Sunday, July 15, 2018

Hexoskin shirt, review and observations with Interval exercise



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 



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