=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<ref name="MillerFattor2002"/> without impairing performance<ref name="EllisSimmons2009"/>.
=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<ref name="Binder-2008"/>. It is sometimes believed that the MLSS represents the maximum clearance of Lactate, but this may not be the case<ref name="EllisSimmons2009"/>. 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<ref name="Billat-1996"/><ref name="Palmer-1999"/><ref name="Baldari-2000"/><ref name="Jones-2000"/> . Lactate Threshold can be thought of as the percentage of VO2max that can be maintained for a protracted time<ref name=" Costill- 1973"/>, though it's not clear what the limiting factor is for exercise above the Lactate Threshold<ref name="Baron-2008"/>. =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<ref name="Bosquet-2002"/><ref name=" Yoshida- 1990"/>, and training can improve performance because of this shift without a change in aerobic capacity ([[VO2max|V̇O<sub>2</sub>max]] )<ref name=" Acevedo- 1989"/>. There is some limited evidence from radio-isotope studies in animals that a benefit of endurance training may be in Lactate clearance<ref name=" Donovan- 1983"/>. 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. [[File:Blood Lactate Curve.jpg|none|thumb|500px|A typically blood lactate curve with lactate level plotted against intensity.]]
=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<ref name="Beneke-2011"/>"/><ref name="Guellich-2010"/>, and polarized training is a better approach<ref name="StögglSperlich2014"/><ref name="Muñoz-2014"/>. For trained athletes, Tempo runs are ineffective<ref name="Londeree-1997"/> and may actually be counterproductive<ref name="Evertsen-2001"/><ref name="Guellich-2010"/>. See [[Tempo Runs]] for more details. Detraining shifts the curve to the left<ref name="Coyle-1985"/> . Lactate levels can be higher at a given intensity after just a few days without training<ref name="Mujika-2001"/>, suggesting rapid detraining effects. =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<ref name="Cook2011"/>. See [[Heart Rate Deflection]] for details.
* '''Respiratory gasses'''. Another method for estimating Lactate Threshold is to measure the respiratory gasses<ref name="Binder-2008"/><ref name="Laplaud-2006"/>, but given this is impractical for most athletes, it's not covered here.
==Lactate Threshold & Maximal Lactate Steady State ==
The best approach to determine Lactate Threshold is to measure the Maximal Lactate Steady State (MLSS)<ref name="Faude-2009"/>. 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% [[VO2max|V̇O<sub>2</sub>max]]. 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<ref name="Urhausen-1993"/><ref name="De SouzaGrossl2012"/><ref name="Beneke-2003"/>. 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<ref name="Figueira-2008"/>, probably based on the mass of muscle engaged<ref name="Beneke-1996"/><ref name="Beneke-2003-2"/>. The difficulty of performing this test makes it impractical in most situations. [[File:LactateMLSS.jpg|none|thumb|500px|Blood lactate levels over time at various workloads. The lower lines are at lower intensities, with the line marked MLSS being the highest intensity that produces stable lactate levels.]]
==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<ref name="Billat-1996"/>. This means that the incremental power test tends to be of limited value<ref name="Foxdal-1996"/>, with 3 minute stages giving low reproducibility<ref name="Morton-2012"/>, the stage length changing the lactate values<ref name="Faude-2009"/>, and even longer stages lengths of 8 minutes having low reproducibility<ref name="Gavin-2014"/>. The lactate level can drop between the 4<sup>th</sup> and 12<sup>th</sup> minute of exercise at a constant intensity<ref name="RieuMiladi1989"/>. 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<ref name="Orok-1989"/>, but this rather arbitrary approach can be used as a guideline (at best)<ref name="Baldari-2000"/>. 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<ref name="GullstrandSjüdin2007"/>.
==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)<ref name="De SouzaGrossl2012"/>, unusually 4.0 mmol/l<ref name="Heck-1985"/> though sometimes 3.5 mmol/l<ref name="DenadaiFigueira2004"/><ref name="Denadai-2005"/>. However, while the MLSS may average around 4.0 mmol/l<ref name="Denadai-2005"/>, there are significant differences for individuals<ref name="Mamen-2009"/>, with variations between 3.0 and 5.5 in small sample sizes<ref name="Heck-1985"/> and has been shown to have a range as wide as 2.0 to 10.0 mmol/l<ref name="Faude-2009"/><ref name="Stegmann-1981"/>. 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<ref name="Heck-1985"/>. 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<ref name="Stegmann-1981"/>. The Fixed Blood Lactate Accumulation is sometimes called "Onset of Blood Lactate Accumulation" (OBLA)<ref name="Mamen-2009"/>, a particularly misleading term in this context.
==Lactate Threshold Estimation From Lactate Patterns==
There have been several approaches to determining MLSS without the difficulty of the full protocol
<ref name="Billat-1994"/><ref name="Harnish-2001"/><ref name="Palmer-1999"/><ref name="Tegtbur-1993"/><ref name="Dickhuth-1999"/><ref name="Urhausen-1993"/>, but their validity is limited<ref name="Kilding-2005"/><ref name="De SouzaGrossl2012"/><ref name="Beneke-1995"/>. 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<ref name="SoteroPardono2009"/><ref name="Sotero-2009"/><ref name="MiyagiLeite2013"/>. However, the effectiveness of the LMST is profoundly impacted by the starting speed of the incremental portion of the test<ref name="Carter-1999"/>and so the results may be coincidence<ref name="Carter-2000"/>. This is not entirely surprising given the initial sprint phase disrupts the metabolism<ref name="Carter-2000"/>. Part of the problem with these approaches may be that MLSS may not represent the point of maximum lactate clearance<ref name="Beneke-2003-2"/>, as injecting additional lactate into the blood of athletes exercising above MLSS did not significantly increase lactate levels<ref name="EllisSimmons2009"/>.Some of the tests could be "p-hacking", where the study looks at a sufficiently large number of variables that some correlation occurs randomly<ref name="HeadHolman2015"/>.
[[File:LactateComp.jpg|none|thumb|500px|The correlation (or lack thereof) between MLSS and the lactate levels at the MLSS intensity seen during 3 or 5 minute incremental power tests<ref name="Heck-1985"/>.]]
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<ref name="Reilly-1999"/><ref name="Yoshida-1984"/><ref name="Maassen-1989"/><ref name="McLellan-1989"/>.
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