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[[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"/>.]]
One approach that looks promising uses three tests to estimate MLSS<ref name="Billat-1994"/>. First, a standard incremental test is used to give a rough estimate of MLSS. Then two 30 minute tests are performed, one above and one below the rough estimate of MLSS. The relative difference in rise between the two tests is then used to estimate the crossover point. For instance assume running at 7:00 min/mile produced a blood lactate that fell from 4 mmol/l at 5 min to 3 mmol/l at 20 min, a 1 mmol/l drop. Then a run at In the run at 6:20 min/mile the blood lactate rose from 4.0 mmol/l at 5 min to 6.5 mmol/l at 20 min, a 1.5 mmol/l rise. The interception point would then be about a MLSS pace of 6:26 min/mile. This is not much less effort than the full MLSS test, but it is an improvement.
==Lactate Threshold And 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 . 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 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"/>.(It's been suggested that SmO2 is probably only viable in lean individuals.)
===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.]]
Another approach used by<ref name="Snyder-2009"/> was defined as the workload immediately prior to a drop of 15% that lead to a continuous decline in SmO<sub>2</sub>. This is shown in the image below, showing a recording with and without a defined breakpoint.
[[File:SmO2 Breakpoints With-Without.jpg|center|thumb|300px|]]
===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 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 smo2SmO<sub>2</sub>. 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" style="margin-left: auto; margin-right: auto; border: none;"
! Subject
! MLSS
| 12.85
| 12.75
| | | | |
|-
| SD
| 1.51
| 1.55
| | | | |
|}
===SmO2 and MLSS===
Sadly, there does not appear to be a difference in SmO<sub>2</sub> during an MLSS test above and below the MLSS threshold pace<ref name="Snyder-2009"/>. If running above the MLSS threshold pace does not result in a drop in smo2, then the ability to use SmO<sub>2</sub> for finding threshold seems rather dubious.
[[File:SmO2 MLSS Test.jpg|center|thumb|300px| A graph of lactate and SmO<sub>2</sub> above and below the MLSS threshold.]]
===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.
