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Wednesday, January 30, 2019

Best metrics for tracking cold weather exercise - HR, Watts, Vent, O2 sat?

The choice of which physiologic metric is the "best" indicator of current effort or overall fitness is an interesting subject of debate.  For instance, one athlete may say that peak heart rate is a great index of intensity as well as state of conditioning.  In fact, in support of this, some fitness trackers use heart rate as such to calculate VO2 max or lactate thresholds. On the other hand, cyclists know quite well the value of power (watts), in demarcating training zones as well as comparing month to month overall physical conditioning status.  Another small group of enthusiasts like to use muscle O2 saturation change of the leg muscles for similar purposes.  However, which parameter would be most useful while doing an intense interval in the cold is another matter entirely.  A ride session in the recent cold wave has brought new insights into which metric is best (for me) and it's none of the above!

Some comments on using heart rate and/or power for fitness status and zone training.  Under ordinary circumstances these are very helpful parameters for effort tracking.  However as we saw in the post on cold weather exercise, the max heart rate may be several beats down, submax heart rate slightly higher and power (watts) diminished from multiple causes.  Even so, we should get some ballpark idea of our status.  However, what if both power and heart rate were reduced on a 3 minute VO2 peak interval?  This could be due to carbohydrate depletion, high RPE with central fatigue, over reaching or muscle strength loss.  Remember that over reaching generally is associated with lower power and heart rate vs fitness loss which is usually associated with a maintained max heart rate but lower power.  Exercise in the cold definitely alters these dynamics making conclusions difficult if not impossible.
Is there an available metric we can look at to say, "yes that interval was the best I could do under the circumstances from the cardiovascular standpoint"?  In other words, can we track some numeric value to help decide if we reached maximal intensity given the constraints of cold related changes.  I recently had a ride session on a very cold day, reviewed all the sensor tracings and will make a recommendation.

The following was from a difficult day in the cold, but rewarding from the physiologic data and understanding viewpoint.

The course and time of day were the same as in prior posts - my usual loop containing the 3 minute VO2 peak (I certainly did not hit max on this day) as well as the Wingate 60 second all out burst.  The ambient temperature was 40 degrees F but the wind chill temp was probably about 30 degrees.  

A word about wind chill
We have all heard of wind chill factors and such.  The wind does not actually make the item (person) colder but since the wind is taking away heat (by virtue of air to surface heat exchange), a certain amount of energy is lost.
Here is the conventional wind chill chart:
Riding at 20 mph in 40 degree F temp yields a "feels like" of 30 degrees.

But even more important is the heat loss in watts (a unit of work, just like in bike power):
Since adult males are about 2 meters squared, that means I'm losing about 2000 watts riding in that scenario.  But since I'm wearing clothing it's much less, but you get the idea.
The reason I'm bringing this up is that most lab hypothermia tests are not in a 20+ mph wind so direct comparison is difficult.  Clearly, on the road bike riding (or skiing) will introduce significant cooling and energy loss depending on your state of dress, wind exposure and surface area.

Back to my 3 min VO2 peak
I was careful to have plenty of carbs during the warmup as well as keeping power low to conserve stored energy for the intervals.  Subjectively I was feeling very well pre interval. At the finish I was quite winded and pretty exhausted (at the end of the 3 minutes).  I pedaled as hard as I could, and it was my best possible effort.
Here is the power tracing:
Power in red, Yellow is costal O2 sat, Blue is Moov HR
The first minute was a moderate fast start at 370 watts followed by only 260 watts for the next 2 minutes, with final average of 300 watts.  

This was way less than the usual 350-360 watts. 
Here is what it should have looked like on a warmer day at a similar average power (7/23/18).  Incidentally, this was not anywhere near a max effort, but I wanted to compare monitored values:
With Heart rate and Ventilation (7/23):

Comparing the two -
I was not surprised that VO2 peak was reduced in the cold after writing the previous post on cold weather exercise.  However, two metrics were odd.  The maximum heart rate was about 20 BPM lower than peak (and 10 BPM lower than the same interval power comparison in warm weather) and the costal O2 was 7% (was over 30% in the summer interval). 
If the poor power result in the cold was related to muscle power loss, fatigue, substrate deficiency (glucose) I would have expected low average power, lower heart rate but not a severely desaturated costal O2. That costal O2 desaturation is only seen with true maximal efforts, high work of breathing and cardiac output redistribution.  Under ordinary conditions, the costal O2 at a 300 watt average would have not dropped below 25-35%.

For instance here is another ride in very cold temperature early in the season (39F).  I was unable to even do a 3 min 300+ watt effort.  This is 250 watts for 5 minutes and costal O2 is clearly not dropping:

Here the limitation to a max effort may have been more related to glucose depletion or severe muscle weakness.  There does not appear to be any sign of cardiac output redistribution or high work of breathing.

Back to the 3 minute 300 watt interval at 40F:
The hexoskin data confirmed the Moov heart rate and showed near peak ventilation rates of 200-220 L/min:

So it looked like I certainly was performing at my maximal best effort for that set of conditions from the costal saturation drop and ventilation standpoints.  The only major discrepancy was the heart rate drop of 20 BPM. Since the cardiac output is stroke volume x heart rate, the constrained max heart rate significantly reduced the output and therefore the VO2 peak.  Less cardiac output also means that output redistribution will occur earlier, at a lower power than usually seen (which is what seems to have happened).

Another site measured was the calf using the Humon sensor.  Oxygen extraction was a bit less than usual:
Here is the 3 min x 300 watts at 40 degrees:
360 watts x 3 min at 60 degree temp:
Both pre interval saturation's were in the low 40s, but the higher wattage session dropped the calf a much greater amount (12 vs 25%), despite both being at my volitional maximum.  According to the literature, cold related systemic vasoconstiction should have reduced flow, enhancing extraction in the cold tracing.  Some reasons this did not occur could be related to less muscular effort or cold induced Hb curve shifting. 

Regardless, I don't think observation of the locomotor muscles will be helpful in deciding if the interval power was at our maximum capability under the given conditions.  There are many conflicting factors potentially leading to more or less desaturation.  In addition, if you are not able to fully desaturate in the cold, observation of this metric can't indicate if you have reached your full potential that day.

Wingate 60
Later on that ride, I did an all out Wingate 60 sec interval.  
Once again, the average was considerably reduced (about 60 watts), heart rate max about 12 BPM below usual, but costal O2 desaturation profoundly low at 3%.

Hexoskin data:
Peak ventilation was near max, especially at end interval.  Subjectively, it was a 10/10 perceived effort.  For the first time in doing that route over the years, I considered stopping to rest.

Calf O2 data:
Slightly better desaturation than on the 3 minute effort but it had warmed up a few degrees.
From a practical basis, not very helpful in deciding if I had any extra "gas in the tank", or if this was the best I could do.

So far the major physiologic discrepancy from normal temperature riding appears to be a markedly reduced heart rate (with a commensurate reduction in cardiac output).  Is there any literature addressing this?  We saw in the earlier post that max heart rate is only off by a few beats per minute.  However, as stated before, depending on how one sets up the test and hypothermic conditions, results could be quite different.  For example,
the conditions in this study, were the following:
Very different from a summer athlete wearing inadequate clothing.  One could argue that these subjects were simply doing their routine "thing" and did not undergo any novel hypothermic stress.

An older study done in 1979 used the following protocol:
Different core (esophageal) and muscle temperatures
were induced by swimming in cold water (l3-15°C) or
by submaximal bicycle ergometer exercise. The rate of
work (both in swimming and cycling) corresponded to
40-60s of each individual’s maximal oxygen uptake. The
duration of these exercise periods was 15-25 min. In three
of the subjects mean skin temperatures of 27 and 31°C
were obtained by setting the room temperature to either
+5 or +2O”C. After achieving a predetermined value for
body temperature 
Then the testing was done: 
the subjects moved to a bicycle ergometer
constructed for combined arm and leg exercise
(2). On this ergometer the subjects exercised at a rate of
work, which at control conditions (“normal” temperatures)
exhausted them within 5-8 min
. The rate of work
for each individual case was the same in
It then follows that power selected was a near VO2 peak  since this would be the max power over 3-4 minutes whereas they chose 5-8 min.  Coincidentally, this is very similar to my 300 watt power range.

What they found was the following:
With progressive reduction in body temp, the heart rate curve was shifted lower and the time of work was reduced.  The baseline heart rate in cold subjects was also lower.

 As temp fell, both heart rate, oxygen uptake and max work time were affected.  

At the greatest temp disparity, heart rate max was near 30 BPM lower than usual.  VO2 was less but ventilation was not markedly different.

From the paper:
The heart rate at 1 minute was reduced in all cold subjects to a similar degree, but this progressively deviated from the norm as the temperature was dropped.

And finally:

Bottom line from the study:
  • Huge drops in heart rate during intense exertion are possible with sufficient hypothermia.
  • Commensurate reduction in VO2 is seen.
  • Only minimal changes occur in ventilatory rates with hypothermia.
  • Work time is reduced. 
Back to my data:
What about the Deltoid and Biceps?
I did have sensors on each with the following data for the 3 minute 300 watt interval (at 40F as noted above):
Interestingly there was less than usual desaturation of each compared to a warmer day.  Biceps dropped to the 15% but is capable of 2-3% nadirs on a normal day at max effort.  Deltoid went from 73 to 51%, also less than usual for a max effort.  Could this have been related to the cold induced shift in the O2 dissociation curve?  I can't say, but my Fenix 5 watch was reading 57 degrees wrist temperature instead if 80 to 90 degrees (usual during warmer rides).  The watch was under the clothing layers and jacket, so the limb temp drop was real and not trivial.

Where does this leave us in regards to intense exercise in the cold?
It appears that attempting to do a maximum effort in the cold will be limited by a major reduction in cardiac output (heart rate) - with the following additional summary points.
  • Major reduction in heart rate
  • Less than normal locomotor muscle desaturation
  • About the same (or slightly less) ventilation rates
  • Similar patterns of deltoid and biceps desaturation but not reaching as low a value as usual on normal temp conditions
  • Similar pattern and values of costal O2 desaturation possibly related to both the factors of cardiac output redistribution and high ventilation rate.

Looking at this another way, let's go through each individual metric and make a value judgement in relation to reaching VO2 peak, training zone intensity and fatigue:
  • Heart rate monitor - poor, given the likelihood for substantial suppression of maximum values.
  • Power - poor, given the expected reduction in normal peak values and reduced time to exhaustion.
  • Locomotor muscle O2 saturation - poor, values may be the same or show less desaturation related to interaction of O2-Hb curve shifts, reduced muscle power, systemic vasoconstriction.
  • Biceps/Deltoid muscle O2 saturation - poor, same as above.
  • Costal O2 saturation - appears to be valid and similar to normal temperature conditions.  Potential reasons - could be acting as both an index of cardiac output redistribution at high loads as well as an index of high work of breathing (high ventilation rates).  Significant desaturation at this site can be a sign of impending decompensation and effort intensity.
  • Ventilation rates - appears to still be a valid marker of intensity zones as well as VO2 peak.  There may be a slight reduction form usual max values.

Several conclusions can then be inferred from the above, when exercising in the cold:
  • Heart rate values (by themselves) will not be indicative of training load, or attainment of VO2 peak.  Using them as such can be very misleading.
  • Locomotor muscle O2 desaturations will not be very informative regarding power zones or loading.
  • Deltoid and Biceps O2 saturation pattern may have some utility with regards to recovery from intense loads.  Persistent biceps desaturation (even at somewhat higher than usual values) may indicate a less than optimal state for another intense bout.
  • Ventilation rates do offer a reasonable measure of attaining near peak potential for that given time frame/temperature zone.  They appear to track more closely than heart rate for a given range of relative intensity.
  • Costal O2 desaturation appears to be a valid indicator of peak potential effort by combining both cardiac output redistribution factors as well as the effect of high ventilation rates (work of breathing increase).  Both the curve pattern (downward without plateau) and absolute values (near historic lows) can be used to help judge whether a given effort was at or near physiologic max.  Conversely, lack of costal O2 desaturation provides evidence that the effort was unlikely to have taxed the cardiovascular system to a significant degree.
  • Finally, during an intense interval, the presence of a subnormal heart rate in the face of normally high ventilation rates and/or significant costal O2 desaturation may indicate cold induced effects resulting in the impairment of VO2 peak, and the myriad issues of hypothermic exercise.

Thursday, January 17, 2019

Moov HR Sweat - part 2

In the previous post the Moov HR Sweat preformed extremely well during intense interval exercise.  Since consistent data is important for session to session comparisons, I wanted to explore some more tracings done with the Moov as well as the Hexoskin.  In addition, we will see some raw signal information from the Moov during higher exercise intensity.  I want to emphasize that the intervals below represent a torture test for an optical sensor.  Body motion is severe and road vibration is higher as well. 

Raw data screenshots of the Moov sensor:
This was done on an indoor trainer, with the top pane showing the signal from easy pedaling at 130 watts then followed by 2 different captures at 340 watts.

Although the higher intensity signal is not as orderly as the low power one(top figure), the overall morphology is good enough for an accurate reading.

7 minute interval at 285 watts:
This interval began with 60 sec of a fast start approach at 400 watts.  The Moov (red) tracks very closely with the Hexoskin (green) both early, middle, late and in recovery.  There are some discrepancies at about 2 minutes into the bout of about 5 BPM.

From a practical use case, my purpose in doing a fast start is to reach near VO2 peak quickly.  Part of that assessment is getting my heart rate to about 90-95% max at one minute.  If the heart rate monitor can't track this closely, then the advantage of doing a fast start strategy could be diminished.  One of my criteria for heart rate monitoring, is accuracy at the 30-60 second range of the fast start session to best optimize this pacing approach.

Hexoskin status:
A look at the raw signal in the Hexoskin shows a less than perfect tracing, but good enough for accurate heart rate.

Wingate 60 with extended observation:
The Moov and Hexoskin are still very close, but there is some discontinuity early on:

So there is a 5-6 second gap where the Moov does not seem to discern the rapid change in heart rate.  This also corresponds to the highest external motion of the body.  
However, after the brief mismatch, the correlation is excellent including the maximum peak HR and recovery dynamics.
As mentioned previously, the Wingate 60 should provide an index of VO2 peak, if the maximum heart rate is almost or completely reached.  Failure to track this precisely will therefore cloud whether the VO2 peak was reached or not (as well as information on over reaching).  Therefore some loss of data early on is certainly not a deal breaker for the Moov as long as accuracy returns by 30 seconds or so.

The raw Hexoskin data here is picture perfect:
This was later in the riding session and presumably I was sweating more, creating a better electrical contact for the Hexoskin shirt sensors.

Final thoughts:
  • When it comes to determining accurate heart rate, the Hexoskin shirt is as good as one could hope for (when the electrical contact is proper).  Fortunately, there is a one lead EKG tracing available for inspection after the ride (or in real time using the smartphone app).  Users can be assured of a precise heart rate by reviewing the raw data which is somewhat unique.
  • The Moov HR Sweat appears to be the most accurate optical based unit for interval exercise that I have personally used.  Although not always "EKG accurate", it is pretty close.  Many optical devices are fine for resting and moderate intensity exercise.  However when it comes to road riding, high intensity exertion and severe body motion, they generally do not track properly.  Forehead based measurements take advantage of the preserved blood flow and reduced motion of the face compared to the arms.

Friday, January 11, 2019

Forehead vs Arm Heart rate, the Moov Sweat vs Polar OH1

Across most exercise modalities, it can be argued that heart rate is one of the key physiologic metrics indicating intensity of effort, fitness as well as many other derived parameters.  Unfortunately, the accuracy of most non belt devices is generally poor especially under more challenging conditions such as high intensity movement.  On the other hand, at rest or during sleep, wrist based measurements are usually very accurate.  One of the reasons I became interested in the Hexoskin was the potential for EKG accurate heart rate while doing intense exercise.  This post will examine some practical issues with "gold standard" devices such as the Hexoskin or Polar H10 and a review of a forehead based optical monitor, the Moov Sweat.

Recently a study came out examining the accuracy of several optical based monitors and the Polar H7 chest strap compared to EKG tracings.  

This was done in a lab setting but did include treadmills, bike, and elliptical machines. The results indicated the chest strap was closest to EKG accuracy with various optical devices further down the list.

Interestingly, the Apple watch performed the best of the optical group!

In any case the scatter was relatively high in the optical group indicating less than ideal accuracy.

Why the need for accurate peak heart rates during intensity?
Generally we are not very concerned with some mild deviation at low or moderate intensities where the typical wrist units are fairly accurate. However, at higher intensity there are several potential reasons depending on the particular issue of concern. For example, to reach near VO2 max, one needs to attain a near max heart rate.  If the device can't accurately track this (given the challenging circumstances), it would be difficult to know if VO2 max was reached.  
Another scenario is the differentiation between over reaching vs de-conditioning.  Let's say you have a below average power (or running time) during an intense interval of known power/heart rate.  The question is why?  If you had taken 2 weeks off and were de-conditioned, one could see a normal max heart rate but lower than normal power.  With over reaching the power would also be reduced, but the heart rate would be less than usual:
The over reached individuals had a decline in power and heart rate.  The heart rate numbers were just a few beats off, indicating the importance of an accurate, trustworthy assessment.  So knowing peak heart rate vs power can help guide whether you are over reached or just need to train harder!
The fact that optical wrist based monitoring is not very precise at high intensity is probably recognized to anyone who has ever used them.  Unfortunately, if the results are not precise enough, important training and physiologic status insights could be wrong.

Are belt type units truly the "gold standard"?
One may conclude the the belt type monitor should be equivilant to an EKG derived heart rate given how closely it tracks to the EKG.

However, lets look at this assumption more carefully.  In laboratory medicine, it is very important to have controls and calibrations when using your equipment.  For instance, your glucose monitor may give a numerical result, but it may be erroneous.  This could be related to old glucose strips, low battery, or dirty sensors in the meter as examples.  Coming back to belt type monitors, do we have any feedback data indicating that the tracing is valid?  The answer is no.  One may say that a visible tracing is not needed but the following examples will show why it is important.  Some time back I noted that the Hexoskin was not reading my heart rate properly.  Fortunately, the Hexoskin web site (as well as the smartphone app) can show you the raw data.  The following three tracings of raw data show the problem:

Bad data:
The above basically shows a bunch of random noise.  The output module will transmit a numeric heart rate but obviously it is worthless.  The cause for this was a combination of a cool day, no sweating and no conductive cream.

Some data in noise:
Here there is some information in the noisy signal but the numerical output would be less than trustworthy.  Causes are possibly the same as above

Perfect tracing:
This was taken from a very high intensity interval with a derived heart rate of 170.  It is a well represented 1 lead EKG and the user can be assured that the number is accurate.

My procedure now is to look at the smartphone tracing before I start riding to make sure it does not look like this:
If it does, I apply more conductive cream and make sure there is a clean signal before starting a session.

Even "gold standard" devices can lose precision:
Theoretically a belt strap should have similar accuracy to an EKG, but since there is no raw  data to confirm this (at the time of measurement in the field) it is not a certainty.  Sure, in a lab setting, they would have excellent correlation, but let's say you were interested in monitoring your max heart rate.  How would you know that the signal quality (and numerical value) were associated with a good signal at the same moment as you reached maximum?  
The fact is, belt straps at their best can rival EKG accuracy but we have no guarantee of this at the time of measurement.  

Optical heart rate done right:
Another observation is that wrist/arm based monitoring is near worthless during a session of very intense exercise.  A previous post looking at muscle O2 saturation in upper extremities indicated very poor tissue perfusion at high intensity.  I surmised that this (plus high extremity motion) would make it difficult to accurately measure heart rate by the optical method if the sensor was on the arm.
It was also discussed that moving the sensor to a better perfused, more stable location should be optimal.  I recently came across the Moov Sweat heart rate device which is specifically designed to measure the signal on subject's forehead.  Since I also has a Polar OH1 sensor, I thought it would be interesting to compare both a dedicated upper arm and  forehead unit with the Hexoskin (looking at the raw signal for accuracy confirmation).
Here are the results.

Polar OH1
This is a tracing of a very typical 3 minute interval done at about 360 watts, just above my VO2 peak power.  The Polar is in red, Hexoskin in green with Power in blue.  Although the correlation seems close toward the end, the beginning is certainly not.

Here is a Wingate 60 (max 1 minute power).  This particular type of interval has always given an optical monitor the most difficulty, and I have yet to use one that could properly get the initial phase.  Although there is a slight wrinkle in the Hexoskin at about 20 seconds, the raw data was reviewed and the signal was perfect (an advantage of having the raw data).  The red is the Polar unit which basically misses the entire interval. 

Why does it miss so badly?
Probably a combination of poor perfusion and motion:
Here is a tracing of Biceps O2 saturation (same intervals but different day):
The desaturation is much more pronounced and occurs faster in the Wingate 60 than the 3 minute fast start interval.

A closer look at this was done on a different day, but same interval type (first one is 350 x 3 min, second is a Wingate 60):
The sensor were placed in two locations, each being a recommended spot for the Polar OH1.  The gray area is the medial forearm, green is the medial upper arm.  The relative forearm area hypoxia (compared to upper arm) was deeper in the 3 min interval (near zero forearm vs 20% upper arm), but both sites approached zero during the Wingate 60.  This may be one reason why the Polar unit was able to track during the 3 minute session, but not the all out Wingate 60.

The following is data from the Hexoskin motion data field:
The light blue line is motion and is much higher in the Wingate 60, perhaps by a factor of 2.

Therefore both tissue hypo-perfusion and severe motion are both contributing factors to the inaccuracy of arm based optical heart rate measurement.

The Moov Sweat:
This optical sensor is designed to be worn on the forehead with the supplied case and headband.  According to the designers, it should provide a more accurate reading since motion is less than arm based locations as well as monitoring a site of preserved (facial) tissue perfusion even at high intensity.  Aside from some technical issues in placing this under a cycling helmet, it is very similar in appearance to the Polar (but slightly thinner).

How does it perform?
Here is the 3 min 360 watt interval, with the Moov in red, Hexoskin in green.  The results are quite close, with some minor deviation toward the end (of just a few BPM+-).  Of importance though is the tight fit over the first minute, something not seen with the Polar.

 Wingate 60:

This is amazingly close both early, middle and  post interval recovery.  I was extremely impressed and never thought that this was possible with an optical sensor.  There must have been some motion given how hard I was swinging in the saddle as well as the helmet tapping the sensor with each sway.

The individual data points are presented below:
The correlation is impressive!

As another confirmation, here is a short 25 second maximal burst later that same session:
Still pretty spot on.

For me, there are still some kinks to work out.  The main one is how best to place this under the helmet.  If one was using this to run, swim or ski, there may not be any issue.  For now I am placing this high on the forehead, above the helmet anchor band.  The first tracing above (3 minute interval) used a placement below the anchor, just above the eyebrow.  This was not very comfortable and I would not recommend it.

Signal feedback:
One of the Hexoskin's strengths is the ability to look at the raw EKG signal and make sure it is solid before starting your workout.  Does the Moov Sweat have this ability?  It just may.
In the smartphone app there is a screen showing the signal strength and wave form.

Here are some examples:
Center of forehead, relatively low amplitude waveform.

Lateral forehead low:
Better waveform than above

Left forehead, high near the hairline:
Very good waveform with high amplitude 

In short, there is visual confirmation of signal quality and waveform.  This can aid in placement and potentially (if the company enhances the software) validate peak heart rates if the raw data is recorded.

Summary and Conclusions:
  • Measurement of heart rate continues to be an essential part of ongoing assessment of peak fitness, VO2 peak, zone training, recovery and over reaching.
  • In order to measure this with the most precision, a device should have excellent correlation to an EKG signal.
  • This correlation should be present especially during high/peak intensities that are associated with poor tissue perfusion and severe jerky motion.
  • The motion and perfusion issues interfere in optical wrist and arm based heart rate monitors.  The measured saturation in the recommended Polar unit locations approached zero during an all out maximum 60 second effort.  In addition, the motion sensor data indicated almost twice the motion in the Wingate 60 than a 3 minute VO2 peak interval.
  • A forehead based optical sensor seems to overcome this limitation.  In my examples, the Moov device correlation was excellent compared to a 1 lead EKG (Hexoskin shirt).  The improvement in precision (compared to arm based) was most evident at high intensity with severe body motion, the critical area of interest for physiologic measurements. 
  • Under perfect conditions the accuracy of belt type units also rivals that of the EKG.  
  • Unfortunately, if there is less than optimal electrical physical contact or introduction of other artifacts, there will be signal distortion causing derived heart rate inaccuracy. 
  • Although in many cases the blind faith in a belt sensor's precision does seem reasonable, there is no absolute guarantee that this is correct.  Even the Hexoskin can record poor quality data, but we can assess this by looking at the raw tracing.  
  • If an athlete needs the highest quality heart rate data, only a device with raw signal recording capability should be considered.  
  • For high intensity exercise, the forehead based Moov Sweat will serve as an accurate heart rate monitor comparable to a belt unit.
Part 2 of the Moov HR review

    Friday, January 4, 2019

    Sensor Measurement Variability - Location and Device

    It is well known that muscle O2 sensor use in different locations, contexts and sports applications, will yield day to day numerical variation in results.  This fact makes it difficult to have rigid cutoffs for zone training or threshold monitoring.  For instance, on one day the absolute value for a lactate max steady state in the vastus lateralis could be 45%, but during another session may be 5% different.  This post will explore a major source of this variation, that of sensor position.

    A study done over 10 years ago was one of the first to look at this issue.  They measured the O2 saturation changes in multiple quadriceps muscles during various 6 minute cycling interval sessions.  There were several important observations.

    The speed of desaturation was different for various sites and there was significant heterogeneity in various subjects:
    In subject B for example, the various sites had large variation in degree of desaturation and rate of desaturation which was not seen in subject A.  In fact, in subject A, except for one site (dotted line), the curves look remarkably similar.  Presumably, placement would have little impact in that person.

    Observations in one subject:
    Another example of the difficulty in finding "benchmark" values for training is the following tracing.  They compared all sites as well as site 3 (vastus medialis?) and site 7 (rectus femoris):

    As seen above, the degree of desaturation and speed of resaturation varied at each site.
    Interestingly they did remark that the time constants of desaturation were similar at proximal and distal VL sites:
    Regarding the comparison of the distal- vs. proximal site
    (i.e., S1 vs. S10 and average of S1–S3 vs. S8–S10), there were
    no significant differences of TDp, p, or MRTp for either
    moderate or heavy exercise.

    Proximal vs Distal locations:

    In a given muscle (VL) does a location lower (distal) or higher (proximal) affect the desaturation pattern or absolute values?  
    A look into this issue was done several years ago.  The study was designed to look at proximal, distal VL as well as RF in relation to speed/extent of desaturation as well as any reserve in desaturation potential compared to a tourniquet test.
    Some interesting data follows:
    MOD represents a moderate pace and SVR a more intense (severe) pace.
    This was calculated from the gas exchange threshold (GET) with the MOD being 80% of the GET (GET=resp compensation point) and the SVR 120% of the same GET (closer to VO2 max).
    There was some variation in the baseline saturations of the proximal and distal VL with VLP lower in MOD but higher in SVR, perhaps illustrating the issue of placement, sensor calibration or day to day variation.  There were some small changes in amplitudes but kinetic timing seemed very close.

    The closeness between distal and proximal was evident here:
    Although there may be some statistical difference between tracings, they are very similar in desat degree and shape.  Also of interest is the rapid drop in O2 saturation, followed by a slow rise.  This pattern is very prevalent in my personal tracings.

    However, the O2 "reserve" between VLP and VLD was different.  This reserve potential refers to how much (if any), desaturation ability (compared to a max occlusion value) is present at a particular site under heavy dynamic load (max cycling).
    The VLD (and RF) during dynamic exercise was unable to reach the desaturation peak of an occlusive cuff.  Therefore the VLP is able to desaturate to the max occlusive value (while cycling) but the VLD is not.

     With the conclusion:
    It would not be unreasonable to conclude that although the saturation changes are similar in the VLD and VLP, some fundamental physiologic difference may exist between the two.

    Another important point is that the depth of sensor targeting into the muscle is also a factor in saturation as well as O2 reserves (but won't be covered here).

    My data:
    Armed with the knowledge of the studies above, I decided to measure the VLP/VLD saturations on 2 different days using 2 different BSX sensors.  The sensor positions were switched on the second day in case one sensor estimated differently from the other.

    Day One

    360 watts x 3 min (max aerobic power or VO2 peak)

     Wingate 60 (all out 60sec)

    Although the baseline and amount of desaturation is different, the curve shapes and recovery are remarkably similar between VLP and VLD.

                                               VLD           VLP

    Baseline                             55%          68%     baseline VLD<VLP
    3 min nadir                         30%          47%
    Wingate 60 nadir                31%          48%      

    Max difference                    25%          21%


    Day 2 - sensor position swap

    360 watts x 3 min (max aerobic power or VO2 peak)

     Wingate 60 (all out 60sec)

    Although the baseline and amount of desaturation is different, the curve shapes and recovery are remarkably similar (again).

                                               VLD           VLP

    Baseline                             66%          60%            baseline VLD>VLP
    3 min nadir                         45%          39%
    Wingate 60 nadir                42%          40%      

    Max difference                    24%          21%

    Points of interest:
    • There was a change in comparative baseline saturation after the sensors were switched.  Initially, I thought that the distal site had a lower baseline than the proximal, but that seems to be related to the sensor unit itself (despite the same brand).  Ambient temps, skin thickness and external pressure were about the same.  One of the sensors seems to have a higher baseline than the other, day 1 VLP sensor unit was used in day 2 VLD location (68 vs 66%).
    • Absolute saturation change from baseline was similar at each site.
    • Pattern of saturation drop, then slow rise was also similar.
    • Despite the baseline numbers being quite different between sensor units, the "Max difference" (baseline minus nadir) was virtually identical on each day.
    • Both Wingate 60 and a 3 minute VO2 max interval yielded similar nadir values at each individual location on a given day.  So the nadir at the VLP will be about the same during a Win 60 or 3 min VO2 max interval.

    Final thoughts
    • The literature regarding monitoring muscle O2 for sprint and endurance performance is large and increasing.
    • There are studies indicating that "breakpoints" in the desaturation curve may be indicative of physiologic thresholds of various kinds.
    • However, there will be measurable differences in saturation depending not only on which muscle is chosen, but what part of it, the sensor is upon.
    • Baseline values may differ at nearby locations of the same muscle group.
    • Different sensor units may also show different baseline saturations.
    • It seems however, that the absolute change in saturation (normalized from the baseline and nadir) as well as the pattern of change is similar within the muscle group.  Other nearby muscles may recruit differently, so we can't assume their pattern will look similar.  Either distal or proximal VL locations should show similar patterns and absolute degrees of desaturation.
    • Since real time monitoring of a locomotor muscle can not normalize at a given site, it follows that using O2 saturation info as an "on the bike" threshold, training zone or race guide will be problematic.  One may still be able to retrospectively glean valuable information however.
    • Lastly, day to day, or longer term comparisons may also be of questionable value given the variation in sensor placement that is bound to occur.  Multiple sensor units should also be directly compared and not assumed to be equal in data output. 
    • Additional data : Sensor comparison - BSX, Hex and Moxy