Heart rate falling during sprint

Mine has also done the same thing, HR dropped when going faster. And shortly after locking on cadence.

This is really disappointing. Cadence lock I do kinda understand, but this points to sensor not being able to read HR at all. I wore watch really tight and regardless of the position, the reading is not accurate.

What results have you had when using a chest strap HRM in the past? "not able to read HR at all" is a bit of an exaggeration considering the trace.

Looking at the second activity (first was on older firmware), it's not apparent whether the pace is lagging or what is going on; HR jumps before pace changes, then as HR starts to taper off the pace jumps up and then tapers off. All in a 1 minute segment. The rest of it seems fairly normal without digging into details.

It'd be interesting to see what result you get comparing to a chest strap HRM perhaps using your previous running watch. How do the pace intervals and HR traces match up?

Now if this is your first time with an HRM, realize optical HRM is best suited for more steady stuff and not as good at capturing short bursts. Remember it's measuring a secondary (or arguably a tertiary) effect of the heart's contractions, with additional interference coming from the arm muscle contractions... and what do we do when we do a speed burst? swing our arms harder...

All in all wrist worn optical HRM is a bit of a tradeoff. If you really need a moment-by-moment accuracy for some runs, then you may want to add a chest strap HRM which directly reads the electrical activity of the heart and isn't affected by the things that will throw off optical HRM.

Think it is fairly common for any optical HR device to struggle with sudden changes in HR.

In the 2nd one (probably more relevant because of the more recent firmware) then yeah HR does jump before pace at around 31-32 mins.
But is the pace graph lagging? It appears to suggest you were only sprinting for 30 secs. Is that right? The HR trace suggests could be more like a minute.

What HR were you expecting to see? I see it got to 160 but quickly fell back to your steady state HR in the 130s.

Not so good for HR based workouts then...

This is my first running watch. I must say I really like it but HR sensor just doesn't cut it.

Regarding second activity - pace definitely isn't lagging since I observed my HR during the run and after the spike I wanted to see what would happen after a 30s sprint. I then stopped to measure HR "manually" (30 sec 90 beats so that's 180BPM).

I understand that optical sensors are not as good as HRM but this makes watch completely useless for interval training.

I was thinking of using Garmin training plans based on HR but now I see this wouldn't make much sense. Instead I will create a plan based on pace/speed on gip.is and somehow transfer it to Garmin site since I don't see an option to create a plan based on current results with each run having exact pace for each interval - only easy/steady/threshold pace.

TBH what your HR does in a 30 sec sprint is largely irrelevant for most endurance training plans but of course it is nice if it gave you a HR trace you might believe.

In terms of the Garmin training plans (or similar) then I imagine the interval lengths are somewhat longer. It would be good to try a longer interval to see what happens there.

Another option, if you want to a benchmark for the optical HR, is to use a HR strap for some interval sessions and see what it brings then.

As an example of interval session HR performance for myself then
https://connect.garmin.com/modern/activity/985355200 is from a 235 and optical HR
and
https://connect.garmin.com/modern/activity/985091280 with a 920 and HR strap (actually a HRM-Tri)

The session was 6 x approx 0.5 mile on the road with 60s rest.

Now there is no denying the HR strap gives a "smoother" trace but the optical gives a pretty reasonable idea.

To a certain extent then the choice in intervals is to whether to use a HR strap for maximum accuracy or just use optical and accept it may not be quite as accurate.

I understand the limitations but my issue is that OHRM seems to have some kind of delay, maybe caused by changes on the skin as it is sometimes mentioned.

I just ran 40 mins with 4x1 min sprint with 2 min rest and just after every interval (during which HR is stable or dropping) HR spikes:
https://connect.garmin.com/modern/activity/990759998

Also it seems that during cool down period HR couldn't drop below 150 although 8kph I was barely running.

I will try to borrow a strap and get to the bottom of this :)

... and remember that there's a lot of benefit to being able to skip a chest strap for the steady non-interval runs and just use one for when it's needed.

The 235 wisely allows you to do either as appropriate for a given run; optical or chest strap as you prefer for a given workout. Use optical when you're just doing steady runs, endurance building stuff where you're concerned with averages and totals and short-timeframe variance is immaterial. Wear the chest strap when you need the higher resolution / lower-variance such as short-interval workouts like where you're finding issues. The 235 supports both modes easily, letting you use a chest strap when it would do better for a given workout.

... and remember that there's a lot of benefit to being able to skip a chest strap for the steady non-interval runs and just use one for when it's needed.

The 235 wisely allows you to do either as appropriate for a given run; optical or chest strap as you prefer for a given workout. Use optical when you're just doing steady runs, endurance building stuff where you're concerned with averages and totals and short-timeframe variance is immaterial. Wear the chest strap when you need the higher resolution / lower-variance such as short-interval workouts like where you're finding issues. The 235 supports both modes easily, letting you use a chest strap when it would do better for a given workout.

I've never worn chest strap before so I don't know how much of a pain it is. But to be honest if I knew this I would go with 230.

There is also a matter of spike just before sprint starts so it not just abut intervals.

When it all sums up it would rate OHRM tehnology (at least for my case) as:
1. 3.5/5 stars for REALLY steady workouts.
2. 1/5 stars for intervals

I'll still test it see if it works for me. If not my better looking half will get it and I'll switch to 230.

@ TIM
Your intervals look really good with no HR lag and if I manage to get this kind of result I would be really happy.

I've never worn chest strap before so I don't know how much of a pain it is. But to be honest if I knew this I would go with 230.

My chest strap regularly dropped out/stopped working until I broke and bought the conductive adhesive goop... but then you have to take the strap, add the (cold) goop, put it on without making a mess... My experience with optical has been good enough and it's far easier - 4 button clicks and you're off. No chaffing, no man-bra feeling.

Yes, chest straps are more accurate, but they have their own problems. The 235 is a great option. Use the optical if it works for your activity, use a strap if your exercise won't play nice.