Heart rate zones (Karvonen method)

Karvonen zones — heart rate reserve, not flat percentages

The Karvonen method calculates training zones as a percentage of your heart rate reserve (HRR = max HR - resting HR), not flat % of max HR. This matters because two people with the same max HR but different resting HRs need different zone targets.

The five zones

• Z1 — Active recovery / fat oxidation (50-60% HRR): warmup, cooldown, very easy
• Z2 — Aerobic base (60-70% HRR): the king of endurance training. Build mitochondria, develop fat metabolism, low fatigue cost
• Z3 — Tempo (70-80% HRR): "comfortably hard" — sustainable for 30-90 min for fit athletes
• Z4 — Threshold (80-90% HRR): lactate threshold, the edge of what's sustainable for ~1 hour
• Z5 — VO2 max (90-100% HRR): intervals, 3-8 min on, equal recovery

Why two max HR formulas

• Fox 220 - age is the classic but overestimates for younger athletes and underestimates for older ones. Standard error is ~12 bpm.
• Tanaka 208 - 0.7×age is from a 2001 meta-analysis (Tanaka et al.) and is more accurate across age groups. Standard error is ~8 bpm.

For training zone math, use Tanaka. For wall-clock simplicity, Fox is fine.

The 80/20 rule

The current consensus on endurance training: ~80% of weekly time in Z1-Z2, ~20% in Z4-Z5. The middle zone (Z3) is a trap — too hard to recover from, too easy to drive max-power adaptation. Most amateur runners spend 60-70% of their week in Z3 and wonder why they don't improve.