Lactate and Exercise Load

To fully understand what lactate is, we need to look at our energy systems. Energy systems need fuel to produce energy. One such fuel is carbohydrates. The energy system uses glucose (carbohydrates) to produce energy. The end product of this process is lactate! In other words: lactate is the end product of glycolysis.

The higher the lactate, the more carbohydrates you are using. It is impossible to measure the exact amount of fat and carbohydrates with a lactate analyzer, but lactate reflects the dynamics well — the higher it is, the greater the carbohydrate expenditure.

Lactate Production and Clearance

Lactate is produced by the body all the time, even at rest, since both aerobic and anaerobic energy production (mitochondrial respiration, glycolytic system, or creatine phosphate) are used at any given moment.

Lactate is a by-product of glucose utilization by muscle cells. The higher the glucose flux into the cell, the higher the lactate production — regardless of oxygen availability. During high-intensity exercise, type II-a muscle fibers are fully recruited due to the high contractile demands of skeletal muscles for energy production (ATP). Type II muscle fibers are highly glycolytic (a lot of glucose is consumed), which leads to the formation of large amounts of lactate. This production is a natural by-product of glucose utilization by skeletal muscle cells. During intense exercise, lactate production is many times higher than at rest.

As more and more ATP (adenosine triphosphate — the body's energy currency) is required to meet the demands of increasing workload, the contribution of the glycolytic anaerobic system also increases. The end product of the glycolytic anaerobic system is pyruvate, which is either oxidized in the mitochondria or converted into lactate. Thus, the reduction in lactate production occurs due to the greater capacity of the aerobic system (mitochondrial respiration) to oxidize fat and provide most of the energy demand, reducing the contribution and need for anaerobic energy production at a given power output.

Lactate Removal Mechanism

The removal of produced lactate (and, importantly, associated metabolites linked to the onset of fatigue) involves the transport of lactate from contracting muscle fibers to other locations where it is either oxidized in the mitochondria or used in a process called gluconeogenesis — essentially the reverse conversion of lactate into glucose/glycogen.

Lactate can be exported into the blood for clearance and energy use in virtually every organ in the body. However, this process takes time (minutes), while lactate is continuously produced during exercise.

The higher your fitness level, the more efficient your muscles become and the less lactate enters the blood, as they clear it in large amounts right within the muscles, which takes seconds or milliseconds. This is highly advantageous as it allows the contracting muscles to remove H⁺ more quickly, as well as enabling faster 'recycling' of lactate for additional energy (ATP).

During exercise, lactate is mainly produced in fast-twitch muscle fibers, which use a lot of glucose for energy. It is primarily cleared by slow-twitch muscle fibers. This is a complex process involving various lactate-specific transporters and enzymes.

Adaptations for Improved Lactate Clearance

Some adaptations that lead to improved lactate transport include training volume, training in the lactate steady-state zone, at threshold level and slightly above. The key is to properly dose the workload. Another key factor affecting the ability to clear lactate is VO2max, as it influences the rate of lactate oxidation. Lactate oxidation contributes most to lactate clearance during moderate and high-intensity exercise.

Lactate and Fatigue

One of the biggest misconceptions is that lactate is our enemy and causes fatigue — but this is simply not true. As exercise intensity increases, athletes begin to use more carbohydrates as fuel. As a result, more lactate is produced. This is one of the reasons people have come to believe that lactate is a by-product that causes fatigue: during high-intensity exercise, lactate concentration is high and athletes feel tired. However, there is no causal relationship between lactate and fatigue.

Lactate does NOT cause fatigue — it is not even 'waste.' It is FUEL!

The anaerobic energy system produces lactate. The aerobic energy system uses this lactate as fuel to produce even more energy.

The release of hydrogen ions (H⁺) associated with lactate can lead to a significant decrease in the pH of the contracting muscle, resulting in acidosis. This excessive accumulation of H⁺ — not only from lactate but also from ATP breakdown during muscle contraction (ATP hydrolysis) — can interfere with muscle contraction at various points. For example, H⁺ can compete with calcium (Ca²⁺) for binding to a protein involved in the regulation of muscle contraction. H⁺ can also inhibit the release and reuptake of calcium from the sarcoplasmic reticulum. Both processes are involved in muscle contraction. All of this can lead to reduced muscle contraction capacity, meaning a significant decrease in strength and performance.

Lactate itself is not harmful to the body, but its excessive production leads to a decrease in pH — this acidic environment negatively affects performance.

Lactate Thresholds

There are 2 lactate thresholds.

LT1 is the lowest exercise intensity at which a measurable increase in blood lactate concentration is observed compared to resting lactate concentration. LT1 is the first lactate threshold and should not be confused with LT2 (lactate threshold 2, or anaerobic threshold).

In the context of endurance training, LT1 is marked as the first elevation in lactate concentration above resting levels. The first lactate threshold is typically around 1–2 mmol/L on average.

LT1 is only needed as an indirect marker of fat and carbohydrate expenditure, since nothing fundamental changes when you run faster or slower than the first threshold. When you train slightly above LT1, lactate concentrations will remain in a steady state. LT2, on the other hand, is a true 'threshold' because it clearly distinguishes two intensities from each other:

Above LT2 — lactate concentrations will rise over time. There is no longer a lactate steady state.

Below LT2 — lactate concentrations will not increase over time. A lactate steady state exists.

With LT1, there is no such clearly visible difference below and above the threshold.

LT2 is the highest intensity at which muscle and blood lactate can reach a constant concentration, which is why it is commonly referred to as the maximum lactate steady state. In other words, it is the point at which the rate of lactate production exactly equals the rate of clearance, where any decrease in intensity will lead to a drop in lactate levels, and any increase in intensity will lead to non-linear lactate accumulation. The term 'lactate threshold' is interchangeable with OBLA — anaerobic threshold, LT2, or VT2. However, all these terms have slightly different meanings due to different measurement methods. The second lactate threshold is typically around 4 mmol/L on average. On average, you can sustain this intensity for approximately 40–130 minutes depending on the amount of available energy in the form of carbohydrates.

Training Zones

Looking at the classic 3-zone model, it can be roughly distributed as follows:

Training Zone 1: below LT1 (1–2 mmol/L)
Training Zone 2: between LT1 and LT2 (2–4.5 mmol/L)
Training Zone 3: above LT2 (above 4.5 mmol/L)