Tuesday, April 23, 2019

Sensor value comparison, BSX, Hex, Moxy

Published studies have evaluated the accuracy of the 3 main muscle O2 sensors, but how do they compare in side by side?  If you only have one brand of sensor, this may not be important, however if you have been using a BSX unit and switch to the Hex, will your historic values hold up?  In addition, let's say all 3 are "accurate" in tracking O2 changes, but you want to buy the one with the best dynamic range.  Which one has that property?

I personally have 3 BSX and 1 Hex sensors and was also able to get some data comparing a Moxy to the Hex. This post will be about some observations on all three.  Several issues to consider when making comparisons are the following:
  • Location of each device - The baseline and range of O2 sat will of course be different between muscle groups.  However, the location within the same muscle will also yield different values.  The proximal vs distal site of the quadriceps should be different.  Therefore if we are using two sensors on the RF for example, they may not have the same dynamic range simply because of this factor.
  • Left vs Right legs - Some individuals have leg strength and/or O2 saturation asymmetry.  In my case, my L RF has a much higher dynamic range of saturation values despite having near 50/50 percent power equality.  So, a direct left vs right comparison will not be valid.
  • Light interference -  I noticed that on some occasions the BSX strobe LED caused artifacts in the Hex readings.  The fix is to place them at least a few cm apart, but then you are getting the proximal vs distal difference effect again. 
What I have done in the cycling interval tracings below is a proximal/distal placement on one day to get a general idea of pattern, then moving the Hex to exactly where the BSX was on the L RF on a different day but doing a similar interval (Wingate 60).  The near exact placement location and similar power interval should give us a good idea on dynamic range difference.

Proximal BSX, distal Hex on the L RF - 350 watt x 3 min with a cool down:
The sensors are on the same rectus femoris but one on top of the other.  Pattern should be matched on the time scale.

O2 sat

Total Hb:


  • The pattern of O2 saturation, deoxy hemoglobin appear very similar.
  • Dynamic range of the Hex is markedly less, but this could be related to distal position.
  • There may be a slight difference in THb.

Wingate 60 at 500 watts:
Same day, placement

O2 sat 

 Total Hb


  • The pattern of O2 saturation, deoxy hemoglobin appear very similar.
  • Dynamic range of the Hex is markedly less (55 vs 20%), but this could be related to distal position.
  • The difference in THb is minimal in this max interval session.

Different day, but now the Hex is in the same spot as the BSX sensor was in the tracings above.  
Perhaps the dynamic range will be better?

Wingate 60 at 500w with cool down - Hex only - O2 sat:



  • The pattern and dynamic range of O2 sat and HHb are now a bit closer to the BSX unit.
  • It seems that location does make a big difference in this regards.  
  • On the R RF (not shown) the BSX O2 sat dropped from a baseline of 70 to about 50%.  
  • The total Hb pattern resembles the BSX (in the same location) closely.

Data line up: What I have done here is to take the day of left RF sensor data from the BSX and match it in time to the different day of Hex left RF data (same spot).  The interval was within 1-2% of the same power average and I show both power curves:

Summary of same location, same power profile:

                                Net range
BSX  70 to 15%            55%
Hex   64 to 30%            34%

  • Even with the same placement, the Hex has a more limited dynamic range than the BSX.
  • In my case, a distal vs proximal placement as well as left vs right leg location does impact the result but not the pattern of saturation, total Hb change.

How does the Hex do on a weight training session?
This is a comparison between the Hex and BSX on my L tricep doing body weight dips.  The length of the active interval is about 30-40 seconds.



 Summary of same location, same power profile:

                                Net range
BSX  66 to 1%             65%
Hex   60 to 7%             53%

  • The BSX has a higher min to max range.
  • Start and stop changes are blunted in the Hex due to the time averaging algorithm they employ.
  • Although the Hex has a smaller dynamic range, it is still ample and one could argue that avoiding near zero values is an advantage.  

Moxy vs Hex
Although I don't own a Moxy sensor, I am using data from my friend the XCSkier who does.
The following tracings are from a session of cross country skiing, Moxy on the R RF and the Hex on the L RF.  Although mirror image location was the same, we can't directly compare legs based only this data.  If we had 2 Moxy's or 2 of the same brand that were documented to match, that would be a different situation.  But we can get an idea of trends and will look at range anyway.

Up and down hill.
O2 sat:
  • Both the Moxy and Hex track in a very similar fashion.  This is certainly good to see if you are considering a switch from one to the other.
  • The range of the Moxy is larger than the Hex, but since they are on different legs, it is hard to say if this is based on location or sensor design.

Total Hemoglobin:

  • The tracking of total hemoglobin in a short time frame is not as evenly matched.  Whether this is from the smoothing algorithm on the Hex is unclear.

To better examine the total Hb change, I decided to look for reperfusion changes during a segment of low heart rate, hoping this corresponded to coasting:

Here is the Moxy O2 sat and total Hb.  
I circled in blue where heart rate drops (light gray) and the total Hb, O2 sat markedly rise.

The Hex during the same segment:
  • The first circled area exhibits the same behavior with rapid O2 sat increase and total Hb.
  • However, the base of the total Hb tracing is broader, possibly from smoothing.
  • On the second marked area there is still agreement, but not as much.  My guess is this is also related to the smoothing done on the Hex.

  • BSX, Moxy and Hex sensors all track O2 saturation and total hemoglobin in an overall similar fashion.
  • The dynamic range of the Hex is less than the BSX and probably less than the Moxy.  This can be troublesome in a muscle location that already has a marginal resting to nadir value range.
  • There is a substantial smoothing algorithm employed in the Hex that can mask sharp change in data values.  In my opinion, this should be an optional feature and potentially interfere with interpretation of the sensor's data.  I would personally rather have it off and if I needed smoothing, just design that into the spreadsheet after data extraction.
  • Different legs can have very different saturation ranges despite identical location, it can't be assumed they will match unless tested.
  • As long as these factors are considered, all three devices should be interchangeable for desaturation patterns, making them all useful for MLSS measurement in long intervals.

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