Ground Contact Time (GCT) VERY Different Between Watch and HRM Pro Plus

I saw your post and had a look back as a few days ago I forgot my HRM . My GCT without HRM with my FR965 was 251 and with the HRM is in the 210 range. So mine is the opposite way around to yours . Honestly with all these metrics I look more at the trends over time than the actual figures.

Totally agree on the trends, but when the GCT or any other channel data shows a giant chasm between watch and HRM strap, something is odd. There are minor differences in the other channels, but this one is like a snack is the face, it's so far odd. Bizarre that yours is the exact opposite. Is the pace and cadence the same for the numbers you wrote here? Can you tell me what they are. 

Without - 4.48 Min / km ,  Stride 1.23 , Cadence 167, VO 9.7 , GCT 251

With - 4.48 min/Km, Stride 1.22 , Cadence 171, VO 9.9 ,  CGT 211.5

With - 4.50 min/KM. Stride 1.20, Cadence  173, VO 9.7,  GCT 212.6

I'd have to find another with no HRM as N = 1 is a bit loose. Ill look when I have time

Without - 4:30 min / km, Stride 1.29, Cadence 171, VO 9.6, GCT 241

With - 4:51 min / km , Stride 1.20, Cadence 173, VO 9.5, GCT 213 

Helpful.  Just did a run this morning.  Intervals just a bit slower than your data, but close...see below:

HRM Pro Plus: Stride 1.13, Cadence 176, VO 8,0, but GCT is at 280.  Watch yesterday at same pace showed 250 GCT, which is spot on what you're showing for HRM.

Interesting how close the other items actually are from watch to HRM looking at my info I was surprised.

My watch is higher than the HRM re GCT , yours is the other way around (isn't it?), I see what you mean  re your and my GCT actual figure ...strange!

• That seems to be be the case unless you inverted. And, yes, all other channels have data that is similar. It's just the GCT with HUGE deviation watch to HRM. I did a test that you can do. Run the same tenth of a mile or two at the same pace in the same location. I did it twice with watch only and twice with HRM. So that created a control and the HRM reports 20 to 30 percent more GCT. That's way beyond just a minor difference. 

I'm actually surprised that Garmin watches manage to measure GCT etc from wrist at any accuracy. To my understanding, HRM strap ground contact time is measured in the following way (this is of course not verified by Garmin):

The strap uses its accelerometer to actually measure the time your are not in contact with the ground: during that time you are in free fall, and the accelerometer reads zero (not even measuring normal pull from gravity). Then ground contact time can simply be calculated by subtracting that free fall time from the time between steps.

However, when you have a watch at the end of a flailing extremity called your arm, the problem is that even when your feet leave the ground, your wrist isn't necessarily in zero acceleration situation, because when you swing your arms, your muscles may be accelerating or slowing your wrist during the flight. The watch probably tries to notice footfall by the jolt it causes, but even that is delayed by tendons and muscles on the path from foot to wrist. So GCT from wrist is probably never going to be as accurate as from torso (or feet). And since different people have different ways of moving their hands, there may even be individual biases in different directions.

All I'm saying is that it's just like with optical HR: a HRM strap gives you more accurate results than the watch alone. So if you want to compare your runs for GCT and other running dynamics metrics as accurately as possible, always use the strap. Or never use the strap, then the results are less accurate, but probably they are still comparable with each other (but not as much to other people).

BTW, you can notice the same with running power (at least I can): running power calculated from HRM strap differs from power estimated from the wrist. Luckily with running power you have a setting that only uses power from the strap, so when I run without it, I just don't get any wrist-based power that would confuse me.

This all makes sense.  The watch did "seem" to report a more reasonable number on GCT than the HRM Pro Plus, but hard to know which is more accurate.  Your theory suggests it's the strap, but would be good to get Garmin to confirm.  What is the diff you are seeing in running power FR to HRM?  The FR is about 12 percent higher than the HRM for me.