About the anaerobic (lactate) threshold
Your anaerobic threshold — also called the lactate threshold — is the exercise intensity above which lactate produced by your muscles accumulates in the blood faster than your body can clear it. Below it, you can sustain effort for a long time; above it, fatigue rises sharply and you can only hold the pace for minutes. It's the single most useful physiological marker for endurance training, because it defines the boundary between "comfortably hard" and "unsustainable."
How this calculator estimates it
Lab measurement requires a blood-lactate or ventilatory test, but you can estimate the corresponding heart rate from your resting and maximum heart rates using the Karvonen (heart-rate-reserve) method. Maximum heart rate is estimated as 220 − age, heart-rate reserve is (max − resting), and the threshold heart rate falls at roughly 85% of that reserve above your resting rate: Threshold HR ≈ resting HR + 0.85 × (max HR − resting HR). Entering a measured max HR instead of the age estimate makes the result more accurate.
Why the threshold matters for training
- Tempo / threshold work sits right at this heart rate — 20–40 minutes at threshold is the classic session for raising it.
- Easy aerobic base miles should stay well below it (roughly 65–75% of reserve) so you accumulate volume without accumulating fatigue.
- VO₂max intervals deliberately go above it, in short repeats, to lift your ceiling.
- As you get fitter, your threshold moves to a higher percentage of max HR and a faster pace/power — tracking it is a direct measure of endurance progress.
Accuracy and individual variation
- 220 − age has a standard deviation of about ±10–12 bpm, so an age-based max HR (and the threshold derived from it) is an estimate. A measured max HR from a hard field test is far better.
- Trained endurance athletes often reach threshold at 88–92% of max HR; untrained people closer to 75–80%. The 85% figure is a reasonable middle estimate, not a personal truth.
- Heat, altitude, dehydration, caffeine, and cardiac drift all shift heart rate at a given effort, so treat the number as a starting zone and refine it against how the effort actually feels (you should be able to speak only a few words at a time at threshold).