Exhausted or under fuelled

Maybe a basic question but what is the difference between being under fuelled and exhausted?
When outdoors on long rides I find my quads start to ache first and get to a point where I can’t really stand out of the saddle nor put down minimal power without it being torture (think petit levels of power) and I find myself crawling back at a snails pace. Is this simply being exhausted or am I missing it and not eating enough.

Sounds like muscular fatigue to me. How long rides are you taking about here?

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Yesterday was only 3 hours but I did go hard and took a gel after 30 mins every 20 mins four times. So I probably didn’t fuel enough for a hard effort but I wondered how I know the difference

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It ‘could’ be down to bike fit, or even too low a cadence. Maybe make some small changes and note down what you did

I call exhausted.

Based only on my recent experience of both exhaustion and under-fuelling.

The start of Dec I did a block of 7 Build HV workouts in 8 days.
The 2nd workout was way under-fuelled; I did 1 hour of it just fine, then plummeted.
Finished the rest of the workout at about 60% power.

The 7th workout (sweet spot) was attempted in an exhausted/fatigued state.
I couldn’t even hold target power for 10 min. There was just nothing there. Empty.

If you were exhausted you wouldn’t have been able to go “only 3 hours”.
But you also say you eat during the rides, so it’s probably not a fuel problem either.

Might be just be a case of building up your plain old muscular endurance (i.e. riding hard for long).

I find anything beyond 2 hours requires fueling plus electrolytes in my water. Even my 2 hour ride yesterday I ate a banana on the ride and had electrolytes in water (summer here so electrolytes important when it’s warm).

3 hours rides at tempo / zone 3 pace should be fairly comfortable to do provided you’re doing a reasonable amount of other training during the week.