linear heart rate readings graph / erratic measurements

I understand your frustration

Based on the traces you posted I see a couple of problems.

First, two of those traces show what appears to be a "dropout" in the heart rate function. I had a total "dropout" yesterday that corresponded with my removing my chest monitor after a morning walk up until the time when I got home and put on my chest monitor to take the dog for his evening walk. Obviously the wrist monitor failed to start up after I removed my chest monitor in the morning, most likely due to some "glitch" in the software. Note, after removing the chest monitor after the evening walk the wrist monitor picked up working properly. What bothers me is that this seems to be a random bug in the software because this has only happened to me two times since purchasing the 235 in August.

The second problem I see is that I believe those traces were made up totally from start to finish. Because I have NEVER EVER seen a heart rate trace that linear and smooth using either a chest monitor or my wrist monitor. IMO that trace is an estimate based on what the programming in the watch was designed to display in the event that the wrist monitor couldn't get a "lock" on the heart rate. One tip here would be to make sure that when you trigger the Run application you watch to see that message about the Wrist Heart Rate Ready, this indicates that the wrist monitor has a "lock" on the heart rate. That may help insure that your wrist monitor is actually tracking the heart rate at the start of your run. However I use a chest monitor for one very good reason, that is that I've found the wrist monitor just isn't reliable for any activity involving a lot of arm motion. My second tip here is rather obvious, get a chest monitor and use it.

BTW, I've learned from experience that the chest monitors are not perfectly reliable either. First, the Garmin soft strap isn't very sweat resistant, so use a Polar chest monitor strap. The Garmin monitor snaps onto a Polar strap perfectly and the Polar straps are much much more sweat resistant than the Garmin strap. BTW, when the strap become salt saturated the chest monitor will tend to go into a "runaway" where the heart rate reading just keeps going up, even if you stop and lay down. The solution to this is to drop the chest strap in a bowl of water for an overnight soak after every workout and run them through a washing machine at least once per week. I've also found that even the Polar Straps won't last forever, I have one that has become a bit unreliable when not fresh out of the washing machine so have two more straps on order and due to arrive Monday. BTW, I get my straps from B&H Photo and Video. Also have a new chest monitor coming on the same order because I've also found they also only last for a year or a bit more. As for the cost of having to replace these items, it's nothing compared to what shoes cost me.

linear heart rate readings

Based on the traces you posted I see a couple of problems.
The second problem I see is that I believe those traces were made up totally from start to finish. Because I have NEVER EVER seen a heart rate trace that linear and smooth using either a chest monitor or my wrist monitor. IMO that trace is an estimate based on what the programming in the watch was designed to display in the event that the wrist monitor couldn't get a "lock" on the heart rate. One tip here would be to make sure that when you trigger the Run application you watch to see that message about the Wrist Heart Rate Ready, this indicates that the wrist monitor has a "lock" on the heart rate. That may help insure that your wrist monitor is actually tracking the heart rate at the start of your run. However I use a chest monitor for one very good reason, that is that I've found the wrist monitor just isn't reliable for any activity involving a lot of arm motion. My second tip here is rather obvious, get a chest monitor and use it.

Thanks for your thoughts. I agree that apparently the measurements are already wrong from the very beginning of my run. Interesting though is that since I noticed this, I deliberately slowed down in yesterdays run to have my heart rate drop - this was in fact reflected directly on the watch. Anyhow after finishing the run and uploading to Garmin Connect I got the posted graph (without the drop being shown). First I thought that this is probably a Garmin Connect upload issue but then this wouldn't explain why my rate wouldn't go above 148 during the run. As for your suggestion: I actually do wait until I have a GPS and HRM as well as a Tempe lock before I speed off.

When I first got the watch (in about Feb 2016) I had the issue that the HRM would not pick up any readings before about 10 mins of running (or waiting). But at least no wrong measurements afterwards. Since the firmware upgrade this issue went away but now Garmin keeps me annoying with this - and it also makes me question any other measurement derived from the heart rate (VO2max etc). I mean what's the use of a heart rate monitor that is as erroneous as a stock market graph? In both cases I don't want to see my health depending on :-)

If you want to see your VO2 Max readings really tank start using an Elliptical Trainer. One thing to keep in mind about any of these types of functions is that the programming assumes that you are running outside using the GPS to determine both distance and elevation changes. Use any type of indoor equipment and the lack of any means of entering the resistance level in that workout will throw a monkey wrench into all of these calculations.

If you look for it you'll also find a notice in Connect and probably Connect Mobile that these values are Estimates Only and shouldn't be treated as being even remotely accurate. However, if you do nothing but running outside using the GPS you actually will get a fairly accurate estimate, so it can be useful if this is kept in mind.

BTW, due to foot problems running is out of question for me at present so most of my workouts are done on an Elliptical Trainer and both the VO2 Max and Recovery advisor are pretty much useless for me. What I pay attention to is the Training Effect rating for my workouts. Because experience has shown that working up towards level 5 for extended periods will cause my resting heart rate to spike up and reduce my 1 and 2 minute recovery heart rate counts, both of which are indicators of over training.

(not so smart) solution

It's not that I am specifically focused on my VO2max - I was rather questioning the sense of any measurement using the HR as input when the HRM delivers wrong results.

Anyhow, I found that each time I completely switch off the watch and restart it before running I don't see my "linear heart rate issue" any more. Still this is in my opinion not really an optimal solution and I am looking forward Garmin investigating this issue. Any other (better) solutions are strongly appreciated!

happy running!

Make sure you select the Acitivity.... then wait for GPS lock....AND wait for it to say "Wrist HR Ready" (or similar) before hitting START

Only time I see a result like yours on my workouts or my wifes is when starting the activity right away. If you are not sure if you missed the notification...happens to me. You can check the Heart icon at the top, if locked into HR it should be a solid white...instead of 'searching' blinking white.