab VO2 max test	Very high	Maximum effort + lab	Supervised graded exercise test
Wearable device tracking	Moderate (improves with training data)	Low	Device records peaks across many sessions

For a field test: after a thorough 15-minute warm-up, run hard uphill for 3 minutes as fast as you can sustain. The highest heart rate observed on your monitor within the final 30 seconds is approximately your MHR. Repeat twice for reliability.

Example zones for a 35-year-old with MHR of 185 BPM:

• Zone 1: below 111 BPM (below 60%)
• Zone 2: 120–139 BPM (65–75%)
• Zone 3: 139–157 BPM (75–85%)
• Zone 4: 157–167 BPM (85–90%)
• Zone 5: above 167 BPM (above 90%)

The five-zone model is the most widely used framework in endurance coaching, though some organisations use a three-zone or six-zone model. All systems divide the same physiological reality into slightly different segments.

Zone	% of MHR	Example BPM (MHR 185)	Feel	Primary Adaptation	Typical Use
Zone 1	50–60%	93–111 BPM	Very easy; full conversation	Active recovery, fat oxidation	Recovery rides/walks, warm-up
Zone 2	65–75%	120–139 BPM	Easy; can speak in sentences	Mitochondrial density, aerobic base, fat oxidation	Base building, long runs/rides
Zone 3	75–85%	139–157 BPM	Moderate; broken conversation	Lactate clearance, aerobic capacity	Tempo runs, steady-state efforts
Zone 4	85–90%	157–167 BPM	Hard; few words at a time	Lactate threshold, VO2 max	Threshold intervals (10–20 min)
Zone 5	90–100%	167–185 BPM	Maximum effort; cannot speak	VO2 max, neuromuscular power	Short intervals (30 sec – 4 min)

The lactate threshold — the intensity at which lactate accumulates faster than it can be cleared — typically sits at the zone 3/4 boundary. Training at and slightly above this point is key to improving sustained high-intensity capacity.

Zone 2 training has emerged as one of the most discussed concepts in endurance and longevity medicine. Popularised by physiologist Iñigo San Millán and physicians like Peter Attia, it targets the intensity at which mitochondrial function is maximally stimulated while fatigue accumulation remains low.

Metabolically, Zone 2 is the highest intensity at which the body primarily uses fat as fuel through the aerobic system. Training here:

• Increases mitochondrial density and efficiency (more energy-producing machinery per cell)
• Improves fat oxidation capacity — the ability to burn fat at higher exercise intensities
• Builds the aerobic base upon which all higher-intensity work depends
• Reduces resting heart rate and improves heart stroke volume over time
• Has the lowest injury and recovery cost of any training zone

Metric	Zone 2 Target	Notes
Heart rate	65–75% MHR	Should feel "comfortably uncomfortable"
Talk test	Can speak 5–6 words comfortably	If gasping, slow down; if singing, speed up
Lactate level	1.7–2.0 mmol/L	Lab measurement; correlates with talk test
Weekly volume for adaptation	Minimum 3 hrs/week	Elite athletes do 8–15+ hrs/week in zone 2
Session length	45–90 minutes	Shorter sessions are valid; frequency matters

Tip: Zone 2 should feel almost boringly easy. If you're listening to a podcast and finding it hard to follow, you're likely in zone 3. Slow down. The aerobic adaptations from zone 2 compound dramatically over 3–6 months of consistent practice.

A balanced cardio training week combines aerobic base work (zones 1–2), threshold development (zone 3–4), and VO2 max intervals (zone 5). The exact distribution depends on your goal and available training time.

Goal	Zone 1–2 Volume	Zone 3–4 Volume	Zone 5 Volume	Total Weekly Time
General health / longevity	150–200 min	30–45 min	0–20 min	3–4 hrs
Improve aerobic base	240–300 min	45 min	20–30 min	5–6 hrs
5K / 10K performance	180–240 min	60 min (tempo)	30–40 min (intervals)	5–6 hrs
Half-marathon / marathon	300–420 min	60–80 min	20–30 min	7–9 hrs
Triathlon	400–600 min	90 min	40–60 min	9–12 hrs

Sample 4-day/week intermediate plan:

• Monday: Zone 2 easy run or cycle — 45–60 min
• Wednesday: Threshold intervals — 10 min warm-up + 4 × 8 min at zone 4 (2 min rest) + 10 min cool-down
• Friday: Zone 2 easy — 45 min
• Saturday/Sunday: Long zone 2 session — 75–90 min

Warning: Adding more than 10% to your weekly training volume per week significantly increases injury risk — particularly for runners. Build volume slowly and prioritise consistency over any single big session.