Muscle Oxygen Training

Tuesday, August 7, 2018

Observations just above the lactate threshold...the RCP

The boundary between sustainable and unsustainable exercise is an important parameter to be aware of. Several terms have been used in the literature including critical power, maximal lactate steady state, and various flavors of the ventilatory threshold. In the last post, I reviewed a simple "lazy man's" way of very precisely determining cycling power at the lactate threshold. Knowing this power figure, I thought it would be interesting to look at the various metrics already discussed at a slightly higher power. The logic here is that at a slight boost in power, the steady state is lost, and perhaps we will be able to capture that departure while riding on the road (not in a lab). Which physiologic metric (in my test case) best indicates the "breakpoint", will also be addressed. Instead of going into detail about critical power, lactate threshold and respiratory compensation point, I have linked to some good reviews on each subject to streamline the post.

The following data is from a road ride using BSX sensors on the L VL, R RF, L costal area, Hexoskin shirt, Assioma DUO power meter. After about an hour warm up, a 5 min relatively constant interval at my previously measured lactate threshold was done, followed immediately by another 5 minutes at that power +25 watts. Although I could have used a smaller power increment, I wanted the results to be obvious and as shown on the prior post, 10 watts over the limit did not show much in the way of change (but only over 4 minutes).
In addition, I wanted to get closer to (or exceed) the RCP (respiratory compensation point). This should cause a substantially higher respiratory rate/ventilation, work of breathing, thus taxing the costal muscles to a greater extent.

Two views of the same data (Cycling analytics and Garmin):

The costal O2 drops after about 90 sec to a very stable 50%, and remains there until the second interval of LT+25 watts. At that point, it progressively drops on a near straight line trajectory to 38%. Historically, (in my case) I can get down to about 5-10% potentially, so I still had some reserve left here.
The RF did drop in each case, but only from a baseline of 60% to 54% in the first, 52% in the second interval. So although probably real, within the noise range somewhat.
The VL data is here:

The VL also drops, with the second interval downsloping, but the net percent change is again quite small, curve slope shallow.

Hexoskin data:

The green arrow is the LT interval and the orange arrow LT+25 watts.
I attempted to put straight lines under the numbers to indicate some degree of stability over 30 seconds or so.
Minute ventilation was a steady 172 +- in the LT effort vs 222 at the end of LT+25. As seen in the prior post, minute ventilation appears to be a very sensitive indicator of effort around the lactate threshold.
There was a change in end interval heart rate of about 10 bpm. The extra 25 watts definitely caused a difference here as opposed to just 10 watts.

A close up of the ventilation data:

I drew a green line showing how steady minute ventilation was on the LT interval.
The yellow line is tracking ventilation toward the end of the LT+25 effort, rising rapidly then plateaus at about 220 is my historic max.

What does all this indicate?

Although locomotor muscle O2 does drop from LT to LT+25, it is a minimal amount. The information is not particularly useful while riding since it's so trivial.
Both minute ventilation and costal O2 change in predicted direction as the RCP is approached (or exceeded). Both are visibly significant and can give the rider real time clues on loss (or regaining) steady state kinetics
There is a change in heart rate with LT+25, but given effects of heat, hydration and cardiac drift, it may or may not be reliable. As noted with the previous post, a 10 watt boost above LT does not seem to change it.

A few comments about the RCP
There have been attempts to equate the MLSS/CP as somewhat equal to the RCP.
A recent excellent "rebuttal" to that was recently published. The authors argue that the RCP must appear after the LT by definition (you need an acidosis to cause the RCP which occurs after LT).
Here are the relevant passages:
First, the important understanding of the RCP

Second, they also address the use of NIRS, namely muscle O2. The difficulties and pitfalls of using this are discussed:

Lastly, the order of breakpoint occurrence:

Interestingly, they mention that the RCP generally occurs about 20% higher than the MLSS. That coincidentally is near the LT+25 power tested above (I used 10%). When I performed my lactate threshold, 10 watts (5 percent) did not seem to cause the changes noted here.

Conclusions:

The Costal O2 desaturation curve appears to relate to the RCP. As seen in the previous post, 10 watts above or below the LT do no change the costal O2 to a noticeable degree. However, with 25 watts above LT (10%), a very obvious drop in costal O2 and curve shape downslope will occur. This may provide a real time indicator of respiratory compensation, acidosis and alert the rider to adjust their pace or prepare for the consequences.
Minute ventilation again provides a nice stratification for zones of effort as well as the loss of steady state work. Although I was looking at this parameter after the fact, it does show promise for real time monitoring.
Locomotor muscle O2 does not have the dynamic range to be helpful for a given single individual in real time interested in avoiding loss of steady state, reaching either LT or RCP. There are numerous papers showing a lactate threshold can be calculated from this metric, but on a practical basis, it does not have the ability to discriminate RCP/LT while actually on the road.

Lactate related posts

Saturday, August 4, 2018

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

Sunday, July 29, 2018

Trainer vs road cycling, psychological and physiologic comparisons

We know all too well the hesitation to use indoor sports equipment such as treadmills and exercise bikes. Let's face it, they just are not pleasant to use. I'll take a ride in the rain any day rather than get on a good turbo trainer. However, putting preference aside, do they provide the same degree of physiologic challenges to the cardiac, respiratory and local muscular systems? While using my own bike on the turbo trainer, I noted the perceived effort was way higher than my physiologic metrics indicated. However, the metrics were similar to the road riding data. I decided to look into this further and report on some literature as well as my personal measurements that help reconcile this.

First off, is my perception of a higher relative effort valid?
A study was done comparing an outdoor 40 km time trial with an equivalent indoor simulated time trial (subjects personal bike on a computrainer). The instructions were:

Therefore, same bike, same perceived effort but no feedback such as power, heart rate were allowed.
Results were quite interesting:

The outdoor trial was associated with a much higher average power (205 watts) than the indoor trainer (163 watts). With the higher power came a higher average heart rate in the outdoor trial (152 vs 143). The RPE and core body temp were statistically the same. Therefore, despite the same bike/riding position, an indoor ride will have a drastic reduction in power at the same perceived effort. Conversely, an outdoor ride, at the same perceived intensity, will yield greater physiologic stress and presumed benefits from the work out. Other studies have not shown dramatic difference in physiologic metrics, but were done differently. Either instructions were given to do a max effort, or heart rate/power was available for the subject to see.
For example:

The above study was done at maximum time trial effort:

"After a short rest the subject was asked to
cover a distance of 40km in the shortest time possible achiev-
ing the highest average power output. Feedback in the form of
heart rate response (b ´ min±1), elapsed time (min: sec), and
percentage distance covered were the only visual cues given
during the trials"

Bottom line:
At equivalent power and heart rate, the relative perceived effort will be higher on an indoor trainer. To reach the same physiologic targets, a much higher discomfort level must be endured. No wonder most people don't like riding indoors!

It appears that the physical benefits can be equivalent on a trainer but only with sufficient effort.

However, what about issues such as muscle specificity and overall cycling efficiency while riding a turbo trainer?
This study looked at exactly that using EMG as well breath by breath analysis for calculations of efficiency. The test condition was cycling on a treadmill vs using an electronically controlled turbo trainer.
From the paper:

"Pedalling technique was quantified by a novel parameter that
described the distribution of power during the pedal revolution.
The minimum power outputs during the pedal revolution (i. e.,
the dead centres) were expressed relative to the overall power
output and described as ‘dead centre size’ (DC) [ 21 ] . Leirdal and
Ettema [ 22 ] showed a positive correlation between efficiency
and DC and thus pedaling technique."

The EMG data did show some differences:

The RF was the main muscle of difference and theoretically could be an issue especially for Powercrank users (such as I), since studies have shown that the RF is more heavily utilized with that pedaling modification. What the significance of these differences would be in practical terms is not clear.
Here is a further look at mean muscle EMG activity as power is increased. There is again differences in the RF and BF (hamstrings):

Dead center data:

And finally their conclusion:

Their bottom line:
Turbo trainer EMG and pedaling technique are subtly different than road cycling but do not appear to impact efficiency.
One condition they did not test for was the effect of a road ride up a slight incline. The trainer ride does feels more like a mild steady incline than a flat road (the flywheel helps of course but is not the same as a road feel). Perhaps that condition would have yielded different EMG patterns closer to the trainer? Arguing against this simple explanation are studies showing that pedaling mechanics and muscle EMG patterns are not altered with uphill riding. This is a figure from the above paper showing that seated uphill and level cycling have very similar muscular patterns (as opposed to standing):

On close inspection, there are tiny subtle changes of unclear statistical and physiologic significance between incline and level seated riding.

On to my data.
My original purpose in getting a trainer was to recreate the interval efforts I have been doing and then determine finger-stick lactate values over this time. Outside blood measurements in the heat/humidity of Florida are just not possible with the Lactate Scout (error 1 always seen).
As I was using the trainer, I was struck by how hard I was seemingly pedaling but the "numbers" were not showing costal O2 drop, heart rate elevation as they should. This now makes sense in view of the first discussion above.
Lets now look at my physiologic metric comparison, specifically Hexoskin cardiac, respiratory data, costal and leg muscle RF SmO2 vs power.

The following interval tracings pairs were done in the same session, first a road ride of about 2 hours (1 hr warm up, interval, 1 hour back), then immediately putting the bike on the turbo trainer. Another 10 minute or so warm up is done then the same interval was repeated.

Even start on the road at 330 watts:

Measurements include
Costal O2 dropping from 58 to 23%
L RF SmO2 dropping from 47 to 8%
Heart rate max 168
Minute ventilation at 30 seconds - 105 L/min
Minute ventilation at end 210 L/min, with transient rise to 226 afterward
Respiratory rate at 30 seconds - 46/min
Respiratory rate at end 59/min

Even start on the trainer:

Measurements include
Costal O2 dropping from 56 to 13%
L RF SmO2 dropping from 46 to 6%
Heart rate max 167
Minute ventilation at 30 seconds - 118 L/min
Minute ventilation at end 214 L/min, with transient rise to 233 afterward
Respiratory rate at 30 seconds - 39/min
Respiratory rate at end 55/min

Allowing for some random fluctuation and noise, remarkably close.
There is some additional O2 drop on the costal area, but the hypoxic shaped curve is very similar.
The RF data was pretty close as well, but since O2 extraction was near complete in both cases it is difficult to judge in my opinion.
Certainly, heart rate (single lead EKG) and respiratory rate/volume were close

Next case, Fast start at 310 watts:

Fast start Road interval with bilateral costal O2:

Same graph interval but looking at R RF muscle O2:

Measurements include
R Costal O2 dropping from 73 to 36%
R RF SmO2 dropping from 75 to 54 rising to 61%
Heart rate max 163 early in the interval
Minute ventilation at 30 seconds - 100 L/min
Minute ventilation at end 209 L/min, with transient rise to 219 afterward
Respiratory rate at 30 seconds - 40/min
Respiratory rate at end 56/min

Now for the trainer data
Bilateral costal O2 and power:

Hexoskin:

R RF muscle O2:

Measurements include
R Costal O2 dropping from 72 to 36%
R RF SmO2 dropping from 69 to 55 rising to 58%
Heart rate max 162 later in the interval
Minute ventilation at 30 seconds - 100 L/min
Minute ventilation at end 208 L/min, with transient rise to 219 afterward
Respiratory rate at 30 seconds - 44/min
Respiratory rate at end 50/min

Comments:

The R RF deoxygenation curves are very similar.
The costal deoxygenation curves are also extremely close to each other.
Both respiratory rate and minute ventilation are on par with each other, although rate seemed a bit less on the trainer toward the end.
Heart rate curves are slightly different but max values about the same. The heart rate rapid rise is present in both cases but there is a slow rise on the indoor trainer as opposed to a slight decrease riding on the road.

One of my continuing interests is in the Fast start pacing strategy of Dr Stephen Bailey. A difference in technique (amount and duration of fast start) may not be the only variable when comparing my observations to that published. They use a trainer indoors (understandably considering what they are measuring):

Would the results be different on an outside road condition? Probably not, but I need to keep testing conditions in mind as further trials are done. One of the major differences that I am seeing with the Fast vs Even start pacing is that of heart rate trajectory. At least on this limited look at indoor trainer vs outdoor comparisons that differential may become blunted.

Road/Outdoor vs Trainer/Indoor cycling generalizations:

Perceived effort is higher on an indoor trainer. Whatever the reasons (distraction riding outside reducing RPE), it is something to consider in attempting to simulate an outside road ride. It also helps to explain why many of us consider indoor riding so unpalatable.
Overall cardio-pulmonary impact at equivalent power outputs is very similar in both conditions. Heart rate, respiratory rate and minute ventilation were all similar between test conditions. However they may be some subtle differences in heart rate patterns in the fast start road vs trainer. This may also translate to lower costal and RF SmO2 at interval end. I need to explore this further.
There were subtle alterations in pedaling mechanics and muscle usage with a turbo trainer. Whether this is going to impact road riding specificity is not clear. It is also possible that these changes resemble outside riding up an incline however literature does not support the EMG changes that are seen with this. The RF is the muscle group that seems to most affected by trainer use. However, NIRS measurements of this muscle did not show marked difference while using a trainer.
Lastly, the Hexoskin, BSX sensor combo is a helpful tool for home physiologic testing and metrics.

This post is dedicated to my son Kipp who uses a trainer exclusively (performing multiple intense intervals) and has become a gifted cyclist as a result.

Sunday, July 15, 2018

Hexoskin shirt, review and observations with Interval exercise

Also of interest:

MLSS determination by doing long intervals with the Hexoskin

Data download and presentation - using Hexoskin .csv web data

The past several posts have revolved around the interaction between locomotor and respiratory muscles in relation to their respective muscle oxygen saturation during exercise. As the exercise intensity rises, acidosis ensues and a higher rate/volume of breathing is needed to help remove the CO2 that results from acid buffering. However even before that, at moderate work loads, more O2 is needed and respiratory rate and volume are boosted. To better quantify this process it may be interesting to follow the respiratory rate and minute ventilation alongside the costal O2, power and heart rate. The Hexoskin shirt has been available for several years to do just that with study data to back up it's accuracy. It provides the following:

One lead EKG for rate and HRV
Respiratory Rate
Minute Ventilation (estimated by an algorithm)
Activity sensor/accelerometer

The placement of the costal O2 sensor is such that a conventional belt monitor like a Polar H10 will get in the way of the device. As a result, I have been using a Polar OH1 which is ballpark at best, and worthless in a sprint. The Hexoskin may also be the answer to better heart rate monitoring for me since I can't wear a belt and O2 sensor simultaneously. The following post is meant as a commentary of my initial experience using it for both heart rate and respiration monitoring.

The shirt basically has sensors for an EKG, 3 axis accelerometer and of course sensors for monitoring chest/abdominal expansion.

The fit was reasonable (size large for me at 178 lbs), comfort good (even in hot weather), and there really is no software to set up except the PC app to transfer the data from the bluetooth module (which is small). If you are going to use the android/iphone app, you may not be able to pick up the BTLE heart rate on another device (watch, bike computer) or even on the same device using a different app (Ipbike). Since this can be a limitation of bluetooth pairing, I decided to just use it as a heart rate sensor for live data, and analyze the respiratory particulars after the ride. Eventually I would like to get both Ipbike to record heart rate and run the android app in real time concurrently on the same device.

On my initial use I had issues with the O2 sensor moving around under the Hexoskin. My solution was to layer a Skins shirt over the Hexoskin. A coban wrap light sensor on top of the Hexskin but under the Skins shirt was also used to block light and keep things tight. There are 2 "belts on the Hexoskin, the sensor is placed between them, as noted in red.

The results:
The following tracings are from 2 rides, both in very hot, humid weather (90+). The minute ventilation was probably calculated on the weight/height I put in (which were not accurate, I was initially in a rush and didn't bother converting into exact metrics). However, the breathing rate would not be affected and the ventilation is comparable within my personal data (on a relative basis).

The first interval is the typical 1 min max on the same hill as previously shown in older posts.

Several comments:
Heart rate (blue) seems spot on, consistent with no drop outs, recorded with a Sony Z5 premium with Ant+, BTLE using Ipbike
Excellent costal/deltoid O2 drop.

Now the Hexoskin data

(A potential shortcoming, at least with cycling is the lack of power metrics on the tracing. We can get around that by looking at the Activity data (green arrows). During a significant interval or sprint, the cadence and activity are higher as shown.)

The heart rate track is identical of course(same data).

The respiratory rate starts at 32 (was as low as 20 before that) and rapidly rises. The spike of 63 may be real. At that point I was getting very winded and probably taking shallower breaths at too high a rate for maximum effectiveness. Interestingly, the minute ventilation rises relatively smoothly and is highest after the interval stops. No wonder the costal O2 stays down even with low power (150 watts) and immediately after ending, the respiratory muscles are working quite hard. From previous observations, the leg muscles are rapidly resaturated post interval, but they are now resting whereas the respiratory muscles are still at maximum load. There appears to be no "coasting" for the respiratory system!
Take home point:
This may be a good illustration of what happens when respiratory muscle O2 drops below a certain point leading to less than optimal muscular efficiency as well as the prolonged time to recovery after such an event.

Several days prior I did a similar 1 min max interval but the O2 sensor moved. Here is the Hexoskin data:

Unfortunately I had a difficult flat and tire change (60 min walk to get air-a long story) well before this point and was probably dehydrated leading to the high heart rate.
Again we see a spike or 2 in respiratory rate without corresponding bump in ventilation. Although artifact is possible, I think it is from shallow, fast breathing. Minute ventilation smoothly rises despite that.
(This was also my first time using the shirt and band fit slightly off.)

Also on my first outing, a 5 min Fast start interval was done (very similar to previous posted data).

The costal O2 drop was a bit higher than usual, either related to the lower baseline, or from the dehydration. Regardless, it was relatively stable as was the chest/pectoral O2. Notice the last 30 sec or so, power was boosted and costal O2 took a nosedive.

Hexoskin data:

The activity (in yellow) gives a good guide to the start, stop of the interval.
Heart rate rapidly and smoothly rises, falls a bit after the power backs off then rises at the end as expected (higher power).
RR starts at 21 (in the box), rises to 49 after the fast start is over, but continues to rise (slowly) throughout.
Ventilation follows the same path, a gradual rise, peaking at the end, then rapidly drops after coasting.
It does not appear there is anywhere near the degree of acidosis as in the 1 min interval where respiration rate and volume remained high (and was highest) after power was turned off.
Also of interest, the RR and ventilation were not higher during the initial fast start segment.
It will be interesting to redo the even and slow starts to compare further.

Several days later a 3 min Fast start was done (also same place/hill).

I also put a sensor on my L costal area (all previous costal measurements are on the R) to double check accuracy since the Hexoskin may cause some motion as discussed above.

It was really nice to see that the tracing was within 1% of the R side!
In addition, there is a relatively smooth leveling out of the desat after 2 minutes (especially on the deltoid).
Will this reflect on the Hexoskin data?

The green arrows (activity data) indicate the interval. The start of the fast start intensity is manifested by the green cadence line.
The respiratory rate smoothly rises as does the minute ventilation. The absolute value of the minute ventilation was near my historic max despite the respiratory rate being well below historic max (see above 1 and 5 min tracings). Could this be an example of more efficient usage of respiratory muscles? Since hypoxia of the respiratory muscles was minimal, perhaps the muscular system was more effective.
Something to look at in the future as well.

Usefulness for "next interval readiness"
One of the claims of both current muscle O2 sensor vendors is that by looking at leg muscle oxygen kinetics one would have a clue as to when to "hit it" hard again. That's misleading on many levels (previously covered) and certainly not supported by any literature. But what about looking at respiratory parameters? That may make more physiologic sense. If you became acidotic, plus high O2 debt, your rate and volume will be high after the effort. Although there are many other causes of fatigue (especially central factors), having your ventilation volume and rate near baseline probably heralds return to physiologic baseline (if that was what you are aiming for).
Here is a look at post interval data, first from the 3 min fast start:

Several interesting observations. The costal O2 promptly returns to baseline, as does heart rate. The respiratory rate and ventilation also return to "active baseline" within a couple of minutes, but it does take a full 10 minutes to get back to resting (coasting).
It seems that time to readiness would be relatively short in this case.

The one minute max data is more extreme:

The minute ventilation stays higher for a very prolonged time (O2 debt, acidosis) and even 8 min later despite almost no pedaling is still high. Breathing rate does not appear to so affected, therefore the per breath volume must be higher during this time. Time till next interval/effort in this situation would be far longer (if you wanted a more baseline condition to start with).

Now for some zoomed looks at the raw EKG data.
This is a tracing during coasting down a hill (before the interval-see orange circle on bottom), but still outside, in the heat and real world:

I was extremely impressed with the quality!
This is as good as a tracing done in an ER or hospital. Well formed QRS complexes and stable baseline.

There is a bit of deterioration at max efforts(bottom circle in orange) but considering how I was moving, still very good.

The chest movement is also seen and appears physiologic, regular with clean contour.

A last tidbit
After my 3 hr ride in 100+ heat index conditions, I put the bike on a trainer (Elite Muin) and tried to do a 3 min interval with a lactate measurement(a future series of posts). Unfortunately, I was pretty beat and could only do 2 minutes but here is the data:

Hexoskin data:

I circled the 3 min, 1 min and trainer intervals in green on the bottom time line to give an index of timing.
The activity sensor gives us the start, stop of the interval even on a trainer.
Heart rate started higher than usual and rapidly hit near max (dehydration, vasodilatation).
Both minute ventilation and resp rate smoothly rose, similar to the 3 min fast start (but this was an even start) and costal O2 fell relatively rapidly. Lactate was 4.6 mM/L within a few seconds of finishing. I don't want to make too much of this, except as proof of concept. I think the limit was more central fatigue with some component of dehydration mixed in.

Thoughts about Carre Technologies
One of my major gripes of muscle O2 sensor companies is the over the top, distorted and many times baseless claims about what their product will do for you. Since I covered this in prior posts I won't dwell on specifics. How does the parent company Carre Technologies position the Hexoskin? Is it the next miracle product that you must have? Is there any innuendo that one will have some breakthrough in fitness, training, racing? In one word NO!
Let's look at the web page:

What they claim is what you get. The honesty is refreshing. I also had several followup emails with tech support for helpful suggestions, background info and details of the web interface. I tremendously respect this approach and hope that other outfits take note and follow this example.

Some concluding remarks:

The Hexoskin shirt accurately measures heart rate, HRV via a single lead EKG method. Wave shape even with high rates and motion (with some deterioration at high activity) appear physiologic.
It has a 3 axis motion sensor, useful for figuring start and stop of interval efforts.
Breathing rate is measured and available either in the Phone(IOS, android) app or after download of the bluetooth module. I have no benchmark for accuracy but studies have shown it is quite accurate.
Minute ventilation is calculated via the breathing rate and chest/abdominal expansion sensors. It would be internally consistent in one person but conversion algorithms are needed for absolute values.
Looking at respiratory parameters alongside heart rate and costal O2 is of potential value. One scenario is obtaining a better idea of steady state exercise intensity. The optimal values of both costal O2/power outputs along with breathing rate, minute ventilation when they are at their most efficient would be helpful. As noted in comparing the 1 min vs 3 min fast start intervals, end respiratory parameters are very different. Very high breathing rates with less than peak minute ventilation may correlate with reduced neuro muscular efficiency. It will be interesting to compare the Fast/Slow/Even start protocols again with this approach, looking at parameters through the effort.
Another use could be in knowing when you are ready for another hard effort, at least from the respiratory standpoint. We know from previous observations that leg muscle re-oxygenation happens quickly but costal is much slower. This is partly due to the higher respiratory rate and minute ventilation, both of which can be observed with the Hexoskin in the field. As illustrated above, the post 1 minute max effort with severe costal hypoxia leads to a prolonged period of high minute ventilation. This can help plan proper rest periods between bouts, as well as provide potential data on improvement in fitness (shorter time till normalcy). In addition, looking at post interval data of Fast, Slow and Even starts may also be different.
The parent company does not make any unproven claims which should be applauded.

Next: Can the Hexoskin be used as real time training tool during a road ride?

See also:
Analysis of Hexoskin binary RR interval and respiratory .wav data

And: An overlay for the Hexoskin app to overcome some of it's drawbacks