LT1, aka the first lactate threshold, is a popular metric in lactate testing. Whilst being popular, the definition is somewhat lofty: it is described as the first increase of blood lactate concentration (implicit linking it to an incremental load test). However this explanation of LT1 is not answering the question of: first rise of blood lactate compared to what baseline AND: what defines an increase in lactate concentration: 0.01 mmol/l or 0.1 mmol/l or even more ? Furthermore the lactate kinetics are linked to the protocol used to determine it. Step duration (for example 3min vs. 6min efforts) and initial load and load increment all influence the lactate concentration and therefore the determination of LT1. This article delves into the concept of LT1 and explains how it is measured using the INSCYD system.
The Science Behind LT1
During low-intensity exercise, the aerobic metabolism efficiently clears lactate as fast or even faster than it is produced. This results in lactate levels that remain steady or even fall below resting levels. However, as exercise intensity increases, lactate production grows exponentially, while lactate clearance follows a near-linear trajectory.
At a specific intensity, lactate levels will increase, marking the onset of LT1. This intensity is perceived as a critical indicator for athletes.
Traditional Methods for Determining LT1
LT1 is typically identified during a graded exercise test, where the intensity is incrementally increased, and blood lactate levels are measured at each stage. However, because lactate levels depend on both intensity and time, defining LT1 can be challenging and varies significantly with the testing protocol.
For example, lactate curves derived from different step durations (e.g., 3 minutes, 5 minutes, and 6 minutes) show variations. Longer step durations shift the lactate curve to the left, while shorter durations cause a rightward shift. Consequently, LT1 values derived from differing protocols can not be compared, making the definition of LT1 ambiguous and questionable.
The identification of a lactate rise furthermore depends on the measurement device used: each lactate meter comes with a specific error or measurement. In some table top lab devices it can be as small as 0.1 mmol/l. In some handheld devices it can be up to 0.5 mmol/l. Therefore a rise of lactate smaller than the error of measurement can not clearly be identified as a valid increase of lactate - it could also just be an error of measurement.
Furthermore the resolution of lactate meters comes into play: most handheld devices measure with a resolution of 0.1 mmol/l.
The INSCYD Approach to LT1
INSCYD offers a standardised method to determine LT1, eliminating protocol-related inconsistencies and taking into account device measurement error. By leveraging an athlete's metabolic profile, INSCYD generates a lactate curve based on a fixed set of parameters:
Load-increment (step) duration: 6 minutes.
Resting lactate concentration: 1.25 mmol/L.
Lactate analyzer typical error: 0.35 mmol/L
Lactate analyzer resolution: 0.1 mmol/L.
This 6-minute duration aligns with INSCYD's recommended step size for lactate testing, as it closely reflects steady-state conditions—a critical aspect when applying LT1 in training scenarios.
Calculating LT1 in INSCYD
Using the athlete's metabolic profile, INSCYD calculates a lactate curve starting at a resting lactate concentration of 1.25 mmol/L. LT1 is defined as a significant increase of lactate above the lowest lactate concentration. Significant increase means an increase which is distinguishable above the typical error of a lactate meter, which means it needs take into account the error of measurement and the resolution.
using the lowest lactate concentration value instead of resting lactate makes the determination of LT1 independent of the resting lactate values. This is important as -in practical application - the resting lactate is often influenced by nutrition and pre test activities
From the lowest point of this lactate curve, INSCYD adds the average error margin of handheld lactate devices (0.35 mmol/L) and a tolerance value (0.1 mmol/L). These fixed parameters ensure a reliable and consistent LT1 definition across tests (in a future upgrade the typical error of the actually used lactate analyser will be
used).
Key Takeaways
INSCYD's methodology ensures that LT1 is calculated independently of testing protocols, providing athletes and coaches with a precise, standardized benchmark. This approach supports more accurate assessments and better-informed training plans, tailored to individual metabolic profiles.
By eliminating protocol-dependent variability, INSCYD redefines how LT1 is measured, making it a robust and practical tool for optimizing athletic performance.