Today I sprinted 2:07/km 6 times and got 0.0 anaerobic affect because the GPS inaccuracy (in an open park) says I ran slower?

How are your heart rate zones set?  It looks like the intervals aren't getting your heartrate into your anaerobic zone, hence the lack of benefit to your anaerobic score.

To me it looks like your HR is not recorded correctly. Are you using the integrated optical HRM? If yes, make sure the watch fits snugly or, use a HRM strap.

In my experience, OHRM are not great at recording big HR changes, they need time to adjust to the new HR

Your HR during the Intervalls didn’t exceed that during warmup and cooldown. If HR felt like 150 (what do you mean? Did you put a finger on your pulse?) the HRM would have collected it. 

overall, you seem to be knowledgeable enough to know that the devices rely on HR (zones) to judge an effort and to know that your HR is not responding fast enough when you do only 15s intervalls. This is why I don’t understand  why you asked your initial question. It seems that you know the limitations of the technic. 

To get anaerobic „credits“, you need to rIse HR fast enough. Pace is not used for it. For example, you can do high anaerobic  exercises with body weight training indoors and the watch will recognize it as such. 

Dor running: either do the warm up with lower HR than the Intervalls or do longer Intervalls for HR to ride higher. 

But it's not as easy either as saying that just because you are doing sprints, your HR will not rise quickly enough and therefore your workout will not be gaining you anaerobic credits.

I had a similar session as 9386496 where my watch suggested a dailiy workout session of Sprint, running at 3:10 (I'm not as fast as him, for sure) for 10 seconds only (a favorite sprint session of mine ). My HR obviously rose, but just a little bit above when I did my warmup.

Still the watch has labeled my workout as Sprint, with an anaerobic benefit of 2.0

Since Garmin suggests sprints, it must have a mechanism to detect that and correctly label the type of training as that, what's else the point of such suggested workouts?

First point, It's important to understand that it's physiologically impossible to get your heart rate high enough in such a short time, never mind trying to get a reading on a strap or the wrist.

Second point. There's absolutely no point trying to pace a 15s sprint. Chances are you're accelerating for the first 10s, perhaps reaching your maximum pace just in time to stop sprinting. If a sprint of 15s or less is programmed forget about pacing just run!

Third point. Doesn't matter whether you believe instant pace to be accurate or not there is no chance any GPS will be able to give you the pace information you want instantly when accelerating over a short distance for a short time.

You are of course free to rant and rave about what you perceive to be short comings in your Garmin watch but you also need to understand your physiological responses and the physical capabilities of your measuring system.

olni65 said:

Since Garmin suggests sprints, it must have a mechanism to detect that and correctly label the type of training as that, what's else the point of such suggested workouts?

A reasonable assumption but there also needs to be some understanding of the real world limitations of the body and the watch as already noted. It's not clear where the heart rate was for the majority of time in the image presented but looking at the recovery drops between the sprints it would appear to be quite high. Analysis of this type of workout means looking at the whole not just the sprints. Just looking at the numbers that result si pretty pointless.

The aim of these type of sprint workouts is primarily to improve oxygen kinetics. Calculations are based on step changes in pace/heart rate that in reality cannot be achieved. However, as your fitness and muscular strength improve the rate of change of pace/heart rate increases and, while getting closer to a step change will never truly be instant.

Thing is my heart rate didn't go high at all, it felt like I did nothing hardly breathing. I'm a distance runner so doing 15 second sprints feels easy and I don't have the muscle to sprint faster as a distance runner to get my heart rate up. It was still 12 second 100m speed so wearing a strap wouldn't matter it would still report a low heart rate = no training effect.

My question is why is the watch telling me it's time for some anaerobic training suggesting 15 second sprints when it's incapable of recording the effort because 15 seconds is too short duration for my heart rate to climb up and the instant pace is too inaccurate to have a correct average 15 second split time. So I go and do this workout hoping to increase my anaerobic metric and it says 0.0. I was hoping it would just use pace and my V02max vdot to get the effect. 

Honestly my HR really didn't increase during the sprints because it's too short and I run like 120 miles a week so my aerobic system is way too developed and my legs can't go as fast as a sprinter to get my heart rate up unless it's a proper interval like 400s. My mate runs 3:00/km with a low heart rate it's hard to get the heart rate up when you max your aerobic system. 

Thank you best answer I've seen. "First point, It's important to understand that it's physiologically impossible to get your heart rate high enough in such a short time, never mind trying to get a reading on a strap or the wrist." Finally someone who understand and not just saying "Should of worn a chest strap". 

My FR10 instant pace will read my sprint speed, it locks on fast and updates fast. I've done sprints with it at 2:03/km for 150 meters on the roads and it jumps straight to the right pace. 

My heart rate reached 145 for the sprint efforts. 

I thought sprinting was to work on raw speed the nervous system and biomechanics. 

It's a shame that doing sprints doesn't add to anaerobic, is it not anaerobic then? Does your heart rate have to go close to max even if you're running max speed? I mean I could feel in my legs I hit an anaerobic system afterwards heavier quads 

i am not sure, if i am right, when i do sprints, i do always all out sprints, after the sprint my HR is very high and i think Garmin uses the recovery time after the sprint to say if anaerobic.

mean, the recovery after every sprint is very important, i don't make any move in the recovery cycle