Originally Posted by An old Guy .
As I understand this stuff:
Both a 1 hour 100% FTP ride and a 2 hour 70% FTP ride yield 100 TSS.
Let's say that the first ride requires 100 units of energy. 66 units (66% of 100) are produced from glycogen. The the second ride requires 140 units (2*100) of energy. 47 units (33% of 140) are produced from glycogen. (How about that not Joules just unidentified units of energy. That is how people do science.) The 66% and 33% come from studies where glycogen and fat metabolism during exercise has been studied.
You have clearly demonstrated in the above that you don't understand it.
1 hour at FTP = 100 TSS
2 hours at 70% of FTP = 98 TSS, so close enough.
A 1-hour ride ride at FTP will be nearly all fueled by glycogen metabolism. Refer to the fuel substrate utilisation chart below.
We'd TT typically at say 80-90% of VO2max, which gives a glycogen utilisation of around 90-100% of total energy metabolism but let's say 90-95%.
So, at FTP, 90-95% of energy demand would be met by glycogen metabolism.
Let's say 1 hour at 100% FTP = 1 unit of energy, of which 0.90 - 0.95 energy units are met by glycogen metabolism.
A steady paced ride at 70% of FTP for 2 hours = 2 hours x 0.7 = 1.4 units of energy
But glycogen utlilisation (as a ratio of glycogen to total fuel substrate utilisation) for such a ride would be in the 65-70% range, give or take depending on fitness. 60-70% of 1.4 = 0.84 - 0.98 units met by glycogen metabolism.
The fact that the amounts of glycogen metabolised for each ride are very similar is actually not that remarkable (it just demonstrates you haven't understood some basic exercise physiology).
What is remarkable is the correlation between TSS and glycogen utilisation for rides of quite different types - something that's actually been studied and published.
That's how people do science.