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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 =Lactate Threshold And Near Infrared Spectroscopy==A promising technology for measuring Lactate Threshold is Near-infrared spectroscopy (NIRS) which shines infrared light into the skin above an active muscle and measures the reflected light. NIRS measures the oxygen saturation in the capillaries of the muscle and has the potential to test for Lactate Threshold without any blood sampling. Because NIRS can monitor continually, it is possible that it may be able to determine the Lactate Threshold during an incremental test rather than requiring the multiple tests of MLSS. ===Introduction to NIRS and SmO2===Near-infrared spectroscopy (NIRS) has been shown to measure the oxygen saturation of blood in muscle (SmO<sub>2</sub>) or other body tissues (StO<sub>2</sub>)<ref name="Kek-2008"/><ref name="Torricelli-2004"/><ref name="Mancini-1994"/>. (This works on similar principles to a [[Pulse Oximeter]].) Medical NIRS systems for monitoring StO<sub>2</sub> use Infrared LED or Lasers at 2, 3, or 4 frequencies<ref name="Hyttel-Sorensen-2011"/>. SmO<sub>2</sub> reflects the balance of oxygen delivery and consumption during exercise<ref name="Chance-1992"/> and there are some initial indications that relative SmO<sub>2</sub> may reflect changes in performance capacity<ref name="Neary-2005"/>. There is generally a four phase response of smo2 during incremental exercise from rest to maximum intensity and the following recovery<ref name="Belardinelli-1995a"/>:# An initial increase in SmO<sub>2</sub> above resting levels to supply the now active muscles. (This may influence be due to increased blood flow<ref name="Bhambhani-1997"/>, but computer models do not support this<ref name="Fuglevand-1997"/>.) # SmO<sub>2</sub> decreases linearly or exponentially with increasing intensity, followed by a leveling off as the subject approaches maximum intensity. There is some evidence of a breakpoint where the rate of decline increases (see below). # During the first 1-2 minutes of recovery there is a rapid increase in SmO<sub>2</sub> which usually exceeds resting levels. # SmO<sub>2</sub> then declines to resting levels over a further few minutes. Skin should not impact SmO<sub>2</sub> readings more than 5%<ref name="Hampson-1988"/>, but surface fat can interfere with smo2<ref name="van Beekvelt-2001"/><ref name="Homma1996"/><ref name="BenaronMatsushita1998"/><ref name="BenaronYamamoto1998"/>. Because the penetration depth of NIRS is about 50-60% of the distance between the emitter and receiver, the site must be selected so that the fat layer is much thinner than this depth<ref name="Bhambhani-2004"/>.===SmO<sub>2</sub> Breakpoint===As the intensity increases during incremental exercise SmO<sub>2</sub> will remain constant or decline, with the rate of decline being greater near the Lactate Threshold<ref name="Belardinelli-1995a"/><ref name="Bhambhani-2004"/><ref name="Belardinelli-1995b"/>. This has led to several studies using the concept of an SmO<sub>2</sub> "breakpoint"<ref name="Grassi-1999"/>. This breakpoint is a change in the slope of the line plotting SmO<sub>2</sub> against work intensity in an incremental intensity test. This increase in the rate of desaturation can either be visually determined or based on bilinear regression. (The bilinear regression iterates over different combinations of two regression lines to find the lowest sum of squares of the residuals. I could not find the details of the constraints placed on this approach.) [[File:SmO2 Breakpoint1.jpg|none|thumb|500px|A graph showing the SmO<sub>2</sub> breakpoint.]]===SmO<sub>2</sub> and Lactate Threshold===A number of studies have looked at the relationship between SmO<sub>2</sub> and Lactate or Lactate Threshold* A study of five mountain climbers found a relationship between the "point of inflection of lactate" and the SmO<sub>2</sub> breakpoint during an incremental cycling test<ref name="Grassi-1999"/>. The definition they were using for the inflection of lactate was a blood lactate reading that is more than 0.5 below the subsequent value, with typical lactate levels below 2 mmol/l. This is closer to the "aerobic threshold" concept than the typical Lactate Threshold. The study did not find any correlation between the SmO<sub>2</sub> breakpoint and the 4 mmol/l value of blood lactate. The breakpoint was determined from bi-linear regression. * A study of 40 sedentary undergraduates showed a correlation between SmO<sub>2</sub> returning to resting levels and Ventilatory Threshold (VT) in 65% of subjects during an incremental cycling test<ref name="Bhambhani-1997"/>. While the text refers to Lactate Threshold as the point at which Lactate rises above resting levels (aerobic threshold) the method used to determine VT appears to be the anaerobic threshold. This study did not use the breakpoint mentioned above, but the point where the SmO<sub>2</sub> drops below the level detected at rest. * A study of 11 subjects of varying fitness levels showed a correlation between SmO<sub>2</sub> breakpoint determined visually and Ventilatory Threshold (VT) during an incremental cycling test<ref name="Belardinelli-1995a"/>.* A comparison between 12 healthy subjects and 7 suffering from chronic heart failure (CHF) showed a correlation between the SmO<sub>2</sub> breakpoint and Ventilatory Threshold (VT) during an incremental cycling test<ref name="Belardinelli-1995c"/>. * A 2012 study compared the results from NIRS on the calf (Gastrocnemius Lateralis) and quads (Vastus Lateralis) in 31 active but not highly trained college students during an incremental cycling test<ref name="Wang-2012"/>. The study found a correlation between Lactate Threshold (determined from the log-log method<ref name="Davis-2007"/>) and the SmO<sub>2</sub> breakpoint (determined from bi-linear regression) in both locations, but the quads corresponded better. (I suspect the results from running could be quite different.)* A 2009 study used MLSS (the gold standard for Lactate Threshold) with running to evaluate NIRS<ref name="Snyder-2009"/>. The 16 athletes performed between 2 and 5 tests of 30 minutes each to determine MLSS, separated by at least 48 hours each. The subjects then performed an incremental treadmill test using 6 minute stages with the 4th stage at the pace they estimated they could maintain for an hour (around Lactate Threshold). The first 3 stages where then 0.66, 0.44, & 0.22 meter/second slower, and the subsequent stages were 0.22 meters/second faster each time. SmO<sub>2</sub> breakpoint was defined as the workload immediately prior to a drop of 15% that lead to a continuous decline in smo2. A Lactate breakpoint was also determined based on the incremental test using the workload prior to an increase of 1 mmol/l as the criteria. Both the SmO<sub>2</sub> and Lactate breakpoint were determined visually. Of the 16 subjects, 1 did not reach MLSS, 2 did not have both a SmO<sub>2</sub> breakpoint or a Lactate breakpoint (based on the criteria used) and 1 did not have either. The study found that SmO<sub>2</sub> is as effective as Lactate breakpoint tests for determining true Lactate Threshold (MLSS). The table below shows the values for each of 12 subjects, with the paces shown as KPH, then min/mile, then the error as a percentage. This shows that while smo2 is as good as the lactate breakpoint, the individual differences from MLSS are not insignificant. For instance, subject 5 had an MLSS of 6:21, but an SmO<sub>2</sub> breakpoint of 6:57 and lactate breakpoint of 6:59, which is a big difference. {| class="wikitable" ! Subject! MLSS! SmO<sub>2</sub> ! Lb! MLSS! SmO<sub>2</sub> ! Lb! SmO<sub>2</sub> err! Lb err|-| 1| 13.5| 12.9| 13.7| 7:09| 7:29| 7:03| 4.4%| -1.5%|-| 2| 14.3| 14.2| 13.4| 6:45| 6:48| 7:12| 0.7%| 6.3%|-| 3| 9| 9.5| 9.5| 10:44| 10:10| 10:10| -5.6%| -5.6%|-| 4| 13.4| 12.9| 12.9| 7:12| 7:29| 7:29| 3.7%| 3.7%|-| 5| 15.2| 13.9| 13.8| 6:21| 6:57| 6:59| 8.6%| 9.2%|-| 6| 12.9| 12.7| 13.5| 7:29| 7:36| 7:09| 1.6%| -4.7%|-| 7| 15.4| 14.5| 15.3| 6:16| 6:40| 6:19| 5.8%| 0.6%|-| 8| 11.5| 11.7| 10.9| 8:24| 8:15| 8:52| -1.7%| 5.2%|-| 9| 10.8| 10.9| 10.8| 8:56| 8:52| 8:56| -0.9%| 0.0%|-| 10| 14.2| 14.2| 13.2| 6:48| 6:48| 7:19| 0.0%| 7.0%|-| 11| 11.6| 12.2| 12.2| 8:19| 7:55| 7:55| -5.2%| -5.2%|-| 12| 14.8| 14.6| 13.8| 6:31| 6:37| 6:59| 1.4%| 6.8%|-| Mean| 13.05| 12.85| 12.75| | | | | |-| SD| 1.88| 1.51| 1.55| | | | | |}===Thoughts on SmO2 and Lactate Threshold===I think that the currently available research indicates that NIRS and SmO<sub>2</sub>hold promise for simplifying the measurement of Lactate Threshold. However, the research is at a fairly early stage, with only one study using the gold standard of MLSS. There are also indications in the research that the indicators of Lactate Threshold are not always evident in all subjects. There is no indication if this is a problem that occurs in specific subjects so that they will never get a valid test result, or if it's a problem that simply occurs randomly. Currently there are two consumer products available; [[BSX]] and [[Moxy]]. BSX is a fully automated approach to analyzing the data and estimating Lactate Threshold, whereas the Moxy is intended to provide the end-user with the underlying data to evaluate.=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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</references>
