Lactate threshold: definition, formula, example

Your muscles produce lactate at every pace, even walking, and clear it just as steadily. At easy speeds the two balance and the concentration in your blood stays close to resting, around 1 mmol/L. Speed up and, at some point, production starts to outrun clearance. That point is what people mean by lactate threshold, and it is the reason a pace you can hold for an hour feels so different from one only 15 seconds per kilometre faster.

The trouble is that “the” lactate threshold does not exist as one agreed number. Faude, Kindermann and Meyer (2009) reviewed more than twenty threshold concepts in the literature, built on different protocols and different rules for reading the same curve. In practice they fall into two families. The first threshold (LT1, often called the aerobic threshold) is where lactate first rises above its resting level. The second (LT2, the anaerobic threshold or, in its strictest form, the maximal lactate steady state) is the highest intensity at which lactate still settles at a stable value instead of climbing until you stop. When a coach says “threshold run”, they nearly always mean LT2.

How it is measured

The classic test is a step test on a treadmill or a track: stages of 3 to 5 minutes, each 0.5 to 1 km/h faster than the last, with a drop of blood taken from a fingertip or earlobe at the end of each stage. You then read the threshold off the curve. Two rules are common: a fixed concentration, usually 4 mmol/L for LT2, and a rise above baseline, often baseline + 1 mmol/L for LT1. The speed between two stages is found by linear interpolation:

The maximal lactate steady state is the reference the others try to approximate, but it takes several days of 30-minute constant-pace runs to pin down, which is why few people outside laboratories ever measure it (Jones et al., 2019).

A worked example

A runner does 4-minute stages from 10 km/h, adding 1 km/h each time. Lactate at the end of each stage: 1.1, 1.2, 1.4, 1.8, 2.6, 4.1 and 6.3 mmol/L at 10, 11, 12, 13, 14, 15 and 16 km/h.

• LT2 at 4 mmol/L: between 14 km/h (2.6) and 15 km/h (4.1). v = 14 + (4 − 2.6) / (4.1 − 2.6) × 1 = 14.9 km/h, or 4:01 per km.
• LT1 at baseline + 1: the lowest value is 1.1, so the target is 2.1 mmol/L, between 13 km/h (1.8) and 14 km/h (2.6). v = 13 + 0.3 / 0.8 = 13.4 km/h, or 4:29 per km.

Same test, same blood, and the two thresholds sit almost 30 seconds per kilometre apart. Read the curve with another rule (the point of maximal distance from a line joining the first and last stages, for example) and you would get yet another value. That is not a flaw of the test; it is the reason to always ask which threshold a number refers to.

How to use it in training

Without a lab, a good field estimate of LT2 is the pace you could race for about an hour, which for most trained runners sits between their 10K and half marathon pace. The pace charts give that pace from a recent race time. Heart rate at LT2 is usually 85 to 92% of maximum, but individual spread is wide.

• Threshold sessions: 3 × 10' or 5 × 6' at LT2 pace with 1 to 2 minutes of jogging, or 20 to 30 minutes continuous. The pace should feel “comfortably hard”: you could speak a few words, not a sentence.
• Easy running stays below LT1. That ceiling is what zone 2 tries to capture, and in a polarized week most of your minutes live there.
• Retest every 6 to 10 weeks, or use races: the threshold moves with fitness, and old zones drift into the wrong place.

OpenAthlete does not measure lactate. You enter or sync your zones, and its statistics show how your time splits below, between and above your thresholds over weeks and months, next to the training load curves.

Limits

The number depends on the protocol: shorter stages give a higher threshold speed, because lactate has not caught up when the blood is taken. Portable lactate meters differ from each other by a few tenths of a mmol/L, which shifts a fixed-value threshold noticeably on a flat part of the curve. Low muscle glycogen lowers lactate at every speed and makes the threshold look better than it is. And a speed found on a treadmill at 18 °C does not transfer as is to a hilly course in the heat. If you want a pace anchor that comes from your own races rather than a blood sample, look at critical speed, which sits just above the maximal lactate steady state.