Maximal Lactate Steady State (MLSS) represents the highest exercise intensity an individual can maintain for a prolonged period without a continuous rise in blood lactate. At this specific point, the body’s ability to produce lactate is matched by its ability to clear it. Operating at or near this equilibrium allows an athlete to sustain a higher work rate for longer durations in activities like running or cycling. This metric delineates the boundary between a sustainable heavy effort and an unsustainable one that leads to rapid fatigue.
The Physiology of Lactate Balance
Lactate was once incorrectly labeled a metabolic waste product responsible for muscle burn. Scientific understanding now shows that lactate is a significant fuel source, shuttling energy between cells. During high-intensity exercise, glucose is broken down for energy, producing lactate faster than it can be used locally in fast-twitch muscle fibers. This excess lactate then enters the bloodstream.
From the bloodstream, lactate is transported to other tissues to be converted back into energy. The heart, for instance, is efficient at using lactate for fuel, as are slow-twitch muscle fibers. The liver also takes up lactate and converts it back into glucose through a process called gluconeogenesis. This system of production, transport, and reuse is known as the lactate shuttle.
MLSS represents the peak capacity of this shuttle system. It is the highest intensity at which the rate of lactate appearing in the blood equals the rate of its disappearance. Below this intensity, the body can effectively clear and reuse lactate, maintaining a stable internal environment. Once an athlete pushes beyond this point, lactate production outpaces clearance, leading to its accumulation and the onset of fatigue.
Determining Your Maximal Lactate Steady State
The most precise method for identifying MLSS is controlled laboratory testing. This protocol involves multiple lab visits where an athlete performs a series of constant-load exercise bouts, lasting 30 minutes each, at varying intensities. During each session, blood samples are taken periodically from an earlobe or fingertip to measure lactate concentration. The highest intensity that can be maintained without a significant rise in blood lactate (an increase of less than 1.0 mmol/L over the final 20 minutes) is identified as the MLSS.
Because direct measurement is often inaccessible and costly, practical field tests provide reliable estimates. A common alternative is a 30-minute time trial. After a thorough warm-up, an athlete performs an all-out effort for 30 minutes, recording their average heart rate, pace, or power output. The average of these metrics during the final 20 minutes of the test is a close approximation of the intensity at MLSS.
While field tests are estimates, they offer a useful benchmark for structuring a training plan. Consistency in the testing protocol is important to accurately track progress over time. For example, using the same route or piece of equipment under similar conditions will yield more comparable results. These methods provide athletes with actionable data to target their MLSS.
Training to Improve Maximal Lactate Steady State
Improving your MLSS involves training at intensities at or very close to your determined MLSS heart rate, pace, or power. The goal is to stimulate physiological adaptations that enhance the body’s lactate clearance capabilities. By repeatedly stressing the lactate shuttle system, the body becomes more efficient at transporting and using lactate as fuel. This raises the workload that can be sustained before lactate begins to accumulate.
A primary method for MLSS training is the “tempo” run or ride, which consists of sustained efforts performed at an intensity slightly below MLSS. These workouts involve continuous efforts of 20 to 60 minutes. This training builds the capacity of muscle fibers to oxidize lactate and improves the efficiency of lactate transport proteins (MCTs), which facilitate the movement of lactate across cell membranes.
Another effective method is threshold interval training. These workouts involve repeated bouts of exercise at or slightly above MLSS intensity, separated by short recovery periods. A classic example for a runner might be four repetitions of a 10-minute run at their MLSS pace, with two to three minutes of easy jogging in between. These intervals push the lactate clearance system to its maximum rate, prompting a strong adaptive response.
MLSS in Relation to Other Performance Metrics
MLSS fits into a broader landscape of performance metrics. One such term is Lactate Threshold (LT), which refers to the onset of blood lactate accumulation. MLSS is conceptually similar to the second lactate threshold (LT2), which marks a more rapid rise in lactate and is often used interchangeably with MLSS, though they are determined through different testing protocols.
Functional Threshold Power (FTP), a popular metric in cycling, is an estimate of the highest power output a cyclist can sustain for approximately one hour. Physiologically, FTP is an attempt to approximate the workload at MLSS. While FTP is determined through a single test (often 20 minutes), it provides a practical and accessible proxy for MLSS intensity.
VO2 max represents the maximum rate at which an athlete’s body can consume oxygen during intense exercise, a measure of the aerobic “engine.” In contrast, MLSS represents the highest sustainable percentage of that engine’s capacity. While a high VO2 max indicates high aerobic potential, MLSS often better predicts endurance performance because it reflects the ability to sustain a high fraction of that potential.