Wednesday, May 1, 2019

Polar OH-1 accuracy on the forehead

Recently, the Polar OH1 optical heart rate sensor was given a firmware upgrade and marketing change.  It is now Ant+ as well as BTLE capable which is a nice addition if you want to transmit data to both your Garmin watch and Ant equipped android phone (something I do).  In a prior review, the accuracy of the Polar unit was not up to the same quality as the Moov Sweat forehead sensor.  However, the placement of the Polar for that comparison was not on the forehead, but on the arm.  Continued usage of the Moov device has continued to impress me with it's accuracy compared to the Hexoskin shirt (single lead EKG).  The recent firmware change and marketing information now say that the Polar can be used on the forehead (for swimming).  Since the forehead is not prone to either motion artifact nor decreasing blood perfusion as is the arm location, the accuracy should theoretically be better.
Here is the new advertisement from Polar's website:
They show a swimmer using the OH1 under a goggle clip.

I decided to take a look at the heart rate tracking of the Polar OH1 vs the Hexoskin, giving it a second chance to redeem itself.

The test session:
Typical 30+ mile cycling ride with 2 HIT intervals, the first a VO2 peak 4 min, the second a Wingate 60.  Hexoskin shirt with PolarOH1 on the forehead.

  • On initial glance - Looking at the whole ride, there does seems to be reasonable agreement between both units.
Does this hold up under detailed scrutiny?
Let's look at the net difference at each time point.  What I have done here is simply subtract the Polar from the Hexoskin value, so we get an idea of how many BPM the Polar is off from the gold standard at each second of measurement (yes, the values were perfectly synchronized):

During the beginning of the intense intervals, the Polar lags (negative BPM) and throughout the ride is off about 5-10 BPM at times.  It seems like the Polar reads higher than the Hexoskin except for early interval times.

4 min VO2 peak session:
As discussed in a prior post, the VO2 peak power (Maximal Aerobic Power) can be estimated by a 3-5 minute, all out session.  It is important that we reach a near maximal heart rate (toward the end of the interval) as an indicator of reaching peak VO2.  Can the Polar unit do this accurately?:

Net difference at each time point:

  • There is a lag in the Polar heart rate in the beginning of up to 20 BPM.
  • However by 60 seconds, it's within 3 BPM of the Hexoskin.
  • The last 2 minutes are pretty much spot on, +- 2-3 BPM.
  • Recovery values are still close, 0-6 BPM.

Verification of the Hexoskin:
The great thing about the Hexoskin is the ability to look at the raw data, to make sure the tracing is of high quality.

Early interval:

Late interval:
Both tracings are of superb quality, as good as a resting EKG at your physicians office!

Wingate 60:
This is the most demanding of any heart rate monitoring device.  There is severe unpredictable motion, blood flow redistribution away from the arms and non essential areas making both optical and even electrical sensors show artifact.

Here is the actigraph from the Hexoskin:

The smooth lines become increasingly chaotic over the first several seconds of the all out effort.  This leads to motion artifact issues in optical heart rate monitoring. 

The comparison tracings of HR:

Net difference:
Unfortunately, the Polar misses the boat.  In a similar fashion to the 4 min tracing, it does not catch the first minute or so of activity.  This leads to an error of up to 50 BPM.
Also in the same vein as the 4 minute comparison, it then does pretty well with near perfect accuracy later on.

Hexoskin verification:
Despite my body twisting all over the place with max effort, a remarkably good signal.
I did yellow in either an aberrant beat or more likely an artifact.

Moderate power climb:
This was a segment at the end of the ride of neither all out nor high intensity.  This gives an idea of how the Polar will track for the majority of moderate training intensity.
Net difference:

  • There was some lag (again) at the beginning of the segment where the Polar is under reading the Hexoskin.  The mid and end of the power interval are very closely matched.
  • There is also some over reading at the beginning and the end (non powered coasting).

The Polar is thicker than the Moov Sweat unit and if worn under a helmet for any length of time leads to significant discomfort as well as an indentation of the scalp where it was placed.

  • The Polar OH1 placed in the forehead location still does not have the tracking accuracy of the Moov sweat.
  • There is significant lag behind the gold standard Hexoskin (1 lead EKG) during intense and even moderate intervals.  This does not occur with the Moov device.
  • However, it is now Ant+ capable and will work with more than 1 recording device simultaneously.  You could record HR to both an Ant+ smartphone and a Garmin watch.
  • It seems to achieve near gold standard accuracy after 60 seconds of a HIT interval, making it acceptable for VO2 peak testing
  • Comfort under a bike helmet is a major issue for the Polar.
  • But, if near EKG accurate heart rate for an entire ride session is important to your physiologic monitoring, it is probably not the device for you.

In the next post, a review of Firstbeat technology for VO2 max determination will be discussed, including why accurate heart rate (even at submaximal intensity) is essential for accurate measurement.  Firstbeat is responsible for the VO2 metrics of Garmin products, including the new Marq line of watches.

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