The Lactate Threshold
Introduction: Few topics in exercise physiology have been more frequently investigated, or more vigorously debated than the lactate threshold. The details create the biggest debates. However, it is the basics that have great application to training and performance. So, we'll stick to those.
What is Lactic Acid and Where Does it Come From?
The carbohydrates you consume consist of several different sugar molecules; sucrose, fructose, glucose to name a few. However, by the time the liver does it's job, all of these sugars are converted to glucose (see figure to left) which can be taken up by all cells. Muscle fibers take up glucose and either use it immediately, or store it in the form of long glucose chains (polymers) called glycogen. During exercise, glycogen is broken back down down to glucose which then goes through a sequence of enzymatic reactions that do not require oxygen to proceed. All of these reactions occur out in the cell fluid, or cytosol. They proceed very rapidly and yield some energy for muscle work in the process. This glycogen/glucose breakdown pathway is called the anaerobic (no oxygen) glycolysis (glucose breakdown) pathway. Every single glucose molecule must go through this sequence of reactions for useful energy to be withdrawn and converted to ATP, the energy molecule that fuels muscle contraction, and all other cellular energy dependant functions.
The Metabolic Fork in the Road
There is a critical metabolic fork in the road at the end of glycolysis. At this fork, glucose has been converted from one 6 carbon molecule to two, 3 carbon molecules called pyruvic acid, or pyruvate. This pyruvate can either be shuttled into the mitochondria via the enzyme pyruvate dehydrogenase, or converted to lactic acid via the enzyme lactate dehydrogenase. Entry into the mitochondria exposes the pyruvate to further enzymatic breakdown, oxidation, and a high ATP yield per glucose. This process inside the mitochodria ultimately requires oxygen molecules to proceed and is therefore "aerobic." Conversion to lactate means a temporary dead end in the energy yielding process, and the potential for contractile fatigue due to decreasing cellular pH if lactic acid accumulation proceeds unchecked. Like a leaf floating in a river, the pyruvate molecule has no "say" in which metabolic direction is taken. The conditions in the muscle determine that.
Which way will MY pyruvate go during exercise?
I am sure you have surmised that that is a critical question with big implications for performance. I will try to answer the question at three levels: a single muscle fiber, an exercising muscle, and the entire exercising body.
The Muscle Cell at Work
In a single contracting muscle fiber the frequency and duration of contractions will determine ATP demand. ATP demand will be met by breaking down a combination of two energy sources: fatty acids and glucose molecules(ignoring the small contribution of protein for now). As ATP demand increases, the rate of glucose flux through glycolytic pathway increases. Therefore at high workloads within the single fiber, the rate of pyruvic acid production will be very high. If the muscle fiber is packed with lots of mitochondria (and therefore more Pyruvate Dehydrogenase), pyruvate will tend to be converted to Acetyl CoA and move into the mitochondria, with relatively little lactate production. Additionally, fatty acid metabolism will account for a higher percentage of the ATP need. Fat metabolism does not produce lactate, ever! If lactate is produced from glucose breakdown, it will tend to be transported from the area of high concentration inside the muscle cell to lower concentration out of the muscle fiber and into extracellular fluid, then into the capillaries.
The Whole Muscle at Work
Now let's look at an entire muscle, say the vastus lateralis of the quadriceps group during cycling. At a low workload, glycolytic flux is low (fatty acid breakdown ins relatively high at low intensities) and the pyruvate produced is primarily shuttled into the mitochondria for oxidative breakdown. Since the intensity is low, primarily slow twitch muscle fibers are active. These fibers have high mitochondrial volume. As workload increases, more fibers are recruited and already recruited fibers have higher duty cycles (more work and less rest). Now ATP demand has increased in the previously active fibers, resulting in higher rates of pyruvic acid production. A greater proportion of this production is converted to lactic acid rather than entering the mitochondria, due to competition between the two enzymes LDH and PDH. Meanwhile, some fast twitch motor units are starting to be recruited. This will add to the lactate produced in and transported out from the working muscle due to the lower mitochondrial volume of these fibers. The rate of lactate appearance in the blood stream increases.
The Body at Work
The vastus is just one of several muscles that are very active in cycling. With increasing intensity, increased muscle mass is called on to meet the force production requirements. All of these muscles are contributing more or less lactic acid to the extracellular space and blood volume, depending on their fiber type composition, training status and activity level. However, the body is not just producing lactate, but also consuming it. The heart, liver, kidneys, and inactive muscles are all locations where lactic acid can be taken up from the blood and either converted back to pyruvic acid and metabolized in the mitochondria or used as a building block to resynthesize glucose (in the liver). These sites have low intracellular lactate concentration, so lactic acid is transported INTO these cells from the circulatory system. If the rate of uptake, or dissappearance, of lactate equals the rate of production, or appearance, in the blood, then blood lactate concentration stays constant (or nearly so). But, when the rate of lactate production exceeds the rate of uptake, lactic acid accumulates in the blood volume, then we see the ONSET of BLOOD LACTATE ACCUMULATION (OBLA). This is the traditional "Lactate Threshold" (LT).
The Traditional Lactate Threshold
We have previously discussed the value of a high maximal oxygen consumption for the endurance athlete. A big VO2 max sets the ceiling for our sustainable work rate. It is a measure of the size of our performance engine. However, the Lactate Threshold greatly influences the actual percentage of that engine power that can be used continuously.
Most of you will never have this measured in a laboratory, but a brief description of a lactate threshold test is still useful, because it will lead us into some specific applications for your racing and training. The test consists of successive stages of exercise on a treadmill, bicycle ergometer, swimming flume, rowing machine etc. Initially the exercise intensity is about 50- 60% of the VO2 max. Each stage generally lasts about 5 minutes. Near the end of each stage, heart rate is recorded, oxygen consumption is measured, and a sample of blood is withdrawn, using a needle prick of the finger or earlobe. Using special instrumentation, blood lactate concentration can be determined during the test. After these measurements, the workload is increased and the steps repeated. Through a 6 stage test, we would expect to achieve a distribution of intensities that are below, at , and above the intensity where blood lactate begins to rise, or the lactate threshold. This point is often defined as a 1mM increase from baseline values. The data from a test would generally look simililar to the example below.
Interpreting the Data
For purposes of interpretation, let's say that the athlete above had a maximal heart rate of 182, and a VO2 max or 61 ml/min/kg. These were also determined using a bicycle test. So they are good values for comparison. Looking at the green dots, we see that blood lactate concentration does not begins to increase until during the 4th workload,from a concentration of abouu 1 mM to 2.5 mM. This is the break point. The subjects VO2 was 45 ml/min/kg at this point. So we determine that his LT occurs at 45/61 or about 74% of VO2 max. If we look at the heart rate at this point, it is 158. Now we have a heart rate at lactate threshold. 158 = about 85% of his max heart rate. This is useful for the athlete. When he is cycling, he can judge his training intensities based on this important value. If he is a time trialist, this would approximate his racing heart rate for the hour long event.
An Updated View on the Lactate Threshold
When I was in school, the textbooks basically presented the lactate threshold as a single point on the exercise intensity scale where blood lactate concentration started to increase. This is the kind of picture you see above. Once you exceeded this "threshold" intensity, fatigue was just around the corner. Over the last 25 years, a great deal of research has demonstrated that this was an oversimplistic representation of things. First of all, taking a blood sample during exercise is like seeing a photo of a bathtub; The picture cannot tell you whether the tub is filling, stable in water level, or emptying. During exercise, lactic acid is being simultaneously produced by working muscles and removed by other muscles as well as the heart, liver, and kidneys. If production rate equals removal rate, then blood lactate concentration will be stable. If production exceeds removal rate, lactate concentration increases. The picture below depicts a more modern view of lactate thresholds and their relationship to exercsie intensiity. The green zone represents an exercise intensity range where lactate production is low and lactate removal easily matches production. The yellow zone represents a range of intensities where we see a marked increase in blood lactate prododuction. But, lactate removal also increases so that a new stable blood lactate concentration is achieved. Finally, the red zone represents intensities where lactate production now exceeds the maximal rate of blood lactate removal. Exercise in this intensity range results in accumulation of lactate acid and fatigue. I have used this 3-zone exercise intensity model to quantify how good endurance athletes organize their daily training intensity. You can download one the research articles I have published on this topic here.For most athletes, the LT1 corrsponds to about 2mM blood lactate. And, as a rough roule of thumb, the LT2 occurs at about 4mM. BUT, there is substantial individual and exercise mode variation here! There are numerous published examples of athletes who can work for 30-60 minutes at an intensity producing a STABLE blood lactate concentration of up to 10mM or even higher. The LT2 blood lactate concentration can range from 3mM to 10mM depending on the individual. And, the LT2 value seems to be higher for activities involving a smaller active muscle mass. Running, rowing and skiing tend to have more typical LT2 lactate concentrations (3-4 mM) while cycling, kayak paddling, etc. may show higher average LT2 values(4-6mM). What we can conclude from this is that it is risky to just assume that a fixed blood lactate concentration like 4mM always corresponds to the lactate threshold. It does not.
Performance Implications
Lactic acid production is not all bad. If we could not produce lactate, our ability to perform brief high intensity exercise would be almost eliminated. However, As I am sure you are aware, lactic acid is the demon of the endurance athlete. Cellular accumulation of the protons (increased acidity) that dissociate from lactate results in inhibition of muscle contraction. Blame those heavy legs on the protons! The bottom line is that exercise intensities above the LT2 point can only be sustained for a few minutes to perhaps one hour depending on how high the workload is above the intensity at which lactate production exceeds maximal rates of removal. Exercise between LT1 and LT2 intensities are often sustainable for 1-2 hours, depending on glycogen availability and where within that range we are exercising. Exercise below LT1 can be potentially sustained for hours, if hydration status and other factors are controlled.
Factors that Influence the Rate of Lactate Accumulation in the body
Absolute Exercise Intensity-
for reasons mentioned above.
Training Status of Active Muscles-
Higher mitochondrial volume improves capacity for oxidative metabolism at high glyolytic flux rates. Additionally, improved fatty acid oxidation capacity results in decreased glucose utilization at submaximal exercise intensities. Fat metabolism proceeds via a different pathway than glucose, and lactic acid is not produced. High capillary density improves both oxygen delivery to the mitochondria and washout of waste products from the active muscles.
Fiber Type Composition-
Slow twitch fibers produce less lactate at a given workload than fast twitch fibers, independent of training status.
Distribution of Workload -
A large muscle mass working at a moderate intensity will develop less lactate than a small muscle mass working at a high intensity. For example, the rower must learn to effectively distribute force development among the muscles of the legs back and arms, rather than focusing all of the load on the legs, or the upper body.
Rate of Blood Lactate Clearance-
With training, blood flow to organs such as the liver and kidneys decreases less at any given exercise workload, due to decreased sympathetic stimulation. This results in increaed lactate removal from the circulatory sytem by these organs.
So, Do I race at My LT Intensity?
This depends on your race duration. If your are rowing 2000 meters, running a 5k race etc., your exercise intensity will be well above the LT2. Consequently, the blood lactate measured after these events is extremely high in elite athletes, on the order of 15mM (resting levels are below 1 mM). In races lasting from 30 minutes to 1 hour, well trained athletes also perform at an intensity right at or even slightly above LT2. It appears that in these events, top performers achieve what might be termed a "maximal lactate steady state". Blood lactate may increase to 8 to 10 mM within minutes, and then stabilize for the race duration. A high but stable lactate concentration may seem to contradict the idea of the LT. But, remember that blood lactate concentration is the consequence of both production and clearance. It seems likely that at these higher lactate concentrations, uptake by non-working muscles is optimized. At any rate, measurements in cyclists, runners and skiers demonstrate the fact that elite performers can sustain work levels substantially above the traditional lactate threshold for up to an hour.
Specificity of the Lactate Threshold
It is important to know that the lactate threshold is highly specific to the exercise task. So if this cyclist tries to get on his brand new, previously unused, rowing machine and row at a heart rate of 158, he will quickly become fatigued. Rowing employs different muscles and neuromuscular patterns. Since these muscles are less trained, the cyclist's rowing LT will be considerably lower. This specificity is an important concept to understand when using heart rate as a guide in "cross training activities", as well as for the multi-event athlete.
Effect of Training
For reasons mentioned above, training results in a decrease in lactate production at any given exercise intensity. Untrained individuals usually reach the LT at about 60% of VO2 max. With training, LT can increase from 60% to above 70% or even higher. Elite endurance athletes and top masters athletes typically have LTs at or above 80% of VO2 max. Values approaching 90% have been reported. The lactate threshold (or thresholds) is/are both responsive to training and influenced by genetics.
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