Hi. During the last days I was thinking about the difference between peak power and high intensity energy. I was thinking about it by considering the analogy to a fuel tank which gets drained if my power goes over the threshold power and refueled if my power is under the threshold. I consider the MPA as a fuel indicator which tells me how much is left in my tank. Since my initial MPA is identical to the peak power, I get the impression that the difference between peak and threshold power is the capacity of my tank. This would mean that a higher difference between these parameters means a larger tank. Now I am wondering, how the tank capacity is influenced by the high intensity energy. Can this parameter be also interpreted by the fuel tank analogy? I also assume, that the draining and refueling speed or resistance also needs to be modeled by the fitness signature. Is that right?

Hi Markus,

Thats a great analogy! In fact, thatâ€™s where the basis of the CP/Wâ€™ comes from. Wâ€™ (sometimes also called the anaerobic work capacity) is the integration of power vs time curve above TP, resulting in units of energy, kJ). The difference between Xert and CP/Wâ€™ modeling is that CP/Wâ€™ assumes that there is no limit on how fast you can empty the fuel tank. Therefore, at the point of failure - using that methodology - your Wâ€™ = 0kJ, and you have literally nothing left above CP. However, we would argue that this is not the caseâ€¦ In Xert, we have HIE, which is similar to your â€śfuelâ€ť remaining, but the way in which you can spend that fuel is rate limited by MPA, which lowers as you spend HIE while working above TP. As you mentioned, when MPA is equal to PP, you can hit very high peak numbers. As MPA decreases as you work above TP, not only is the reserve in the tank smaller but itâ€™s now also rate limited by MPA. Therefore, you canâ€™t expend all of your HIE, since you become rate limited by MPA before HIE = 0 kJ.

So, how does HIE fit in here? Increasing your HIE m eans that it takes longer for MPA to decrease and a prolonged time to exhaustion at a given power above TP (if we were to compare 2 athletes with the same PP & TP, but different HIE). Conversely, this also means that it takes that athlete with a larger HIE to fully recover MPA to PP at a given sub-threshold power. Thereâ€™s a general correlation between PP, and HIE where a larger PP generally suggests a larger HIE. Hope this helps, cheers

@scott_steele if I have read HIE correctly, it is the area under the power curve. That is why, for a sprinter, they may have a huge PP but a smaller HIE compared to other cyclists. So if we can â€śstretchâ€ť out the power curve (be able to hold 5, 10, 20s etc power longer) we increase the area under the curve and hence HIE. That is why training fatigue resistance is important. That the way I understand it but I am no sports scientist and may have it completely wrong.

*Fatigue resistance* isnâ€™t a thing in Xert. Those with larger HIE can handle *more* fatigue. Thatâ€™s good in this case. Long-term fatigue is another dimension that is very different than short-term fatigue because it canâ€™t be readily replenished whereas short-term (HIE) fatigue can. Long-term fatigue depends on managing intensity and proper fuelling. Not really sure what *fatigue resistance* really means in the end. Perhaps during world-tour stage races, seeing less accumulated fatigue from days of racing could be classified as fatigue resistance.

@armando. I donâ€™t know exactly where the phrase comes from but Tim Cusick from WKO basically describes it as being able hold x wattage for longer. So progressively over loading on time vs increasing watts. Here is his presentation: https://help.trainingpeaks.com/hc/en-us/articles/115000273271-Building-fatigue-resistance-strategy-into-training-with-WKO4

Hi Scott,

thank you again for the detailed and precise explanation!!

In dynamic racing, you can perform the same power with different signatures each implying different â€śfatigueâ€ť. Being resistant could mean higher TP, HIE or TP depending on many factors including the course itself.