LTP = TP - 2.5 x HIE. What is the physiologic reason behind this?
I’d be interested in the answer too
The equation works for my numbers too. Here’s the explanation provided in the support material. I don’t understand the explanation.
“LTP represents the lowest possible value for Threshold Power when all Endurance Energy reserves have been depleted”
p.s. I tried to post a link to the Xert support material, but the forum wouldn’t allow it, saying it was suspected spam. I also got 15 emails informing me of the reason the post was flagged.
(I believe this was the link you were looking to post? No such issues . Just a usual link…)
LTP is something we had seen in the data very early on and it had become so useful as a training metric that we decided to incorporate it into the system more directly. It provided a way to encapsulate a way to identify how well an athlete would withstand longer events: the higher the LTP, the better the athlete would do later in rides. It made sense to train at LTP since it would mean you’d do better at longer events, if these were part of your goals.
Physiologically, it made a lot of sense. LT1 is a value that aims to represent the onset of blood lactate accumulation which accumulates when glucose (i.e. glycogen) is used to generate power. This value is difficult to assign a value to, since the lactate levels increase continuously:
(From https://www.researchgate.net/figure/Representative-blood-lactate-curve-with-14-LTs-calculated-from-GXT-4-participant-9_fig1_326693764)
Hence physiologists have ongoing arguments on how to calculate it and what to do with it once you do.
If LT1 is low, it would mean you’d run out of glycogen sooner than another athlete with a higher LT1 riding under the same conditions, assuming it’s calculated the same way and represents the same for both.
We decided to use our method and call it LTP and use it as a indicator of long-term fitness and use it in training prescription. The feedback from this has been almost universally positive, some using it in place of their LT1 calculation. However, others that have not found it to coincide. We don’t make claims that it does, only that it is a useful metric to monitor and train with, much better than traditional zones. In practice, LTP has been extremely useful since it captures the difference between those that are gifted with lots of top end power (Sprinters, Pursuiters, Puncheurs) with those that are gifted with great endurance (Climbers, Time Triallists, Triathletes). In the end, if you’re an athlete with lots of high end power and you need to prepare for a longer event, if you follow traditional the Zone model based on %FTP, you’ll likely be training above this value and may find yourself frequently tired as a result. Similarly, if you’ve got a strong diesel engine and train by zones, you may not be putting sufficient strain to make your engine stronger if you follow the zone method. See Sweet Spot, Threshold and Polarized Training … By the Numbers – Xert to understand how the differences play out.
Interestingly, there are other patterns in the signature that we’ve discovered after looking deeper into the data. For example, this formula:
HIE*1000 / (PP - TP)
is roughly 30. Work it out for yourself and see where your number falls. What do you think this means?
The next iteration of the Xert model will likely see an improvement to how the lower intensity system works and will have a more continuous calculation, similar to how the body actually works.
Cheers.
10 Likes
based upon what is being said and the formula as LTP= TP-2.5*HIE then the best way to improve your durability for long endurance rides would be to decrease your HIE so that your LTP gets closer to your TP?
You need to reduce you HIE to get them closer together.
Mike
I think this is the second or third reference I’ve seen to the next iteration of Xert (or Xert 2.0). Is that going to be a total rebuild, or just adding on top of and refining the original model?
1 Like
good question and no idea how they came to that number of 2.5 it was given in the original post but it does seem to be true in my case.
i’d like to point out the dimensions. the constant (2.5) has units of kilohertz.
I’m not sure but generally if you want to assign dimensions to a constant then that constant can generally be measured in nature as a fundamental physical constant? for example, we can measure the value of boltzman’s constant to a bunch of decimal places and it has units of J/K…
2.5 kilohertz is probably not a fundamental physiological constant. but who knows… perhaps it is lol… like the number of kJ we can regenerate per second or something like that…
lol idk man the mathematics and theory behind critical power models is something I do not pretend to understand.
You can also use TP - HIE/400 where HIE is in Joules. 400 is in seconds. TP + HIE/400 is roughly VO2max.
3 Likes
Units of watts are joules per second… and units of hie are kilojoules.
Units of hertz are 1 over second…
Watts=watts + ckJ
J/s = J/s + ckJ
J/s = c * kJ
kJ/ks = c * kJ
1/ks = c
c=1/ks= 2.5 kilohertz
2 Likes
is that VO2 max formula correct? I am no math scholar but it seems more likely that VO2 max is TP+ (400/HIE)? or have I made some math error? to my reading HIE is a number between say 10 and 20 then that divided by 400 is a pretty small number.
1 Like
Yes I was thinking the same thing but didn’t want to say anything 
HIE x 1000 converts it from kilo Joules to Joules.
Okay now I’m totally confused, which is easily done. Where did this 1000 appear from? As its not in the equation above.
TP + HIE/400 is roughly VO2max.
so in other words the actual math formula you wrote is" TP + HIE/400 is roughly VO2max".
then you mention HIE * 1000 so if you multiply HIE* 1000 then divide by 400 it is actually HIE/4???
so wouldn’t the formula be TP + HIE/4 is roughly VO2max. where TP and HIE are the numbers from your signature?
ie for example TP 200 HIE 16 gives an approx VO2 Max of 204…
but if you go by some general concepts of VO2 Max it is often said to be in the range 105- 120% of TP so then 201 vs 200 doesn’t fall within that relative range ie it is only 102%? but if the math was TP + 400/HIE the result is 225 and that is in that range at 112.5%… not certain it makes much difference in all reality but anyhow as I say I am no math whiz.
2 Likes
Ron,
You’ve got your maths wrong:
pVO2max = TP + HIE / 400 in your example would be 200 + 16,000 / 400 = 240 Watts
LTP = TP - HIE / 400 = 200 - 16,000 / 400 = 160 Watts
HIE needs to be in Joules rather than kJoules, hence the 1000 multiplier. This is the same as multiplying by 2.5 (1000/400) in the opening post.
Mike
1 Like
see said I wasn’t a math whiz… that makes sense then.
You can get that from your power duration curve. In this case the time to failure is about 6:30 but that does depend on max power.
Mike
I think it might be useful in these sort of things to have 2 terms
1- HIE in kJ
2- hie in J
therefore HIE/1000=hie
then the numbers make sense. HIE is what is displayed in your signature in kJ and what the OP multiplied by 2.5
hie (or HIE/1000) is what @xertedbrain used to make the equation where he divided hie/400 NOT HIE by 400
then the thing makes sense to my way of math…