Garmin MPA data massively innacurate

I just tried to use my Garmin Edge 1000 with the MPA filed to pace a 10 mile Zwift TT. I tried to estimate how much MPA I wanted to lose per km, understanding decay is not linear.
All going well, I thought. Initially not dropping as much as I wanted, ,but thought ok, it’s not linear, no prob, Then after about 14 mins it dropped massively, and I got a breakthrough alert. It then continued to drop until it hit 265 and then stayed there for ages. I pushed on feeling strong til 20 mins, then had a short recovery before pushing onto finish.

I was expecting a massive breakthrough,having seemingly had MPA below my power for at least 5 mins.

When I looked on Xert online, The MPA decay was completely different, by about 200 watts…!

As such, the garmin MPA decay was no where near the online one, and hence not usable as a pacing tool.?

Am I doing something wrong…??


What you are seeing in the post-ride analysis is your MPA development with the new signature after the BT, hence the difference to the one in the ride. That’s assuming you had configured your garmin correctly (I.e. it was your signature and not the default) of course.

You can select ‘advanced MPA’ in the activity, then click on previous below the chart to see how it looked with the old signature. Should more closely match your recollection. (Just don’t click save) :wink:

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Thank you, that helps… a bit confusing though why Xert would show against your new signature. Surely the breakthrough should be against previous signature and the new signature should apply for next ride?

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That has always been confusing for me as well.
Can someone explain what the idea behind showing the activity aginst the new signature that is calculated from it is?
I think Andy has a good point here. There is much more information against the previous signature, sometimes there are more than one ‘breakthroughs’ in one activity and this one shows a long time above MPA so it should be a big change in signature.

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Technically you can’t ride that long above MPA according to Xert’s model. The fact you could triggered the breakthrough analysis to kick in and adjust your signature.
Essentially you were more fit than your current signature reflected so you blew past the markers Xert predicted should be your limits. That is why the chart flatlines if you look at the workout with your old signature.

Kinda like going to a race event where you are classified below the group you should be riding with so you break free at the halfway point and finish ahead by a wide margin. Next time they’ll bump you up in classification. Had you rode with that group the race results would have been different. :wink:
Anyway that is how I look at it. YMMV

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But there is only one point of failure and that is what the new signature shows you.

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No, I do not believe that is correct.

In the new signature there is not really a ‘point of failure’, that’s what the new signature is for. To correct the old one so that current power is never more than MPA. Because that is the assumption of the Xert system, that current power cannot be more than MPA. If it is for some reason MPA and thus the signature is wrong and needs to be adjusted.

If you have an activity where the current power is several times above MPA, for example in a sprint and in a longer effort it will most likely result in a gold breakthrough and you do have two points of failure (or maybe none, because you just stopped without really falling dead off your bike :wink: ) while sprinting and when going for a new TP. The new signature would only show one point where MPA meets the current power but that’s just the ‘bigger’ breakthrough. The other one is not insignificant or less interesting to see. I always look at the ‘previous’ first to see where and for how long I actually broke through my Xert signature.

Thus my confusion :confused:

What you describe there is the point of failure - the point at which you cannot continue to ride at an intensity that would otherwise create a breakthrough.

You need to think about this in a different way. Your fitness signature is a mathematical model of your physical capabilities and when you actually start your breakthrough ride you already have those physical capabilities, meaning that if the signature generated after the ride (because Xert isn’t really trying to predict things), doesn’t show a breakthrough on the first sprint, or the second, but only on the third. The first two were not actually points of failures, just hard efforts.


My “strength” is in TT, ie long steady efforts, and that was the first time I have done one since using Xert, so I am not surprised that my signature has been updated, I am just surprised about HOW wrong it was. I guess it should be better next time I do a TT. I like the idea of MPA depletion so you know what you’ve got left, but if it’s so far out, not that much help…

You should try to keep your signature as up to date as you can. Every 3 or 4 weeks would be good.


Or if you aren’t planning BT efforts for a long time, put it on ‘no decay’ and you are very unlikely to have the problem of massive breakthroughs again. I think what’s happened here is just a lot of decay

I don’t race but wouldn’t TTE be more useful for doing that?
I can’t see how MPA would be useful for pacing on a time trial (long continuous effort).
I think of it more as a tool for intervals and max efforts or targeting breakthroughs, none of which you are going to attempt on a time trial. :smiley:
Perhaps someone using it for pacing will chime in.

I understand what you are saying Mike, and I’m not interested in fighting.

The whole concept of “point of failure” or “maximal effort” is as far as I’m concerned not real. It is the point where the rider decided to stop, in this case it was the 10 Mile finish line of Andy’s TT. If a wild anymal was trying to eat any of us, we’d all be faster, I’m pretty sure. These “maximal efforts” we do at home on our trainers are what we can somehow motivate ourselves to do, not what we actually are capable of doing. Winning a race is not trying to survive and even that might not motivate everyone to do what is actually possible, depends on how much you want to live but that’s another thing entirely. Apart from ‘until you drop’ implying that death is the goal because if you’re still breathing you can do more, you just really don’t want to.

What I wanted to say was, that when I look at these two screenshots Andy has posted the first one does not really say much, exept that after 25 minutes of a race there was a breakthrough that updated the signature. The second one really tells the story, namely that the breakthrough occured after 15 minutes and went on for 10 minutes, with a bit of a dent for a few minutes recovery and that this breakthrough should give a huge boost, since the signature was obviously really wrong before the race.

And I still think that if you have a sprinting breakthrough and a 5MMP breakthrough in one workout (some of the breakthrough workouts do try to achieve that) to get a new well rounded signature it is not a good idea to just look at the updated signature chart that will by definition only have one point where current power and MPA meet and just ignore that you had two breakthroughs of different kind in one workout and both played a part in your new signature. At least as far as I know.

So I would prefer to see the second one as a default since that’s the one with much more information. I’m all about the data :slight_smile:
I can’t hardly ride a bike compared to you guys.

Well, you’ll be glad to know that not one member of this forum ever comes looking for a fight - this is a place for healthy debate and learning.



In the interest of this debate :slight_smile: could you please post your breakthrough report?
Click on the breakthrough icon badge next to the activity title.

I noticed you joined the forum about a month ago.
Does that mean you have been using Xert for about 4+ weeks?
How many breakthrough events are present in the historical power data you loaded into Xert versus the activity posted since you joined?

If you are a beginner with Xert you might benefit from information provided in this post –
Beginner questions - Support - Xert Community Forum (
If you provide answers to the 11 questions there, we may have some additional advice for you.

Here is my breakthrough report and some further info.
I have been using Xert since the start of the year, so about 6 weeks,
I have historical data from October with about 10 Breakthroughs from October to end of 2020.

Interestingly, vast majority are from Team Time Trials in Zwift which are 1 min efforts followed by recovery

Only one breakthrough was from a long duration “steady” effort, in my new year FTP test.

What I think I now understand is that my signature was not fully accurate… I hadn’t done enough representative long duration TT type efforts. My fitness has gone up and so has my TP/ FTP by about 10 watts as a result of Saturdays effort, which seems about right.

My MPA has stayed the same,

So here is the bit I still don’t understand… the thing that seems to have changed in my new signature is “only” the rate of decay of my MPA. I don’t have a higher MPA, it wasn’t a high intensity type effort, so the only thing that has changed is my FTP. Is FTP used in the algorithm that determines rate of MPA decay? And does an increase in FTP of 9 W equate to that level of MPA decay “innacuracy”… that’s a 3% FTP increase, but over 200 W difference in MPA at 16 mins…

I guess I will need to wait until my next steady TT effort to see if is closer to my new signature …

He are the 11 answers

  1. Are you running the 30 day trial or are you a subscriber? Subscriber
  2. What equipment do you have for training indoors and outdoors? (meter type, trainer type, bike type, bike computer or phone). Wattbike atom indoors, road bike garmin vector pedals outdoors, garmin edge 1030
  3. Are you young (20-30s), older (60+), or in the middle? :slight_smile: (no need for exact age). In the middle
  4. How experienced are you with training by power? (newbie or X years using another platform), 4 years power meter on bike, 2 years Wattbike atom
  5. Are you a recreational rider or competitive racer?. Recreational ( but very serious, long mountain gran fondos) but first TT’s last year
  6. What are your goals? (example, special event in 3 months or best fitness for riding this summer). TT s this year,
  7. How many weeks or months of recent power data have you loaded into Xert? Since OCtober
  8. What is your current star status? (example, 2 stars) 3 star
  9. What Athlete type did you select? (determines focus duration target). SPRINT time trial it’s
  10. Which Improvement Rate (IR) did you select? (hours/week training load) Moderate 1
  11. What TED (target event date) did you enter? (determines what phase are you in) 1st May, Just out of base and in first week of Build

I’ll try to explain MPA a bit. Might not be correct though, I hope someone corrects me if necessary.

MPA is the maximum power you could generate for one second at this moment in time. MPA starts at PP when you are fresh. That’s Peak Power or 1s max power, is yours correct? Mabe do some few sec (5-10) sprints to check?
As long as you ride below TP (the power you can hold for a good long while without short term fatigue building up) MPA stays where it is.
Long term fatigue is not modeled yet and that is often confusing for people. You cannot ride for 10h at 99% TP but Xert would not generate a breakthrough if you did.
But the short term fatigue above TP (that’s why it is called threshold, something is supposed to change physiologically at this point) is modeled really well in Xert.
It uses the third signature component to determin the rate of decay for MPA when riding above TP. A high value for HIE indicates more room to breathe above the threshold and slower MPA decay. As far as I understand it affects recovery as well and it does inversely affect LTP (lower threshold).
LTP is supposed to be the value your TP falls to after a long ride and thus something that might be helpful in basing long endurence rides on. If you have a high HIE (shorter focus duration type) you have a low LPT (bad long term endurance) and vice versa. Your muscles are either good at short and hard or at long and steady but it is a scale and a tradeoff you can adjust with training to a degree.

I’m not really sure how to do 25 minutes constant efforts with the help of Xert though. But someone else might. You could look at the power curve of your current signature (open the last activity and there it’s in the third tab) and see how much power Xert thinks you might be able to hold for that long and try to use that to pace yourself, maybe. I’d just try the 30 min power and see how that goes, if you have power left after 20 min you can always go faster, might be better than starting too fast and crashing. But that’s just me.

Looking at MPA is more useful for shorter more dynamic effort/recovery scenarios, I think. The whole Xert system excels at the shorter more dynamic scenarios as far as I understand it. It is not too useful for ultra endurance stuff but some here do just that and they still like to use Xert for training. You have to adapt to your needs a bit but Xert is nothing if not adaptable.

To get an idea what the focus duration and athlete types really mean you can use activities or races you uploaded and look at what focus duration they are according to the Xert system. That’s a common practice to find out what to train for, when you have a goal in mind. In your case Sprint TT seems a good fit but it might get boring and not be ideal in the end, often it is a better idea to go one or two steps up out of the endurance, so Climber or GC Specialist. At least for a while to find out if it is better.

Hope any of this helps a bit.