1 What is Lactate?
Main article: Lactate
At one time Lactate was viewed as a harmful waste product due anaerobic exercise, but research since the early 2000s has shown that Lactate is an intermediate fuel in the metabolism of carbohydrates. Muscles will burn Lactate in preference to Glucose, and will convert Lactate back to Glucose at rest. The level of Lactate in the blood is primarily dependent on exercise intensity. Note that Lactate forms Lactic Acid in the blood, and the terms are used interchangeably. Lactate is used as a fuel source by working muscles, and injecting extra lactate into the blood results in increased lactate metabolism and carbohydrate sparing without impairing performance.
2 What is the Lactate Threshold?
The Lactate Threshold (LT) is the point at which the lactate level in the blood will rise even if the work intensity is kept constant. This is sometimes referred to as the Anaerobic Threshold (AT), or the Onset of Blood Lactate Accumulation (OBLA), though the most accurate term is Maximal Lactate Steady State (MLSS). Even within the scientific community terminology is confusing. It is sometimes believed that the MLSS represents the maximum clearance of Lactate, but this may not be the case. Note that Lactate is normally measured in the blood stream, so the Lactate level reflects the net of the muscles releasing and absorbing Lactate. Lactate Threshold is important as it is an excellent good predictor of race performance. Lactate Threshold can be thought of as the percentage of VO2max that can be maintained for a protracted time, though it's not clear what the limiting factor is for exercise above the Lactate Threshold.
3 Lactate Curve
It is common to plot exercise intensity against lactate level to produce a blood lactate curve similar to the one below, showing an exponential rise in lactate level with intensity. It's generally accepted that a shift of the curve to the right indicates an improved athletic performance, and training can improve performance because of this shift without a change in aerobic capacity (V̇O2max). There is some limited evidence from radio-isotope studies in animals that a benefit of endurance training may be in Lactate clearance. Note that above the Lactate Threshold the Lactate level is not at a steady state, but rises even though the intensity remains constant, so the typical curve that is shown is rather misleading. Some Lactate Curves are plotting Blood Lactate against time during an Incremental Power Test (see below), which is more reasonable, but can still be rather misleading.
4 Lactate Threshold Training
Main article: Tempo Runs
There is good evidence that endurance training changes Lactate Threshold. However, the idea that training at threshold intensity (Tempo Runs) is particularly effective has no evidence"/>, and polarized training is a better approach. For trained athletes, Tempo runs are ineffective and may actually be counterproductive. See Tempo Runs for more details. Detraining shifts the curve to the left. Lactate levels can be higher at a given intensity after just a few days without training, suggesting rapid detraining effects.
5 Determining Lactate Threshold
There are various ways of determining the Lactate Threshold, each with their own problems.
- MLSS. The gold standard test (and the only one that appears to be valid) is to measure Maximal Lactate Steady State (MLSS). The test requires 3 to 5 constant intensity trials of at least 30 minutes' duration, each performed on separate days. Each test is at a different exercise intensity, and the highest intensity that does not have a rise in blood lactate in the last 20 minutes is the MLSS. While this is the best way of determining Lactate threshold, it's obviously time consuming and interferes with the athlete's regular training. Because of the effort of MLSS testing, various shortcuts have been tried.
- Fixed Blood Lactate Accumulation. A simple approach is to assume that Lactate Threshold always occurs at the same Lactate level. Sadly, this assumption is wrong, as Lactate Threshold can occur at vastly different levels.
- Lactate Patterns. There are various approaches that look at the pattern of change in Lactate in an attempt to create a simple test. So far, I've seen little evidence to support any of these approaches.
- Heart Rate Deflection. An indirect way of finding Lactate Threshold is to look for the Heart Rate Deflection, sometimes called the "Conconi test". This test only requires a heart rate monitor to perform rather than blood draws, so it is much easier than the above approaches. However, the validity of the Conconi test has many issues and seems of dubious value. See Heart Rate Deflection for details.
- Respiratory gasses. Another method for estimating Lactate Threshold is to measure the respiratory gasses, but given this is impractical for most athletes, it's not covered here.
5.1 Lactate Threshold & Maximal Lactate Steady State
The best approach to determine Lactate Threshold is to measure the Maximal Lactate Steady State (MLSS). The test is actually several constant load trials of at least 30 minutes' duration on different days at various exercise intensities (in the range of 50–90% V̇O2max. The highest workload that results in an increase of less than 1 mmol/L of lactate between the 10 and 30 minute mark defines the MLSS. Lactate is typically measured using a blood sample, either using a pinprick or a catheter. Note that MLSS for a given individual will vary by sport, probably based on the mass of muscle engaged. The difficulty of performing this test makes it impractical in most situations.
5.2 Lactate Threshold & Incremental Power Test
A common approach to determine the Lactate Threshold is the Incremental Power Test. The subject exercises in stages of increasing intensity, with lactate measured at the end of each stage, with stages typically lasting 3 to 10 minutes. However, blood lactate takes 20-30 minutes to stabilize for a given intensity. This means that the incremental power test tends to be of limited value, with 3 minute stages giving low reproducibility, the stage length changing the lactate values, and even longer stages lengths of 8 minutes having low reproducibility. The lactate level can drop between the 4th and 12th minute of exercise at a constant intensity. Some have suggested using the lactate value measured as an indication of the prior stage's intensity, as it takes more than 3 minutes for lactate to stabilize, but this rather arbitrary approach can be used as a guideline (at best). For running, it is common to pause the exercise for 30 seconds to take a blood sample. These breaks only make a non-significant difference to the testing, though the slight difference tends to be greater at higher intensities.
5.3 Lactate Threshold & Fixed Blood Lactate Accumulation
Because MLSS is time consuming and expensive, a shortcut is often used to estimate MLSS by assuming that it occurs at a fixed Lactate Level (Fixed Blood Lactate Accumulation, or FBLA), unusually 4.0 mmol/l though sometimes 3.5 mmol/l. However, while the MLSS may average around 4.0 mmol/l, there are significant differences for individuals, with variations between 3.0 and 5.5 in small sample sizes and has been shown to have a range as wide as 2.0 to 10.0 mmol/l. This approach also typically uses a blood test, but in some sports (like running), the athlete has to pause to have a pinprick blood sample taken, further confusing the test. The term "Individual Anaerobic Threshold" (IAT) has been used to emphasize that the Lactate Threshold is specific to each individual rather than using a FBLA, though this can refer to a specific protocol for estimating MLSS. The Fixed Blood Lactate Accumulation is sometimes called "Onset of Blood Lactate Accumulation" (OBLA), a particularly misleading term in this context.
5.4 Lactate Threshold Estimation From Lactate Patterns
There have been several approaches to determining MLSS without the difficulty of the full protocol, but their validity is limited. These approaches generally look for some pattern in the change of Lactate level. For example, one approach called the "Lactate Minimum Speed Test" (LMST) uses an initial sprint to elevate blood lactate followed by an incremental power test. However, the effectiveness of the LMST is profoundly impacted by the starting speed of the incremental portion of the testand so the results may be coincidence. This is not entirely surprising given the initial sprint phase disrupts the metabolism. Part of the problem with these approaches may be that MLSS may not represent the point of maximum lactate clearance, as injecting additional lactate into the blood of athletes exercising above MLSS did not significantly increase lactate levels. Some of the tests could be "p-hacking", where the study looks at a sufficiently large number of variables that some correlation occurs randomly.
6 Factors that may influence Lactate Threshold
There are a few factors that may change the Lactate Threshold (other than training)
- Because lactate is produced from the metabolism of carbohydrate, a reduction in carbohydrate intake (or Glycogen depletion) will shift the lactate curve to the right.
- It's not clear if Delayed Onset Muscle Soreness changes the lactate curve as there are reports that it does and reports that it does not.
- Lactate Threshold will vary by sport, probably based on the mass of muscle engaged, or because the inactive muscles consume more lactate as the concentration rises. MLSS may also vary with environmental conditions, with a lower lactate levels at MLSS in hotter conditions.
7 Aerobic Threshold
There is a related concept called "Aerobic Threshold" that is generally used to mean the exercise intensity at which Lactate levels rise above resting baselines. This threshold is believed to be the upper limit of nearly exclusive use of aerobic metabolism that can be sustained for many hours. Intensities just above the aerobic threshold can be maintained for prolonged periods (~4 hours). This aerobic threshold can be hard to determine in untrained subjects as it occurs at very low intensities. Unfortunately, the term "Lactate Threshold" is sometimes used to mean this point where lactate rises above resting levels.
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