VO2 max performance - fenix 5x

What you describe is perfectly normal. Based on how I interpret Firstbeat's documentation (Real-Time Performance Condition), the shown real-time performance condition figure is a real-time "snapshot" of you current condition (i.e. HR/speed ratio, with HR variation baked in, etc). It's perfectly normal for the performance condtion to be better at the beginning of the run (when you are not tired), and then start dropping when your muscles tire (how much this happens depends on your fitness, how long you run and how accustomed you are to running that long). Later, your VO2max is determined by fitting the whole performance condition curve to what Firstbeat has measured in lab conditions.

I wouldn't do anything differently based on the performance condition figure alone, and definitely wouldn't do anything to just affect the VO2max  on the watch (which wouldn't change your fitness, just the figure). But the performance condition might give you information you can use to tune your training. If you performance condition is poor at the beginning, and especially if you also feel a little tired, it might be a good idea to take it a little easier that time. And if performance condition is really good and you feel up to it, maybe it's time for a little longer/faster run. And looking at the performance condition curve afterwards and comparing it to earlier runs might give you information on how your body adapts to longer runs (performance condition drops more slowly).

So, if your performance condition drops quickly in your opinion, then maybe it's time to concentrate on longer lighter runs to increase your endurance?

Watch this video about performance condition, what it is and how to use it:

Here is the help text from Garmin Connect:

Performance Condition
As you track an activity with your Garmin watch, performance condition analyzes your pace, heart rate and heart rate variability to make a real-time assessment of your ability to perform compared to your average fitness level.

During the first 6 to 20 minutes of an activity, your watch displays your performance condition score. Performance condition can be an indicator of fatigue, especially at the end of a long training workout.

Support page: https://support.garmin.com/en-US/?faq=A28UA4k16v1qjjGuvSFgo8

"The Performance Condition metric is a real-time assessment of your ability to perform compared to your average fitness level. This metric ranges from -20 to +20 with each point representing approximately 1% of the VO2 Max set on your device. During the first 6-20 minutes of an activity the device will alert you to your current Performance Condition."

What you should do depending on the score you get is up to you and what the purpose of the workout is.

It is natural that you will get a lower and lower score during a workout. If you get a negative score at the first assessment you will probably not be able to perform at your best in your workout or race.

Thanks for your feedback guys.

The Garmin app keeps mentioning that potentially I'm overtraining, another reason for wondering if I should be cutting short my runs at which point my performance is at its peak.

I usually run for an hour and I was short of time on Wednesday, so did a quick run. Garmin loved it and my performance indicators jumped for the first time in a while.

It can be hard to understand exactly what each training status means and why you get it.

Training status is a measurement of your training over a longer period of time. At least a week.

Overreaching is probably caused by too high 7-day load.

"The Training Status feature gives you an overview of your longer-term training habits to provide you with powerful insight into how your training is really going. Provided by Firstbeat, the calculation utilizes several dimensions of your personal physiology. It considers changes in fitness level ( your VO2 max ), heat and altitude performance acclimation, your current acute (7-day) training load, and any change in your longer-term training load."

support.garmin.com/.../