No anaerobic impact on interval run

Bit of missing details that could help - screenshot or link to the activites HR graph.  Is your maximum heart rate setting manually set based on a known max HR?  (accurate real Max HR).  What % of max did you hit during the hard work intervals and what % of max did your HR drop to during the 1' recovery?  1' depending on your fitness might have been a bit short to drop a lot.  

Based on what I know your HR needs to get up during the work, but it also needs to drop nice and low during the recoveries for best 'effect' score.   Looking for something like 120bpm during recovery... and 170-180bpm during the work.   Looking for a big difference in the peaks and valleys of HR.  But, If you are in real good shape aerobically, more of an endurance guy, intervals don't drive HR super high, so it short changes you a bit.

Thanks very much for your answer Nick. This is the link to my workout https://connect.garmin.com/modern/activity/11670165807?share_unique_id=2

I believe my thresholds were calculated many years ago when I first got my garmin using a chest strap. My resting HR is quite low so I don't hit high HR readings in many of my activities, perhaps that is the issue ? 

Looking at it a bit closer, looks like a good workout, was this with a chest-strap sensor?  Low resting shouldn't effect results much, mainly based on your maximum HR, so that needs to be right.  The fact that you ran for a good long warmup and barely got to low-middle zone 1... :-/  seems odd.   Looking at the relationship of your warmup pace vs your interval work pace.... and HR effect seems about right.  BUT because your heart rate hardly got into zone4, could be that they were not hard enough (or that your max HR setting is based on some flakey/bad /estimated data).  

I would do a hard review of your recent really hard workouts and races, what was the max HR for them?  COuld you have pushed harder?  Likely if you ran a 5k in the past that was all out with a good kick at end, you could take the max of that and add 5bpm unless you were getting the death blur / tunnel vision and wanted to fall down. lol  But 90sec intervals very hard... should kickup HR by the end especially with just 1min recovery.  although it was a cool cloudy/rainy day, which will lower HR.

the anaerobic effect has always been a bit of a mess for me as well - get good results some days, some days i get a decent score (2.5) on tempo intervals (not super hard)... some days on same tempo intervals i get 0.0..... bike outdoor without a power meter has always given me more anaerobic score than a similar running workout. 

Thanks Nick. I checked some of my recent runs, including 2 1.5mile races that I did in the last three weeks where I went all out for most of the race and my max HR was very similar for both races, 162 in one with 22% in zone 4 and 156 the other one but 0% in zone 4, both very hot days.  I don't wear a chest strap by the way, I get the HR reading from my wrist on the 945. I am wondering whether my HR zones are correctly set up in the first place. If we use the formula 220-age or 211 – 0.64 x Age both formulae give me lower HR zones to the ones I have set up in my Garmin, perhaps that is the reason? I will change the max HR to 179, which is what I get from the 2nd formula. This will bump my HR zones down and will see that effect this has in my results. Will keep you posted out of interest. Cheers

Everyone's max HR is different, it isn't age specific besides as an average.  Within every age group it is a bell curve of some sort with a +/- of probably 20-30... so yeah that calculation is a starting point, but should use actual data to set it (manually, not auto by garmin), if the heart rate data of those races looks good, nice even slope upwards and seems to correlate with effort, maybe your max is near 165-170bpm based on that race.  But... as you can see 156 and 162 on two races... is quite different.  So hard to say without looking at the data and knowing how it felt.  Often long speed intervals on a hill are a bit better to hit max, but my max has been in sprint finishes at 5k races.  I would definitly adjust down to 170 or 172.  That will likely improve all of your training metrics, as you should be into zone5 occasionally on super hard workouts and shorter races... but with a max of 179 that will almost never happen(with zone5 starting at 169, which might be at or above your real max HR!!! lol)

So essentially your watch thinks you are not trying very hard... and that you are running your shorter 1.5mi races at your strong aerobic and tempo HR effort!  When really it is nearly full gas zn5 effort most likely for most of it (or should be if it is a 6~10minute race)

Your training effects and training load will be different then (more real) but also likely your vo2max will update to a slightly lower as well, now that your watch will know you are working harder at your everyday paces or on tempo runs.

So I changed my max HR to 170 and the HR zones were automatically adjusted by Garmin. I did another 10x400 workout yesterday and finally got some anaerobic impact.  I tried to push quite hard and ran the sprints at 3'25' to 3'50" which is quite hard for me. I kept an eye to my watch during sll the series and for the first three, I only got to zone 4 for the last 15 seconds of the sprint.  Then as I got tired, I was in zone 4 for slightly longer.  Maximum HR was 169, slightly higher than previous runs. 

https://connect.garmin.com/modern/activity/11704681150?share_unique_id=5

So as you recommended,  I will continue to keep an eye on my stats for the hard runs and adjust my max HR accordingly.  

Cheers!