Training

Heart rate is the most accessible measure of exercise intensity for runners who don't have access to a lactate lab or a power meter. Used correctly, heart rate zones transform vague descriptions like "easy," "moderate," and "hard" into precise, reproducible training targets. Used incorrectly — or with inaccurate maximum heart rate estimates — they can systematically misdirect every session.

Finding Your Maximum Heart Rate

Every heart rate zone calculation depends on maximum heart rate (HRmax). The commonly used formula — 220 minus age — has a standard deviation of approximately 12 beats per minute. For a 40-year-old, this means the formula predicts 180 bpm, but actual HRmax could reasonably range from 156 to 204 bpm. Using an inaccurate HRmax makes every zone wrong.

The most reliable field method:

1. Warm up thoroughly: 15–20 minutes easy running with several short accelerations
2. Run a hard uphill for 2–3 minutes at maximum effort
3. Immediately run a flat 400m at sprint pace
4. Record the highest heart rate shown on your monitor at the end of the effort
5. Repeat on a separate day to confirm — true HRmax should be reproducible within 2–3 bpm

Wrist-based optical heart rate monitors can be used but are less accurate than chest straps during intense exercise due to movement artefact. For HRmax testing, use a chest strap if available.

The Five-Zone Model

Most endurance running programmes use a five-zone model derived from HRmax:

• Zone 1 (50–60% HRmax) — Active Recovery: Walking pace or very easy jogging. Used for warm-up, cool-down, and active recovery days. Does not generate meaningful training stress.
• Zone 2 (60–70% HRmax) — Aerobic Base / Fat Burning: Conversational pace. Primary zone for building aerobic base. Develops mitochondrial density, fat oxidation capacity, and cardiovascular efficiency. Most training volume should be here.
• Zone 3 (70–80% HRmax) — Aerobic / Tempo: Comfortably hard. Can speak in short phrases. The "grey zone" — harder than Zone 2 without the strong adaptation stimulus of Zone 4+. Research suggests many recreational runners spend too much time here.
• Zone 4 (80–90% HRmax) — Lactate Threshold: Tempo/threshold pace. Sustainable for 20–60 minutes. Direct lactate threshold development. Breathing is laboured; speaking is difficult.
• Zone 5 (90–100% HRmax) — VO2max / Maximum: Interval pace. Sustainable for 3–8 minutes per repetition. Maximum aerobic stimulus. Near-impossible to speak.

The Three-Zone Alternative

Stephen Seiler's polarised training research uses a simpler three-zone model that many elite athletes and coaches find more practical:

• Zone 1 (below LT1): Below first lactate threshold. Conversational pace. All easy running belongs here.
• Zone 2 (LT1 to LT2): Between first and second lactate threshold. The moderate zone that polarised training deliberately avoids.
• Zone 3 (above LT2): Above second lactate threshold. Hard intervals and race pace. All quality work belongs here.

LT1 approximates to ~75% HRmax for most runners. LT2 approximates to ~85% HRmax. The 80/20 training principle means 80% of sessions in Zone 1 (three-zone), 20% in Zone 3, with minimal Zone 2.

Heart Rate Drift and Its Implications

On hot days, during long runs, or when dehydrated, heart rate "drifts" upward at a fixed pace — a phenomenon called cardiac drift. A run that starts at 140 bpm (Zone 2) may end at 155 bpm (Zone 3) without any pace change. This means:

• On hot days, maintaining Zone 2 requires slowing down as the run progresses
• Heart rate is not a fixed indicator of effort — hydration, temperature, and fatigue all influence it
• For truly easy runs, pace by feel and confirm with heart rate rather than targeting a specific HR number rigidly

When Not to Use Heart Rate

Heart rate zones work best for longer, steady-state efforts. For short, hard intervals (less than 3 minutes), heart rate lags the actual physiological demand by 30–60 seconds — making it a poor guide during the effort itself. Use pace or perceived effort for intervals; use heart rate as a recovery check between repetitions.

Heart rate is also temporarily elevated by: caffeine, dehydration, heat, illness, poor sleep, and high stress. These elevations don't represent higher training stress — they represent physiological disruption. Running "by the numbers" on a day with elevated resting HR due to illness is not a useful approach.

Putting It Into Practice

Use the NorthLine Running Pace Calculator to derive training paces from your recent race times — these pace zones and heart rate zones should align. If you find your Zone 2 pace corresponds to a heart rate 10 bpm higher than expected, recheck your HRmax estimate. Inaccurate zones undermine everything built on top of them.