Lactate Kinetics, Cycling power, Muscle O2 and Minute ventilation

 More recent Lactate, SMO2 sensor exploration

One of the "promised" features of the Moxy, BSX and now Humon muscle O2 sensors is the ability to accurately measure your lactate threshold.  Yes, there are published studies supporting the concept but many experts in the field still have a problem with the validity of this technique. After all we are talking about a small superficial muscle mass that may or may not be the main recruited muscle group at a given exercise intensity.  Superficial muscle also behave differently than the deeper ones for many reasons (fiber type, usage patterns, energy substrate usage).  However the alternative, doing a ramp style lactate threshold test is a time consuming and arduous endeavor.  In addition, depending of the ramp type (length of time in each zone, watt increment) different results can occur.  After reviewing literature of how to do this sort of testing, I was struck with the huge variation in how to proceed.  What follows is what I finally did as well as correlation with leg muscle O2, EKG quality heart rate, costal O2 and respiratory parameters.

First, lets look at how to do the lactate test.  An excellent review and algorithm for testing came out recently.  They explain very nicely the concept of MLSS and LT as well as the importance of fine tuned regulation:
Although the appropriate use of lactate measurements
and data interpretation is still a matter of debate, the high
correlation between the onset of plasma or blood lactate
accumulation (OPLA, OBLA) and endurance performance
already demonstrated in 1979 by Farrell et al. (1979) and 1981
by Sjödin and Jacobs (1981), supported the idea to use lactate
biochemistry as a diagnostic tool. Since then, a number of lactate
tests have been developed (Billat et al., 2003; Faude et al., 2009),
but for economic solutions, most of them try to measure the
MLSSW with graded WL increments before accelerated blood
lactate accumulation occurs (Billat et al., 2003; Faude et al.,
2009). Moreover, the widely used WL increments of 0.3–0.5
m/s in running or 30–50W in cycling respectively (Svedahl and
MacIntosh, 2003),may constrain the detection of smallmetabolic
events. The maximum variation of running velocity during the
current official world marathon record (0.07 m/s) (Kimetto,
2014) and the fastest ever recorded marathon time (0.07 m/s;
final 2.2 km 0.14 m/s) (Tucker, 2017) emphasizes the potential
fine-tuned regulation of lactate metabolism (Messonnier et al.,
2013). This suggests that elite athletes perform close to their
MLSSW, and that minor WL deviations may result in a rapid
decline of running speed or power output.
Usually, several tests on different days are needed for a precise
determination of the MLSSW (Heck et al., 1985; Faude et al.,
2009). Here, we report a reliable and valid test method for
running and cycling, which employs lactate kinetics to detect the
LT and to subsequently determine the MLSSW in less than 1 h.

In addition they proposed an algorithm more precisely measure the LT:

In short, the method first estimates a starting point of power/speed depending on age.  Next there are 4 minute intervals with lactate tests done immediately afterward.  Depending on the lactate value, power/speed are raised or lowered for the next 4 minute bout.  The goal being a lactate of 4.0 mm/l, and showing as well that a small power/speed cut, drops the number below 4.0.
In addition the test result was validated against a 10 K constant velocity run and a half marathon with excellent correlation:

To get a better idea of the watt range to test, I went back to some older data showing correlation of the 30 min TT with La/anaerobic threshold .

In this study, the goal was to do your best 30 min TT which lead to higher but steady lactate levels.  I estimated my 30 min TT and subtracted 20 watts or so to get near a La of 4.

My modification of the test was the following:

• Guess my lactate threshold.
• Cycle 4 minutes at that pace.
• Measure lactate.
• If >4, cut power by 10 watts until target reached (4 min steps)
• If <4, boost power by 10 watts until target reached (4 min steps)
  
• Do another 4 minutes and re test.
• Once at about 4, a final cut of 10 watts to verify that La drops below 4.

I used my regular bike on a turbo trainer, did the test after a 3 hr ride with some intervals, but was careful to hydrate well with fluid and sugar.  The Lactate Scout meter was used and calibrated.  Baseline La was 1.8.
BSX sensors were on my R RF, L VL and L costal areas.
Hexoskin shirt was worn with a Skins dynamic top over everything to keep it snug.

Here is the data:

At the initial "guess" of 261 watts, La was 4.6 after 4 min.  The next interval was at 251 watts with the La at 4.3 and finally at just another 9 watts down (242) it was 3.3.  About 2 minutes later a final recheck was 2.8 verifying that it indeed was coming down.
Final La threshold is about 250 watts!

Now for some interesting observations (at least in my data):

• Trivial change in heart rate for all three power zones (peak)
• No change seen in last 30 sec avg O2 in the L VL and R RF  
• No major change seen with avg costal O2 in the last 30 secs but a slightly different looking curve with La 4.6

Of note the minimum costal, RF, VL were far lower than the La threshold values (see right hand side).  The minimum levels were from a 1 min max interval done on the preceding warm up ride.
From this observation there does not seem to be a close correlation between HR, SmO2 and La threshold +- 10 watts.  As discussed in the La testing paper, that 10 watts can make all the difference in both race pace and training stimulus. 
So sure, the NIRS studies show that a ball park figure can be obtained, but certainly not with the precision a simple La test can achieve.

Now for the Hexoskin data (I saved it for last on purpose):

Here we may have something to help us determine training zones or race pace.  The minute ventilation did stratify nicely with the last 30 sec averages showing a difference (statistical significance unclear).
To show an example of 4 minutes of easy cycling here is a comparison:

At 165 watts minute ventilation is at 100.  Much lower than the 158.

Another comparison with lactate levels is this:

It's a fast start 3 min interval with a final average of 310 (but the last 2 min are at about 280 watts).  Lactate was much higher after but the wide dynamic range of minute ventilation is plainly evident.  Minute ventilation was approaching max at the end of the interval corresponding to lactate elevation.

From looking at the Hexoskin respiratory data, it appears potentially useful to folks who don't have power meters for their sport (skiing, row etc).  Training zone and race pace selection may be possible with observing minute ventilation in real time, if a prior correlation to La threshold was done.  For example in my case, keeping minute ventilation between 150-158 could be a surrogate marker for La while on the bike (using their app).

On reviewing the literature there are published reports of minute ventilation, VO2 and lactate correlation.  For some reason it appears to be largely forgotten as a dedicated metric.  Perhaps it should be revived.

Conclusions:

• It is neither difficult nor relatively costly to get an accurate La threshold determination that is correlated with cycling power.  The Lactate Scout was about $500 total for an entire starter kit.  That included 50 strips, test solution and case.
• The La test does not need to be done with a ramp approach and there are good reasons not to do so.  Most importantly, precise accuracy may not be possible with the usual ramp increments.
• In my case, there was not a significant difference of La threshold (plus or minus 10 watts) looking at heart rate, muscle O2 in VL, RF.  My original hunch that costal O2 would be helpful here did not pan out.  That is not to say these parameters are not useful, perhaps at higher intensity levels other conclusions could be drawn.  Costal O2 in general may be a better marker for the respiratory compensation point and cardiac output redistribution.
• Minute ventilation may be useful as a surrogate marker for La threshold in sports without power meters.  Comparison of the blood La to minute ventilation would be the way to proceed if interested.
• My personal thanks to the group who devised the practical La threshold test.  Their clever method is both easy, short and highly precise. 

 Lactate related posts

• Issues in using Muscle O2 for lactate testing
• Lactate Kinetics, Cycling power, Muscle O2 and Minute ventilation 
• Observations just above the lactate threshold...the RCP
• MLSS +10 watts, a journal review plus personal data 
• Determining MLSS with longer intervals and a NIRS device 
• Optimizing Lactate Clearance via Power Modulation 
• MLSS retest by both blood lactate, Vent and NIRS  
• Ventilatory thresholds and Heart rate variability testing
• First and second Lactate thresholds, why the confusion, how to measure? 
• Performance lab official lactate testing - my data