Friday, February 7, 2020

Smart trainer usage in physiologic testing and interval training

Over the past few years numerous cycling ramps and intervals have been presented in this blog.  About 6 weeks ago, I attempted to do a ramp on my "dumb", fixed resistance turbo trainer consisting of 30 watts per minute rise, beginning at 100 watts.  Despite my long standing familiarity in performing ramps, there was major difficulty in ratcheting up the power evenly, especially at the higher values.  This was related to several reasons including the need to constantly watch the average lap power (each 1 minute) as well as pressing the lap button (disrupting my power flow) and shifting gears.  Sure, doing a series of 5 minute ramps with 25-30 watt differential is doable, but 20 watt per minute is not going to be easy and 10 watt per minute may not be possible.  I had previously heard of "smart" trainers that would automatically change resistance and "clamp" your power within a narrow range.  This seems ideal in theory, and in fact most physiology laboratories utilize similar equipment.  Here is an example of a lab grade cycle ergometer:


This particular model is about $3000.  Forget the price a moment, does this look comfortable to spend time on?  The pedals have no clips and the upright body position could lead to false ergonomics.  We just don't ride this way!  So if you did testing on this bike, the results don't necessarily translate to real world riding on your own bike. 

Can we do as well from the "watt clamping" standpoint with a smart trainer?  
Let's explore that question.

Before looking at what a smart trainer can do, a look at manually doing a ramp on a fixed resistance trainer is important.  Although I've done them before, it is very difficult to keep steady power, watch your interval time, shift gears and hit the lap buttons to do it well.  Here is a typical attempt on my part, of a 30 watt per minute ramp (20 watts per minute would be much more difficult):
Although I did ok on the cadence, I needed to constantly shift gears, which caused difficulty maintaining a steady power output.  Notice the very unstable power readings especially at the high wattage.

Worse, is the average watt per interval.  Here is a listing of each stage with the desired targets in red on the right:
There were substantial differences in target power, worse at the higher rates.
Coupled with the already uneven pacing pattern, the physiologic results of this session would be untrustworthy to say the least.

Can we do better with a smart trainer?

One of the more reasonably priced "smart", electronically controlled trainer options (about $800) is the Elite Suito.   If you are interested in other models, that's fine and many trainer reviews are available.  I was happy with the Elite Muin basic model I had before and decided to stick with Elite.

The Suito:
It's quiet, has great road feel (even with Powercranks) and comes already set up.  Just pull it out of the box, spread the feet and go.
The trainer needs to be controlled with software if you are interested in either physiologic testing or doing fixed power workouts.  I have tried both the Elite app as well as Zwift.  The Elite app is a no frills program for your phone or laptop.  Both apps allow you to use your own power meter, so there is no reason to rely on the less accurate trainer meter.

Here are a couple of screenshots of the Elite app, the first was the creation of a custom ramp, beginning with warmup, 100w start then 20w per minute until stopping (near exhaustion).

And a screen of custom workout choices that I designed:



  • The top workout was simply a series of 5 minute intervals at my VT1 power, plus or minus 25 watts hence - 150 then 175 then 200w.
  • The middle was a single 5 minute 269 watt interval (after a long warmup) to test some MLSS metrics.
  • The bottom was the 20 watt per minute ramp.

How did the 20 watt per minute ramp look?

Here is the Cyclinganalytics.com display:

With the 1 minute splits (the desired watts are in red on the right column):


Although the average is close to desired, it does deviate somewhat above 200 watts.
A clue to this is the rise in cadence.  


Although the trainer should reduce resistance as cadence rises, there will still be some resistance-time lag and that is probably why I was 20 watts too high at the end.  Still, the results are reasonable, and with some practice (stable cadence) could be even closer.  This is certainly better than the manually done ramp (which was 30w per minute).

Can the smart trainer maintain strict enforcement of interval power?
This was part of a series of repeats of 145, 155, 175 and 195 watts.  As indicated, the cadence and power were near perfect.  Stable cadence and square wave power were the result.

Comments on the Elite training app:
  • There is no entertainment factor, just the nuts and bolts of power/time.
  • There is some lag.  
  • If you reduce the cadence markedly (coasting), there is a dramatic increase in resistance.  I would recommend pausing the active routine and restart the program while pedaling.
  • The workout creation tool is not as intuitive as it could be.
  • The resistance during initial warmup (first 15 minutes) is higher than it should be.
  • But, once you are warmed up, the desired target power will be very close to the realtime power output.

Zwift usage:
There is ample literature on using Zwift, the erg mode option and workout creation (so I will skip that).  What I will review is my own observations and comparison to the Elite app.
However, make sure you are in erg mode when doing the test ramps and intervals.  This mode provides a clamp on power by varying the pedal resistance. A higher cadence will cause the app to lower resistance, providing the same net power output.

Zwift is partly entertainment.  The graphics of a virtual ride are excellent, there is potential real life pedaling resistance up inclines (non erg mode), you can earn points, pass virtual riders plus acquire virtual gear (for the gamers out there - think Diablo armor).

Designing workout and test protocols on Zwift is very easy.  There is a "create custom workout" option (not on the smartphone app, only the laptop/PC version).
Here is what  the workout creator looks like: 


And here is my new custom ramp of 10 watts per minute, ending at 300w:


I have not tried this yet but it's next on my list of tests to do.

Interval power stability with Zwift:
This is an example of 5 minute intervals done in Zwift that was designed to be just below, and above my MLSS:

Here I designed a set of 5 minute intervals in Zwift with a target of 261 and 276 watts:
 
Real world output result:
Looking at the pair of 5 minute intervals, the first is spot on the target and the second is only 3 watts higher (<1%)


  • There was smooth cadence and relatively stable power.

There was a tiny bit more variation with the 155, 175, 195w intervals, but they were within 1% of target power:

All in all, Zwift does a competent job of keeping your power and cadence stable in erg mode.  I did not design a 20w per minute ramp, but it can be done.

Comparison to the Elite training app:
  • There is excellent entertainment potential, great graphics and possibility for social interaction. 
  • There is less lag in resistance.  
  • If you reduce the cadence markedly (coasting), there is minimal increase in resistance.  Therefore, you can stop riding and the resistance reduces to compensate.
  • The workout creation tool is easy to use.
  • The resistance during initial warmup (first 15 minutes) is not higher than it should be.  Somehow, the Zwift app takes this into account better than Elite does.
  • There may be a bit less of interval power stability.  Since the trainer resistance does not change as quickly as using the Elite app, one can deviate more from target wattage.  The trade off is being able to pause at will without hitting any pause buttons.



Putting this into practical usage.

The 20 watt per minute ramp:
Obtaining your MLSS using a rectus femoris muscle O2 tracing should be possible with the 1 minute ramp stage process (using Elite's app).  Remember, we will need to figure out the approximate wattage at the breakpoint, then subtract about 10-15 watts due to the "response time" of VO2 change according to the article of Murias.
Here is what my data showed (from the above 1 minute per stage ramp).  Each point represents the O2 sat of the rectus femoris (per each second) over the course of the ramp.  I found the breakpoint (red line intersection), then noted the corresponding elapsed time.  Using that time marker, the average power over the past 20 sec was calculated at 290w.  Taking 15w off this figure results in a MLSS of 275w, which is very close to my historic data (and official testing):






Using the Zwift ramps above:
Although not spoken of in the literature, this is a favorite of mine to determine MLSS.
We are looking for the transition from stability of RF muscle O2 over time to a constant decline over 5 minutes.

The rectus femoris site had a similar O2 saturation breakpoint between 261 and 279 watts:

  • Somewhere between the 261 to 278w zone is the limit of stable muscle O2 sat.   
  • Potentially, with finer increments, this could be tracked down to a more narrow range.
  • Good agreement with the 20w per minute ramp.
  • Excellent target watt agreement with the custom protocol

Summary:
  • It is very difficult (near impossible) to perform a 20w per minute incremental ramp to (near) exhaustion with a reasonable degree of accuracy using a standard resistance trainer.
  • Smart trainers by virtue of their feedback properties in combination with appropriate software, have a much better chance of approaching lab grade equipment.
  • Advantages in doing so include price, ergonomics, comfort and body position specificity in both training and testing.  Using your own bike to do the testing is priceless.
  • Both the Elite training app and Zwift provide the ability to create custom training protocols and testing schemes.
  • Both apps will result in fine control of incremental resistance at a fixed cadence.
  • With the availability of reasonably priced smart trainers and control software, home physiologic testing and defined interval training are now both possible and fun to do.




Thursday, January 23, 2020

Does the FTP relate to the MLSS - Yes, No, Maybe?

Several paper published recently have tried to elucidate the differences between the MLSS and FTP.  Are the tests equivalent in evaluating current performance status as well as fitness improvement after training.  Since the conclusions of these studies were quite different, i thought it might be interesting to see why.
This post will attempt to point out the definitions of each test, what they measure and the use of these values for training zone boundaries.  In addition, the use of the FTP (or equivilant) for fitness monitoring will be reviewed. 

First, what does each test measure?
The FTP (functional threshold power) is a near constant interval time trial with the average power defined as the FTP for that duration.  So the FTP20 would be your best constant effort of power for 20 minutes.  Therefore, this test is to exhaustion and very stressful.  In addition the performance would be heavily dependent on the subjects motivation, existing fatigue, recent training and other myriad factors.  If you had a poor night sleep or your legs were sore from some intense efforts done recently, your FTP may or may not be affected.
On the other hand, the MLSS (maximal lactate steady state) is generally defined as the best constant power over 30 minutes with little further lactate rise (above 1 mmol) from minute 10 to 30.  The centerpiece of this test is thus metabolic stability (lactate), where the FTP is an all out (potentially non stable) effort aiming for no reserve at the finish.  Technically, the MLSS can go on for a variable time above 30 minutes but the FTP can't.  

Here are a couple of photos illustrating the difference (yes, that's Lance in both):

This is the FTP - all out, maximal effort


The MLSS - generally done in a laboratory, a 30 min lactate steered interval with several repeats


Given the metabolic disparity mentioned (exhaustion vs steady state), various formulas have been proposed to reconcile the difference.  A popular method is using 90-95% of the FTP to approximate the MLSS.  Since many training regimes and zone 3 prediction methods rely on knowing the MLSS (or RCP - respiratory compensation point), it is valuable to getting this number right.  Practical note - Given the measurement issues in obtaining RCP/LT2, one may be better off doing zone 3 efforts well above MLSS so as not to bleed into zone 2 by mistake.  

Now for the 2 basic questions: 
  • Does the MLSS relate to the FTP20 by some constant (90% or 95%)?
  • Can one track the change in fitness over time using either MLSS or FTP?

FTP20 vs MLSS:
Several studies have looked at the relation between FTP and MLSS.  Several months ago a well done study was published showing that both a direct 90% x FTP and alternate formula were correlated with MLSS.  Here is the testing protocol:

Functional Threshold Power and Correction Factors. After standardized warm-up (5-minute pedaling, 80 and 90% of VT1 intensities), subjects performed a TT20 test using the
software provided by the ergometer (Rouvy; Cycleops), with 5%of slope simulation(1). Subjects were asked to produce the highest MPO possible for 20 minutes using their own pacing strategies, cadence and the gear ratio.
Note - virtually no warm up!  This is not a criticism, but it will become important later on.

They found reasonable correlation between FTP and MLSS but the correction factor needed was higher in the superior performing athletes:
To factor this observation into an equation, they proposed the following:

Conclusion:


Bottom line:
  • 90% of the FTP20 power or using the alternate formula above (.7488 x FTP power + 43) is very close to MLSS


Another study showed excellent correlation between FTP20 and MLSS with no correction needed!  Here are some details:
Thus, 7 cyclists were classified as trained (T; V̇ O2max 55 − 64.9 mL/kg/min); and 8, as
well trained (WT; V̇ O2max 65 − 71 mL/kg/min).  

To investigate the concurrent validity between FTP20 and MLSS, the cyclists performed
in this order: an incremental test, the FTP20 protocol, and several tests to determine the MLSS.
The riders were asked to refrain from strenuous exercise in the 48 h preceding each test.
Participants were given at least 2 and a maximum of 4 days between visits and all tests were completed within 3 weeks. 

The incremental test was started at 100 W, with increments every 3 min of 30 W until maximum voluntary exhaustion

The warm up: 

Note - the warm up included a 5 minute time trial!

The results:
  • Excellent correlation of the MLSS and FTP with no correction needed.
  • The 5 minute time trial power was equivilant to the Maximal aerobic power.  One of the recommended validation methods to VO2 max testing is to perform a 5 minute constant TT at the power achieved during the last stage of testing.  It's nice to see this work out here as well.
  • Why was the FTP equal to the MLSS?  I feel fairly certain this was due to the warm up, that included the 5 minute MAP time trial.  An effort of that magnitude will have lasting effects in the short term on the next longer FTP interval.  This type of maneuver should be differentiated from a brief sprint to initialize some "fast start" strategy effect.  A 5 minute bout at MAP will introduce lactate elevation, local and central fatigue, obviously affecting the later FTP interval.
  • This does not invalidate the study, but does illustrate the need to read the details of the methods section carefully.
  • The first study had minimal warmup and needed substantial correction of the FTP20.

The next study was done by the group lead by Dr Murias, who has done excellent pioneering work on muscle O2 kinetics as well as other areas.  In this paper the additional question was asked whether a performance change after a cycling season is measurable in either the MLSS or FTP.  

Methods:
A total of 18 participants (values in mean [SD]; 12 males: 37 [6] y, 180[6]cm,79[8]kgand6females:28[6]y,171[6]cm,68[9]kg) volunteered and provided written informed consent to participate in this study. 
The study was separated into 2 separate parts with identical testing procedures. The first part included all participants (n= 18), whereas the second part included 10 returning participants (9 males and 1 female; 39 [5] y, 178 [8] cm; PRE 76 [10] kg, POST 76 [11] kg). For these 10 participants, the first and second parts corresponded to before (PRE) and to the end (POST) of a 7-month cycling season.

FTP test and warm up
FTP20 Test. The Velotron 3D software (Racer Mate, Seattle, WA) was used for the FTP20 during which the participants controlled the gearing of the ergometer. Participants were familiarized with the gearing system prior to the test.The test was preceded by an 8-minute baseline at 80 W. For the FTP20 test, the participants were familiar with the goal of achieving the highest average PO possible across the 20 minutes, and no verbal encouragement was provided. During the test,participants were blinded to the PO,but they were allowed to see time and cadence to allow for individual pacing strategies.

Note - warmup was only 8 minutes at 80w and subjects did not see their power output.

Results:
Although the results of this study indicate that 88.5% (4.8%) of the FTP20 is more likely to reflect the PO at MLSS, the large amount of variability in the agreement for these measures (limits of agreement=9 to −44 W) prevents the use of this percent value with any confidence as a superior approximation of MLSS. 

  • Again we see that with a short, low power warmup, FTP20 is higher than MLSS as in the first study presented above.
  • With an additional correction to near 90% of FTP20, the match to MLSS is close.

Do either MLSS or FTP change with fitness improvements through the cycling season?
This is complicated and is a topic that is valuable to discuss.  
First, how do we measure improvement in aerobic fitness?  The authors did VO2 max testing in the Pre and Post groups but failed to see a change:
For the 10 participants who completed both phases of the study, no increase in VO2max was observed from PRE (4.32[0.53] L·min−1, 56.6 [4.3] mL·kg·min−1) to POST (4.37 [0.60] L·min−1, 57.7 [7.9] mL·kg·min−1)
However, the MLSS power improved from Pre to Post which is a better parameter of race success than pure VO2 max change:



  • The MLSS Pre power was 252w and increased to 264w after training.  This indicates that on average the subjects were able to increase their power by 12w during the 30 minute MLSS test without change in lactate parameters.
  • However the FTP20 power did not change (286 to 288w).  Does this mean that the FTP20 is incapable of registering the metabolic improvement seen in the MLSS?  A further discussion is warranted.

To better appreciate the FTP20 effort from a cardiovascular standpoint in both Pre and Post training tests, we need to look at additional physiologic parameters.  One metric that would have been helpful would be to actually measure the VO2 (O2 usage) during the FTP tests.  Since the FTP is an effort/motivation/fatigue affected index it is plausible that these athletes were in "better shape" but just were not as physically fresh as before the season.  Cyclists will know this feeling all too well after months of intense, frequent riding.  There does not appear to have been any tapering or post intense training preparation for the Post test.

The cyclist below was possibly in excellent cardiovascular condition, but the consequences of continued high intensity would lead to a poor maximal test.


Although we don't have a measure of VO2, we do have the heart rate to power relationship to go by.  Garmin (Firstbeat) uses this formula to calculate a "Performance Index" that I have discussed before.  Although prone to error with changes in ambient temperature, humidity, altitude, looking at the performance index in these subjects does show a fitness improvementAlthough the Post FTP power was the same as Pre, the heart rate was less (174 to 170 bpm).

Therefore although the conclusion was a negative in regards to using the FTP as an index of fitness/performance, it seems to be as valid as the MLSS, although in a different way.  

From the conclusion:
Furthermore,the results demonstrated that the POs from the FTP95% are not sensitive to small but meaningful and significant changes in fitness level, and thus its use as a tool for monitoring training may be limited
True, power did not change, but the overall picture indicates an improvement in fitness:
  • The ability to cycle at the same power but with a lower heart rate does signify a fitness change.  Whether this is related to better muscle O2 extraction, fiber type shifts, capillary enhancement, mitochondrial change is unclear.  Whatever the reason, in my view this does indicate that the FTP20 can be used to monitor fitness change.  However, it is also monitoring both physical, mental fatigue and potential over reaching as well.
  • The lack of Post test power improvement seems to be more of a fatigue related issue.
  • Perhaps if the subjects tapered in an optimal fashion before the Post test, the FTP20 would have been higher (they would have been able to reach the same HR as in the Pre).
  • The other possible wrinkle is the lack of power feedback during the test.  If the rider was able to see their current and running average power/heart rate, they may have pushed a bit harder.  From personal experience, knowing what power is possible for a given interval is very helpful in pacing strategy and could override elements of fatigue.

Some final thoughts on FTP and longer maximal tests:
Almost 20 years ago an interesting study was published looking at lactate levels, power and gas exchange thresholds in cyclists.  The subjects did a 30 minute time trial with power and lactate values measured throughout.  The results showed that the average power was very near the RCP (respiratory compensation point or VT2):
The average self-chosen work intensity during the ITT30 corresponded to 88% of the subjects’ VO2max (234M11W) and was not statistically different from the energy demand eliciting VT2
This certainly makes sense since both the MLSS and RCP are felt to be at similar intensity.
In essence, this test is basically an FTP30 equivilant.  This finding is also in line with the study above where the FTP20 was similar to the MLSS (without correction) as long as a 5 min MAP TT was in the warm up.  That makes sense if we consider that part of the FTP30 test is encompasses the 5 minute MAP warm up of the FTP20.

Therefore:
5 min MAP warm up then FTP20 = MLSS
Light warm up then 90% of the FTP20 power = MLSS
FTP30 power = MLSS

Here is the interesting graphic form the study:
This is a plot of power and lactate over the 30 minutes.  

  • A key point here is that although power was relatively stable there was some variation in pacing strategy (some higher at the end or beginning).  
  • The metric of most interest is the lactate tracking.  Several subjects had a gradual rise after the 10 minute section and there was significant variation of absolute lactate levels.  The OBLA (onset of blood lactate) of 4 mmol would have substantially different meaning in each of the above  participants.  There was almost a twofold difference in the lactate values during the 30 minute cruise!
  • As a parallel to the FTP20 test we can see differences in metabolic stability during the 30 min TT.  Some but not all subjects have a stable lactate, some continue to rise and a few even have a small decline.  The mean/average response is relatively flat, masking the individual variations of the group.
  • This plot reinforces the key difference between MLSS and FTP testing.

Summary points:
  • The MLSS is a metabolically defined test looking at stability of lactate from minute 10 to 30 of a time trial.
  • The FTP is a maximal effort (presumably to exhaustion) and therefore subject to factors such as motivation, fatigue and pacing strategy.
  • Multiple studies have been done looking at the equivalence of performing an FTP20 in lieu of the MLSS.  
  • The disagreement in equivalence seems related to the "warm up" protocol.  Lack of a significant warm up effort will yield FTP values well above MLSS and correction formulas are needed.  However, incorporating a 5 min TT at the MAP warm up seems to be sufficient to lower the FTP20 to the MLSS power.
  • The FTP20 may not show improvements in average power with positive fitness change.  This may be related to either fatigue, issues in pacing (if power readings are blinded) or even over reaching.  Comparing the heart rate:power relation of the FTP test, pre and post training should help to show a performance improvement (al la Garmin "performance index").
  • The MLSS is not a trivial enterprise to perform since multiple "test runs" are usually needed.  However, the FTP20 test is also physically taxing, given the exhaustive nature of a 20 minute maximal effort.  I personally have done neither.
  • Alternate surrogates for determining the RCP/LT2/MLSS are therefore very attractive and necessary.  Continued investigation into using muscle O2 kinetic breakpoints to estimate MLSS will be a focus of this blog.


Further reading

Sunday, January 5, 2020

Measurement of Hemoglobin saturation breakpoints - use as a fitness monitoring tool

This blog was started as a source of practical guidance in using muscle O2 sensors for both weight training and endurance exercise.  In honor of the New Year, I am going to review a recent article and present a practical guide in using a Moxy, BSX or Hex sensor for monitoring the second lactate threshold otherwise known as the RCP, MLSS, or FTP.  Yes, there are small differences in these terms, but for practical purposes, let's lump them together.  
It has been well established that there are "breakpoints" in the behavior of the oxygenated and de oxygenated hemoglobin signal as measured by sensors over the rectus femoris and vastus lateralis in the leg during progressive incremental lower extremity exercise.  In other words, as one cycles or runs at higher intensity, the muscle O2 declines (or the desaturated hemoglobin rises) in certain patterns.  At about the RCP/LT2/FTP/MLSS, the pattern changes giving one an opportunity to approximate that physiologic threshold.  Some investigators have questioned how a relatively small, superficial area of muscle just happens to correlate with the RCP, but it appears to behave that way (to our benefit).  

This post aims to provide a framework and method for performing your own incremental ramp to obtain the MLSS/RCP breakpoint from muscle O2 data.

A very important observation has been that the desaturation behavior of the rectus femoris is markedly different than the vastus lateralis.  If we are going to put this into practical usage, that is critical to know.  

The rectus femoris has a gradual desaturation with increasing effort then has an acceleration at the RCP.



With the rate of change between stages showing a shift at high power outputs corresponding to the RCP:

On the other hand, the vastus lateralis progressively de-saturates (almost in a linear fashion), until there is a point where it plateaus with no further change:

 
Either muscle group can be used in a progressive ramp scenario.  

Can these ramps be helpful in fitness monitoring?
A recent study (by members of the same team responsible for the figures above) looked at the ability of the HHb (desaturated hemoglobin) breakpoint to track fitness changes through a cycling season in 8 male cyclists.  The question was whether the NIRS derived breakpoint was accurate enough to give meaningful information in an athletes change in RCP.  Why would this be important?  From one standpoint, it could give info on whether or not they are making progress based on a given training schedule.  Perhaps more importantly, it would guide the zone 3 power demarcation needed when adhering to a polarized training regime.
If your fitness improves, you may need to adjust upward the zone 3 cutoff, otherwise you continue to train in zone 2, which may not be wise.

Study Methods:
Over the course of the 7-month season, each participant reported to
the laboratory for 3 to 4 days of testing at 3 separate phases: PRE
(February), MID (May), and POST (August).
Ramp-Incremental Test. The first visit of each phase consisted
of a ramp-incremental test to exhaustion to determine maximal
oxygen uptake (VO2max), gas exchange threshold, RCP, and
[HHb]BP. The ramp-incremental test began with a 4-minute
warm-up at 50 W followed by a 30 W·min−1 (1 W every 2 s)
ramp. The participants were instructed to cycle at their preferred
cadence, and this was recorded and kept consistent for all testing
sessions
On successive visits, the participants
performed a 30-minute constant PO test for MLSS determination.
MLSS was defined as the highest PO at which a stable
blood lactate concentration (Δ ≤ 1.0 mmol) was measured between
the 10th and 30th minutes of the constant PO exercise.26 After a
4-minute warm-up at 80 W the PO was instantaneously increased
to a predetermined value.
 Near-infrared spectroscopy-derived [HHb] was measured in
the vastus lateralis muscle of the right leg (Oxiplex TS; ISS Inc,
Champaign, IL) at a sampling rate of 2 Hz and automatically
interpolated to 1 second by the Oxiplex software. The specifics
of this system can be found elsewhere.12 The probe was placed on
the belly of the vastus lateralis muscle midway between the inguinal
crease and the proximal border of the patella
.
The [HHb]–time relationship from the ramp incremental
test was modeled with the following 2-segment
(“double linear”) linear regression model, as previously described
As an aside, what does the double linear regression mean?  The original modeling was done in this article back in 2012.  Here the same group looked at desaturation kinetics in the VL and compared the fit to either an S distribution (sigmoidal) or a double linear (2 lines intersecting).
Here is the raw data from the 2012 study:
Notice again the somewhat linear rise in HHb (or fall in O2 sat), with a plateau toward the end.

And then it was plotted as a normalized set of values:
It could be argued that either model is valid and that the inflection point could have some wiggle room as well.  
I wanted to show some details since the original paper has some statistical descriptions that may be confusing to some readers.  It is a simple concept, hidden in all the math.

Back to the recent 2020 article:
To account for the muscle–lung transit delay and VO2 onset
kinetics, the mean response time of the VO2 was calculated on an
individual basis and used to align (left shift) the V˙ O2 data to its
corresponding PO as previously described.30 To retrieve the PO
corresponding to the RCP and [HHb], the MRT-corrected VO2
versus ramp PO data were linearly interpolated.
What this is referring to is the time delay of O2 usage (VO2) vs power (MRT or mean response time) seen in incremental ramps vs constant power longer intervals.  Depending on the ramp details, it may be minimal vs moderate.  Since this was a 30w per minute ramp, the effect is large:


If we were to do a slower ramp (10 watts/min = 30/3min) the effect is small (2-10 watts).  We will need to keep this in mind for our testing.

What was the correlation between cycling season RCP power and HHb breakpoints? 

Pretty good!


Conclusions:
The coherence between the VO2 associated with the RCP,
[HHb]BP, and MLSS is retained even after training-induced improvements
in each. Importantly, changes in the VO2 associated
with each threshold parameter are proportional on an intraindividual
level and are strongly related on an interindividual level.
These findings indicate that the equivalence between RCP,
[HHb]BP, and MLSS is not coincidental
Using the HHb breakpoint is a practical way to follow the RCP over time.  

In my experience, the rectus femoris has a larger desaturation response and may be a more suitable target using recreational NIRS devices such as the Moxy and Hex that may not have the precision of higher priced units.
Rectus femoris desaturation will have a different pattern than the VL which is not an issue as long as we know what to expect.  What I would like to show next is the similarity between my RF HHb desat breakpoint with the calculated RCP/LT2 from my previous VO2 max ramp test.  
The protocol was an incremental ramp done with a 30 watt rise every 3 minutes (so about 10w/min).  Although not exactly like the MRT testing referred to above, it is close enough.  As you will see, the results are fairly good and agree with my measured LT2/RCP.

Here is a step by step guide to doing this yourself - VO2 or gas exchange testing is not needed, but you will need a cycling power meter and one of the muscle O2 sensors placed on the rectus femoris.  Make sure the sensor is secure and does not move during the test.

  • Calibrate the power meter (zero it).
  • Warm up for 30 minutes or so.
  • Check recording gear (Ipbike or Garmin watch etc) to make sure metrics are received.
  • Start pedaling at 100 watts.
  • In 3 minutes, boost power by 30 watts.
  • Continue a stable power (130w) for another 3 minutes and keep repeating the cycle (30 watt rise every 3 minutes) until exhaustion (or just before). 

End the recording, save it and import it into Golden Cheetah.  You will need to review the import and data extraction methods from this post.

Create a spreadsheet of time, power, HHb etc (see the post).
Here is an example (with other fields as well).
The goal is to get a running average of HHb over 10 seconds, extract just the 10 second averages and plot against time.
To begin with, make sure the first value in a "time" column is 1. You may need to cut the earlier data out and renumber the rows.



 Create a running 10 sec average:




How to pull just the data every 10 secs:
Make a new column next to it:
Put 9 "X" values in the rows then a "1" in row 10:


Select the 10 rows, and copy to clipboard:

Select the entire series of rows below the initial selection:

Paste the clipboard (the 9 X and 1) into the rows to get this (note Column K):

Go to Data-Standard Filter as pictured above.
Use the Column K = 1 as the filter to get just the 10 second rows to display (remember the other rows had an X in the K column).
Notice the time column - it's every 10 sec now:


We now want to make a new spreadsheet with just the time and HHb results.
Copy each column to a new sheet (important, when pasting into the new sheet, use "Paste special" to remove any formatting or calculation errors (absolute vs derived values).

 

Here is the final plot:
(Plotting methods here)

The breakpoint where the rate of HHb change accelerates seems clear from visual inspection at about 950 plus seconds into the test.

If we were looking at the vastus lateralis muscle, we would be looking for the plateau of HHb rise - a totally different parameter.

The 950 seconds equates to 1 minute into the 290 watt stage:

Although I did not perform interpolation, this seems to correspond to the official RCP/LT2 of 280 watts (290 minus the 10 watt MRT shift).  It also agrees with my unofficial long interval technique to determine MLSS (and here). 
If this was done running on a treadmill, heart rate can be substituted for power.

Summary:

  • It is possible to do muscle desaturation ramp plots to determine MLSS as done in the literature.
  • The result should be fairly close to the "true" MLSS value and has been shown to change as fitness parameters shift.
  • Since true MLSS testing requires multiple long, intense intervals that may disrupt a training program and/or contribute to unwanted stress, alternate assessments are very attractive.
  • Longer, more gradual ramps will show less "mean response time" curve shifting.
  • When comparing your results over time, similar muscle groups and ramp protocols should be employed.  However, small sensor position change should not play much of a role here as in day to day comparisons.