Thanks. It’s a saying I came up with almost 50 y ago, when I was still in high school.
The point of expressing exercise intensity relative to FTP (vs. VO2max, which is the classic approach) is to account for differences in muscular metabolic fitness. All else (e.g., habitual diet) being equal, two individuals exercising at the same percent of FTP would therefore be expected to utilize glycogen at the same rate. TSS is a function of intensity and duration, so when TSS is equal, you would also expect them to use the same total amount of glycogen as well.
Consistent with this, I have found a good correlation between TSS and glycogen utilization when retrospectively analyzing the various studies of trained cyclists we did back in my UT-Austin days.
Cycling economy/efficiency is far less variable than, say, running economy, or especially swimming economy.
In any case, maybe some day some enterprising young sports scientist will get around to testing my claim… after all, TSS has only been around for over 20 y.
You would have to trawl through old posts to the wattage list to be sure, though, as it was quite some time ago, and I no longer seem to have the data file.
ETA: I found a bit of discussion of this topic on another forum, in which someone summarized one of the graphs I shared to the wattage list as follows:
“The fitted line is described by: rate of glycogenolysis = 2.6754(IF^2) - 0.9965
and its R^2 is 0.7862”
But this seems to imply that free glucose would increase your capacity. Carbohydrates consumed and digested during exercise aren’t stored as glycogen afaik, at least not if the glucose demand is >= the available glucose.
And I’m unsure what is meant by rate in this context. Rate as a ratio of carbs to fat, or rate as raw consumption. Surely someone capable of holding 400 watts for an hour (at .9) utilizes more glucose than someone holding 200watts for an hour (at .9). If not my mind is absolutely blown.
I believe he is saying that TSS would give you an idea of the total amount of glucose/glycogen for a ride within the same individual. IE, a 100 TSS zone 2 ride would mean you use the same amount of glycogen on a 100 TSS sprint workout.
That said, would a person that is “fat adapted” and in full blown ketosis have less stress from a 3 hour ride vs when they take on carbs? If anything, I’d imagine the body has more stress while trying to operate on purely secondary and tertiary fuel sources such as fat and ketones.
I’ve been keto for a few months now (losing weight), and I feel shockingly good…for about 3 hours. After that, I start to hit a wall. It’s very repeatable though. I’ve done 3 three hour days in a row without issue.
You are correct in stating that carbohydrate consumed during exercise does not alter the rate of glycogen use during exercise. Depending on the scenario, though, it may delay fatigue (I e., task failure) . I’m not sure precisely why you bring that up in the context of TSS as a predictor of glycogen utilization, however.
(Note that TSS was not designed to predict glycogen utilization, it just happens to do a pretty good job of doing so because of the way physiology work.)
If you compare two different athletes - athlete A who is more glycolytic, athlete B who is less so* - is it better for athlete A to accumulate TSS and lower intensity (to avoid fatigue / glycogen depletion), whereas athlete B can get away with higher intensity work to accumulate that same TSS?
Or said another way, athlete B can tolerate more high intensity work than athlete A to accumulate a given TSS, while not getting as fatigued?
So, presumably, the “ideal” training plans for these 2 athletes would look different from the perspective of training intensity distribution?
*E.g. as informed by looking at RER curves for the two different athletes
Basically, A misunderstanding on the timing and ratio of glucose uptake in muscles.
Edit
To be a bit more verbose. Based on your curiosity around my question I went and looked up and read a few papers on glucose uptake in muscles. Which made clear to me (and led to more questions) that glycogen is the preferred source until depletion.
I know of no evidence that athletes who are more “glycolytic” (whatever that really means) are any less tolerant of a high training load.
What does seem clear is that the more you train, the fitter you will be (to a point, anyway) and also that the more you train, the more you can train. So, if somebody only train a few days a week, constantly takes time off to “recover”, overcompensates by doing the wrong kind of training (e.g., a VO2max “block”) at the wrong time of year, etc., then yeah, they won’t be very fit, and they won’t be able to handle a high training load, simple because they haven’t built up to it.
Muscle definitely prefers intracellular substrates over plasma borne, but you don’t have to run out of glycogen before glucose becomes an important source of energy.
Then to answer the question you posed regarding my reason for asking.
If the cells would prefer blood glucose to glycogen the rate of glycogen utilization would be pretty handily modified by consuming carbs on the bike. I mistakenly assumed that was the case, mostly due to thinking if glycogen is just a well packed glucose, and to use it the cell has to unpack it, why bother, just go to the source.
It seems pretty clear that other aspects of glycogen production/utilization are important (and the barest of scratching at this question reveals a pile of information) but at the very least it’s a lot more complicated than my very basic assessment. I didn’t (and still don’t) know. I have silly hypotheses, but no knowledge/info/data. Answers only lead to more questions.
I guess the lesson is, when it seems easy, check my assumption.
It is important to highlight that skeletal muscle does not express glucose-6-phosphatase (G6Pase).[2][3] Thus, skeletal muscle cannot break down its glycogen to be utilized by other tissues.
I don’t think so. If my RPE is a 10/10 I can’t/won’t go harder-a 10/10 RPE means I can’t go harder by definition. If my RPE is below 10, I can go harder. Lots of things go into that RPE number but I think it summarizes them all fairly well. I guess you could argue that we are capable of more than we think we are, but I would say your RPE meter isn’t calibrated in that case.
Can you think of any counter examples to change my mind?
A bit of trivia just to complicate things further…
Although textbooks routinely state that skeletal muscle doesn’t express G-6-Pase, it can release small amounts of free glucose during very high intensity exercise. The mechanism isn’t clear, but it may be due to low levels of G-6-Pase, the non-specific action of some other phosphatase, and/or the action of debranching enzyme.
Absolutely. Whenever I do a test to failure, power is dropping off a cliff at the end because I’m unable to physically keep turning the pedals at that power. It’s not an RPE issue.
Now, there are workouts that I’ve cut short due to RPE being higher than intended. But if you’re measuring performance it’s about “fitness” and ability, not RPE.
Edit: a specific example , RPE has never been higher for me than the last 20 minute test I did, absolute 10/10 crushed me. Yet, I’ve never set as high a power for that duration, my highest ever performance, and failure was not due to RPE it was legs failing and power dropping…
no wonda!