From my experience AC updates on the T3/T4 are linked to both duration of training sessions and your own cardiovascular performance. 

For example: Your AC goes up as you get fitter and naturally you need to train for longer periods to make further progress. 

If you stop training for one week your AC doens't just drop to zero, it slowly goes down in line with a natural decline in your own fitness level. 

You get a much better idea of how this works when you have access to the Coach function on the T4 and its suggested target duration / intensity for each session.  Some jumps in AC don't make much difference to the PL (Program Level) in terms of duration, the watch has just made a decision that you are now at a new fitness threshold.

I think you're right there - that's why it takes about a month's worth of training log data, when you first use the watch, before it stabilises your AC and PL.

I set my initial AC based on number of hours exercised and it was at least 2 AC levels below what it turned out to be a month later (I was very fit when I was younger). 

During this initial period there was no increase in the time I spent exercising.  The rest of your user entered parameters in combination with the your logs must help it work out where your AC should really be.  There is probably some kind of weighting going on in the algorithm, so I don't know which are the most important or over-riding factors.

Slightly on a tangent, there are also settings on the T6c like METS that factor into calories consumed etc that you can't set manually on the T3/T4.  My guess is that the watch uses some kind of best estimate figure for METS based on your weight/height and maybe your AC level or MaxHR.  I don't know enough about this to understand how it might be calculated.  Perhaps some of you sports science bods can tell us how it might be done.  One day I will finally find time to study this field in more detail!  

Can't begin to tell you how much I've learned through the interesting postings contributed by the more sports science oriented people in this forum.  Many thanks to all of you! 

Folks,

I got some excellent help from Tuomas at Firstbeat Technologies. This is his expålanation on how AC is calculated. I thought it could be of interest to more people than myself:

Hi Borje,



I have now looked through our data and based on your done exercises Athlete and Coach are working correctly.



Athlete calculates the activity class based on the number of done exercises and to the weekly EPOC sum. Activity class is calculated based on to the past 28 days. Athlete has calculated your present activity class from the weeks 6-10. Limits for AC 9 are 24 exercises/4 weeks and 1360 EPOC/4 weeks. Your corresponding values from weeks 6-10 are 28 exercises and the EPOC sum is 1484.



If the week 7 would have been a normal week for you, 4-6 exercises, your activity class at this moment would be 7-7.5 but high number of exercises in that week puts you to the AC 9.



If we look at your week 7-10, you still have a high number of exercises (27), but your EPOC sum is 1328. These numbers corresponds to AC 8.5. So your AC is about to drop to 8.5 soon.


In summary, based on latest 5 weeks of exercises you are a national level athlete J. Values are high because of the high number of exercises in week 7. When this week is not anymore included in the AC calculation the activity class will probably drop to 7.5 or to 7.

Yes, I should probably only record cardio sessions. It's probably the weight lifting I should stop recording. Jogging, biking, spinning and swimming should be there.

lindh - Far be it for me to disagree with FB - the calculation is what it is (and therein lies the problem with 'automatic' calculations) - but with respect would you say you are a national class athlete?  Your VO2max would suggest otherwise.