Why did my power duration metrics change in WKO4? Pmax, FRC, mFTP, Stamina, TTE

Have your PD curve metrics (Pmax, FRC, mFTP, Stamina, and TTE) changed drastically in the hero bar? There are many possible reasons these metrics can change, and your coach is the best person to explain why and how, but there are a few common scenarios that you might find relevant.

Before we look at the scenarios, remember:

  • Your power-duration curve is based on your Mean Max Power (MMP) data. It is unique to you.
  • The power-duration model is an estimate of your power, not a prediction of what you are capable of.
  • There is a difference between the model and your actual physiology. A sudden change in the model doesn't necessarily mean your fitness changed, but rather that you provided newer and more accurate information to the model and it adjusted your numbers accordingly.

Scenario 1: Bad data and/or data spikes

Bad data and data spikes can cause drastic changes in your power-duration metrics or even cause them to disappear completely. Click here for instructions on finding and fixing data spikes.

Scenario 2: Key mean max power (MMP) data points aging out of the 90-day window

The typical time range for calculating power-duration metrics is 90 days. There may be specific data points that are shaping your current power curve (and modeled curve), and your curve will change when those data points "fall off the back" of the 90-day range. Again, the model reflects only the data it contains; it is not a prediction of your abilities. 

Scenario 3: Adding short efforts after doing only easy efforts for a while

This can happen at any point in the year, but it's especially common in the spring when folks begin to train outdoors again after being cooped up on their trainers all winter. When you've been doing lots of easier aerobic efforts for a while and then begin to add in some hard, short training, it can change your power-duration model a lot. When this happens, the model is simply adjusting itself now that it has hard/short Mean Max Power data to fill itself out. Remember, the model does not predict what you might be capable of; it simply reflects the data it contains.

If you have no hard, shorter efforts, the model may overestimate your mFTP (modeled functional threshold power) due to the underestimation of FRC (functional reserve capacity). 

Adding shorter efforts that raise Pmax can increase your mFTP slightly. Think of the curve as a suspension bridge; pulling up the outside supports pulls up the curve.

Adding shorter efforts to increase FRC can reduce your mFTP. Put overly simply, mFTP is an aerobic measure of a sustained transition of energy (along with all the standard definitions of lactate threshold). Since FRC is anaerobic by nature, increasing it in the model can decrease your mFTP (in the model), as mFTP is generally an aerobic measurement.

When you notice your metrics change, take a look at the FTP Contribution with TTE chart and the FTP Review chart pack in the WKO4 Chart Library; these two charts are an excellent place to start when figuring out what is going on.

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Should you change your sFTP to match your new mFTP?

Your set functional threshold power (sFTP) is used to calculate TSS, so be sure to set it to what you (and your coach) think is accurate. mFTP is a good place to start; think of it as a trusted adviser. Don't copy every micro change, but it might be helpful to adjust bumps in multiples of 5 (250, 255, 260, etc.) that correspond to an increase or decrease in fitness. As time goes on, you'll start to get a feel for it and sometimes decide to make tweaks for cases when you've been off the bike due to illness or other situations where you know your MMP curve doesn't represent what's happening at that time.

How often should you test?

We recommend testing once every 3-6 weeks. This is crucial, not only because it will help you maintain an accurate power-duration curve, but also because it will help you understand the dose and response of your training.

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