John Galea's Blog

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Leg based heart rate monitors

I’m always looking around for new ways to track my heart rate. In my mind the heart rate can tell a lot about you and your day. Warning … This post is going to be a bit of a ramble. A couple of new products have come on the market and I have been thinking of splurging on. The new Garmin Vivosmart 3 looks interesting and includes true all day heart rate monitoring. This allows you to get a couple interesting tid bits of information including your resting heart rate and your HRV. HRV (heart rate variability) is a measure of how stressed/tired/exhausted you may be and can be used to guide you in when and how long/hard you ought to work out. A quick read on DC Rainmakers web site reveals, as expected, wrist based heart rate monitoring for cycling is not practical. In Ray’s words “In case it’s not overwhelmingly obvious above (the yellow line): It sucked.  Badly.” On the positive side it would give me more data in the Garmin connect world, and would land all my data including sleep/tracking all in one place, the holy grail. So if I bought this one, I wouldn’t be using it on my rides.

Another gadget that caught my eye and what triggered this post is the Lifetrak Zoom HRV. This is again a wrist based all day heart rate monitor that also provides HRV (thus the name Captain Obvious :)). One of the unique things they did was provide an optional arm/leg band so you can move the sensor to somewhere it might get more accurate data during workouts (ie off the wrist). They claim they can get accurate data from multiple places on the body. So it got me thinking, I wonder if it’s actually possible to get an accurate heart rate from the leg? The lifetrak even recommends using it on the leg for cycling in which case it will also get cadence (rate of rotation of the peddles). Hmmmm.

And thus we have the experiment. So first off, I do not own a LifeTrak Zoom HRV, I do however own a Scosche rhythm + that can be worn on the leg. I’ve never seen anything talking about whether you can or can not do this so …. I wore the sensor on the leg above the calf, below the knee. This insured it wouldn’t fall off when cycling.

One of the first things you have to ask yourself is what are you trying to do with the heart rate? If you are trying to use it to keep your workout in zones then accurate data is a MUST. If all you want is an accurate calorie count then accuracy of the data at a given point is less important, average are all that really matter.

First off lets have a look at simple sedentary measurement. Sitting around not doing much. In this case the data actually looks quite promising. Both the point accuracy and average look good. For this comparison I used a polar h7 chest strap.

Expecting more than a 10% accuracy is unrealistic in this market segment IMHO, however having a variance of 8 bpm is getting up there as impractical for use for heart rate zone management.

Now let’s have a look at quick stair climb. This time the leg will actually be doing something. This time around it looks bad. There’s a short period of time where it’s just an act of fiction. Then a period of time where it lags (somewhat expected) and lastly it seems to somehow catch up.

Again it did reasonably ok from averages point of view, but bad for point comparisons.

Now I went on a short walk 20 mins. In this case for the most part the data tracked reasonably well, although there’s some noticeable lag in the heart rate being detected on the leg, and then something bizarre on the end of the graph.

And last but not least we get to my real use case, cycling. Visually comparing the data during the ride the leg based heart rate monitor was REALLY BAD. Like unusable, an utter act of fiction. If you were using it to guide you in zones you would be completely off. Average wise, shockingly it’s not so bad. I can only imagine the pounding of mountain biking could make this even worse.

I had another thought … What about the ankle, would it be any better? Again the scosche is on the ankle and the tickr is a chest strap. As you can see, other than an odd drop out for a period of time it was not totally out to lunch. Now that said, again, it would not be accurate enough to use for keeping you in zones. There were definitely times when I could see 10 or BPM off. Enough to effect what zone your in. And again, average wise it was not half bad.

So what does all this mean? First up for cycling, the Rhythm can not be worn on the leg for accurate heart rate data. I would also be skeptical that when worn on the leg that the Lifetrak Zoom HRV could be considered accurate enough for cycling. Now I admit to making a HUGE leap having done this test using a different monitor, but I have seen comments around the web that have also indicated that the Zoom on the legs accuracy on heart rate is BAD.

I asked Scosche about using the sensor on the leg, here is what they said: “Greetings John we have had customer use our monitor on their leg. As far as testing, no real testing has been performed on the leg application only the forearm.”

May 11, 2017 Posted by | Uncategorized | Leave a comment