MLSS. Maximal Lactate Steady State.

MLSS, or anaerobic threshold, is commonly used as a reference point for defining training zones. Training intensities are often prescribed as percentages above or below threshold power or speed.

However, the widespread use of threshold-based training zones is partly historical. Incremental exercise testing became one of the most accessible methods for physiological performance assessment, and lactate-derived thresholds therefore became convenient reference points for exercise prescription. This does not necessarily mean that a fixed percentage of MLSS is the physiologically optimal way to define an individual athlete’s training zones.

MLSS is therefore an important performance benchmark, but its usage to define training zones is primarily historical and not based on physiological science.

MLSS represents an equilibrium of the metabolism, a sustainable intensity which doesn’t lead to failure or exhaustion unless the athlete runs out of fuel in terms of carbohydrates. In contrast, racing and competing implies maxing out one’s physiological capacities.

For this reason, no competition is performed entirely and exactly at MLSS. Most events are performed close to, somewhat above, or below this intensity. Depending on the athlete and event duration, performances such as cycling time trials, 10-km running or elite-level half-marathon running may occur relatively close to MLSS, while longer events are generally performed below it.

MLSS is therefore useful not because athletes necessarily race exactly at MLSS, but because it provides a physiological reference point against which race intensity and endurance performance can be understood.

MLSS is not a physiological system, such as the aerobic or glycolytic energy system by itself. Unlike VO₂max or VLamax, it does not describe the maximum capacity of a single energy system. Instead, MLSS is an outcome of the interaction between different metabolic processes.

Two of the most important determinants are VO₂max and VLamax. VLamax strongly influences the rate at which lactate is produced in glycolysis, while aerobic capacity determines the ability to utilize lactate and produce energy oxidatively. MLSS occurs at the exercise intensity at which these opposing processes remain in equilibrium.

Consequently, changing VO₂max or VLamax changes MLSS. With other factors remaining equal, a higher VO₂max allows a higher power or speed at MLSS. Conversely, a higher VLamax increases glycolytic lactate production and can reduce the power or speed at which lactate production and combustion remain balanced.

If the goal is to understand why MLSS is where it is, or how to increase it through training, VO₂max and VLamax need to be measured.