Test physiology

Blood lactate: why the timing of the measurement changes everything

Lactate kinetics and the duration of steps

2 July 2026 · ~6 min read · Arnaud Collet, PhD, CPSS®

When you run a step test to assess an athlete's physiological and metabolic profile, blood lactate concentration is often used as a key indicator. It helps identify certain intensity zones, better understand the metabolic response to exercise, and individualise training prescriptions.

But there is a fundamental point still too often overlooked: the blood lactate concentration measured at a given instant does not necessarily reflect the real metabolic state of the current step. In other words, when an athlete moves from one intensity to another, blood lactate does not adjust instantly.

Kinetics

Blood lactate is not an instant snapshot of the muscle

When you raise the intensity of an effort, the body enters a transient phase. Lactate production, its transport, its diffusion into the body's various compartments and its use by other tissues must all gradually adapt to the new level of effort.

The concentration measured in the blood is therefore the result of a dynamic balance between several phenomena:

lactate production by the active muscles,
its passage from muscle into the blood,
its distribution throughout the body,
its use as fuel by different tissues (it is oxidised to release energy),
and its reconversion into glucose by the liver (the recycling, or Cori cycle).

This balance does not settle in a few seconds. If you measure too soon after an increase in intensity, the value obtained may still partly reflect the previous step, or a transition phase between two metabolic states. It therefore does not necessarily represent the body's stabilised response to the current intensity.

blood lactate intensity (schematic) transient phase

The solid line is blood lactate; the dashed staircase is intensity (it jumps at once). During the transient phase (orange band), lactate hasn't stabilised yet: measured too early, it underestimates the step's true response. This plateau holds for an intensity below the maximal lactate steady state; above it, lactate keeps rising without stabilising. Illustrative sketch, indicative values.

Steady state

Why steady state is essential

In a step test, the goal is not simply to measure a lactate concentration at some arbitrary moment. The goal is to relate a given intensity — a speed, a power, a heart rate — to a representative metabolic response. For that, the body must be given enough time to adapt to the new step.

If the step is too short, the lactate measurement risks underestimating or misrepresenting the real response associated with that intensity. You then believe you are measuring "the lactate at 300 watts" or "the lactate at 14 km/h", when in reality you are measuring a value taken during an adjustment phase, before the body has reached a sufficiently stable state.

This can have important consequences:

a threshold may be mispositioned;
a training zone may be poorly defined;
an athlete may be considered more or less metabolically efficient than they really are;
a training prescription may be offset from the intended physiological objective.

In a performance-optimisation context, this is not a methodological detail. It is a central point.

Steps too short

The problem with steps that are too short

We still frequently see testing protocols based on very short steps, sometimes two minutes or even less. In some cases, lactate measurements are even paired with ramp protocols, where intensity rises continuously without ever giving the body time to stabilise.

The problem is simple: if intensity changes constantly, or changes too quickly, the blood lactate concentration cannot be interpreted as a stable value associated with a precise intensity.

In a ramp, for example, the athlete is never truly at equilibrium. The lactate measured at a given moment is influenced by the previous intensities, by the rate at which the load rises, by the delays in lactate diffusion and transport, and by the overall inertia of the metabolic system. In this context, giving a precise meaning to a lactate value measured during the ramp becomes extremely fragile.

It is not that the measurement is technically impossible. It is its physiological interpretation that becomes problematic.

long steps (steady state) steps too short / ramp

At equal intensity, steps that are too short underestimate blood lactate concentration: the whole curve slides down. As a result, any threshold, zone or model derived from it ends up displaced — the athlete can look fitter than they are. Illustrative sketch, indicative curves.

Precision ≠ relevance

Measuring is not understanding

An analyser can give a precise value: 1.8 mmol/L, 2.6 mmol/L, 4.1 mmol/L. But instrumental precision does not guarantee physiological relevance. A measurement can be technically correct and methodologically hard to interpret.

This is exactly the risk with tests built on steps that are too short, or with ramps used to interpret blood lactate. You get numbers, but those numbers do not necessarily reflect the athlete's stabilised metabolic state at a given intensity.

In physiological assessment, what matters is not only producing data. What matters is producing interpretable data.

Inertia

Let the system speak

The human body is not an instantaneous system. It has physiological inertia. When you change the intensity of an effort, ventilation, oxygen uptake, heart rate, the mobilisation of energy substrates and blood lactate concentration all adjust gradually.

So the system must be given time to respond. This is precisely why lactate protocols must be built with steps that are long enough. The goal is to get close to a situation where the value measured is genuinely representative of the intensity being tested, rather than a mixture of the previous step, the current step and the transient dynamics between the two.

Rigour

A question of rigour

Blood lactate is a powerful tool. But like all powerful tools, it must be used with rigour. A lactate test only has value if the protocol allows a correct interpretation of the data. Cutting step duration excessively to save time (and money) may seem practical, but it can seriously degrade the quality of the information obtained.

In practice, this means a good test is not necessarily the shortest test. It is the one that gives the body enough time to express a coherent metabolic response at each intensity. If you want to use blood lactate to define thresholds, individualise zones or understand an athlete's metabolic profile, you have to accept this constraint: time is part of the measurement.

Key points

Blood lactate does not adjust instantly: after an increase in intensity, it takes several minutes to reach its plateau.
Measuring too early in a step captures a transition value, which underestimates the stabilised response at the intensity being tested.
When you characterise metabolism through blood lactate against intensity, steps that are too short and continuous ramps are unfit for purpose: they cannot pair a stabilised blood-lactate value with a constant, well-defined intensity.
A good test is not the shortest one: it is the one whose steps are long enough to reach a steady state.

In performance testing, the goal is not to go fast. The goal is to measure right. And to measure right, you sometimes simply have to give the body time to respond.

Arnaud Collet, PhD, CPSS®