W' in real time

Now that Garmin has a data field to show W’ balance in real time, are there suggestions for how to incorporate the info in real time while doing intervals?

For me, and the guys I coach, the intervals should be as hard as possible across each interval; sustainable, though, across all intervals. W’ is then reviewed after the workouts to compare to their feel. The next week’s session is then planned based on how they performed this week; another interval if they performed well, or reduced power range of they struggled. In many cases, W’ is depleted in the penultimate interval, and possibly negative on the last one.

If I had to see W’ go into negative while doing an interval, or in a race, it would be a psychological indicator to ease off instead of keep going.

Thanks. FWIW, W’ always struck me as a fancy way of quantifying how many matches you have left, and way back in the days of dinosaurs and no powermeters, riders would intuitively know what it feels like to burn that last one. I like what you’re doing with the post-workout assessment of the intervals, makes sense, especially if the information is used to calibrate the feeling of burning through the matches. I was more wondering, though, if there’s a use for W’ in real time.

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what is interesting is if you are an XERT user they have a similar measure called MPA. They are not identical because technically if you have your setting right you can not draw down MPA to negative numbers while in W’ it is not uncommon. In the XERT world you can watch MPA in real time. They continue to work to refine the MPA metric so that it is more in line with what you really can do on long rides but the idea is that at any time there is a certain Maximum Power Available and that declines with the power output ie in a sprint you may only hold MPA for a short duration and at lesser power for longer. It is I believe related to your individual power curve but others may be more able to explain it than I can. It is intended to be useful in the way you suggest for real time W’.


Hello @Mudge I would agree. I would use xert long before using w prime, it is more accurate and useful for the reason you would likely want to use.

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W’ / W’bal are less intuitive when used for training and racing as compared to MPA. MPA is in watts and tells how much power you have available at any given point in time and is thus very useful for both training and racing. Indeed, Xert’s algorithm can determine how much power you have for any given duration at any given point in time, roughly within the range of 1s to 1 hour, both unfatigued and under fatigue. But as @scooter points out though, PD characteristics beyond this have second-order fatigue effects. MPA. like CP and W’, doesn’t extend beyond this range but new modeling in this respect is in the works :wink:

The most important aspect of MPA and the Xert method is that it is easily validated by simply running the MPA app or examining power with MPA on a chart. What some forget though is that the ultimate goal isn’t to predict CP or W’ but to predict performance, and in our case, even predict performance intermittently. MPA allows you to do this and validate that it’s working first hand.

Armando Mastracci
Xert Breakthrough Training


My question was about using the metric in real time. W’ is available on my Garmin headunit. I don’t use XERT, so have no specific familiarity with MAP. Can it be displayed on the headunit or is it something for post-ride analysis?

@mastracci @Mudge there is an app or ten (many great data fields that are very useful during riding)


@mudge I believe and @mastracci could confirm for certainty that the MPA Garmin app may require that you have an Xert account to work. It need information that is part of your fitness signature to accurately predict the draw down of MPA. It works and is a big part of their algorithm in that when your actual power exceeds you current MPA that is a sign that your fitness has changed and that you need to update your signature. Each individual has a unique fitness signature so the way each affects their MPA will therefore be individual as well. You can give Xert a spin, they have a free trial and I believe it can use data from your Strava or Garmin accounts to determine your signature. Again @mastracci can certainly tell you far more than I on the hows and whys and what is needed to get a starting signature. Full disclosure I have nothing to do with XERT just a user.


For a taste you can use the Garmin What’s My FTP? data field which is free. Other data fields do require an account since they pull down your current fitness data from the server when you start a recording. This gives you up-to-date calculations on the device.

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