Revisiting Cadence

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Cadence Source Order

I came across a few places that stated the order cadence data is sourced is:
  1. Running Dynamics
  2. Foot Pod
  3. Internal Accelerometer
So I decided to test this out. I have one of the original Running Dynamics heart rate monitors from my Forerunner 920XT bundle that I normally don't normally use. So I dug to the bottom of my gear storage box and pulled out the HRM-Run and put the 920XT on my other wrist, all DC Rainmaker-style. 

Of course, I tried to pair the Vivoactive 3 Music to the HRM-Run, even though Garmin doesn't list it as Running Dynamics capable. Eventually, after turning off the optical HR in the watch settings, it found the HRM-Run as a heart rate monitor, but only a heart rate monitor as these sites suggested.

I also looked into the Connect IQ API to see if the Toybox::AntPlus::RunningDynamics class would open up Running Dynamics to devices other than Garmin listed. After finding existing ConnectIQ app, Targets + Running Dynamics, I really can't tell. The Targets + Running Dynamics app states:
Running Dynamics fields included! 
Requires an additional ANT+ sensor. E.g. Garmin HRM-RUN, HRM-TRI, RD Pod. And this is currently only compatible with Fenix 5/5X/5S/Chronos and Forerunner 645/645 Music/935/735XT.
And by this point I had put off my run for long enough and it was time to get moving. I'll have to look into this more later.

During the run, cadence from the HRM-Run was displaying on the 920XT, so things were looking positive that the above stated cadence source order was correct. Afterwards, the graphs also backed that up that same order. The 920XT had its cadence sourced from the HRM-Run and the Vivoactive3 had its cadence sourced from the treadmill SDM "foot pod" (reporting 0, of course).
Vivoactive 3 Music - Cadence sourced by the Treadmill SDM

Forerunner 920XT - Cadence sourced HRM-Run

So, it looks like if you've got an HRM-Run sitting around and a watch that supports it, cadence data can be recorded as part of the activity.

The Treadmill Speedup

Another thing I've been noticing is that my warmup mile pace and cool down mile pace haven't matched up, even though I was setting the treadmill to the same pace. I thought this might have had something to do with the belt flexibility increasing over time. That is, it seems a little less flexible at the start of the run than when it's warm at the end of the run.

But looking at the data while using the HRM-Run might reveal a little more insight on this.
Warm Up - Mile 1 - Treadmill set to 10 min/mile - SDM reports ~9:45 min/mile
Cool Down - Mile 7 - Treadmill set to 10 min/mile - SDM reports ~9:30 min/mile
With the Running Dynamics Ground Contact Time overlaid onto the first and last mile pace graphs, the data suggests that the reason for the discrepancy is that the first mile has a much longer ground contact time than the last mile as my legs get warmed up. As the folks at Stryd noted, ground (or belt) contact slows the treadmill belt down, so minimizing it keeps the belt moving faster.

I'm not ready dismiss the flexibility of the belt during the early minutes as at least a minor contributor yet, but this ground contact time data seems to explain a lot of the discrepancy.

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