Muscle switching to fat stores?

On long endurance rides after about 2h30, I feel like I’m gonna bonk but after maybe 20 to 30 minutes, I still feel some fatigue in my legs but not enough to make me think I’m gonna bonk anymore and I’m able to go faster too.
I can go like that for another 1h30 at best.

I’ve been thinking that it was my muscles switching to fat stores to produce energy.
Am I right or is it something else?

I’m under fueling by the way, still trying to find the right balance. Maybe it could be part of the problem.

I’m similar (although its depends upon the intensity of the ride as to when) and under fuel also I’ve put it down to that. I think if the body is getting a steady supply of fuel it performs well but when it’s not steady its having to fill the gaps from elsewhere, which is ok to a point but eventually it isn’t and your body has restock that fuel before you can continue efficiently. There’s also the physiological aspect if you tell your self you will struggle around 2.5h you probably will and similarly the knowledge that you’ve pulled through before will help you pull through again.

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I think it is the brain and your muscles fighting for carbs. At 2.5 hrs my head feels “light” and feels like a bonk, but then I feel better after. I think this is when the brain wins the “battle” and gets the carbs, and then your muscles start oxidizing more fat. Your muscles are still burning lots of carbs though. Good fueling makes this feeling completely disappear for me.

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