Earlier in Sunday's 100K on our tandem we encountered a long (45 min.) fairly steep climb (8-12+%). I'm pretty max'ed out on these sorts of climbs on the tandem with my HR in the 145-155 bpm range. I'm 64 and while I haven't measured max HR I'd be surprised if it were over 170. I felt reasonable at the top, drank water/Nuun during the ride and ate reasonably at the two rest stops. I couldn't really drink during the climb as balancing/steering the tandem at lower speeds takes both hands.
There were some other small rollers after the big climb and I felt reasonable up to the last rest stop at 42 miles, but had nothing left after that. I felt crampy, but didn't cramp. When I put my foot down at the finish my right quad cramped.
Ate well after the ride and felt fine save for a bit crampy. Had no trouble on the 4 hour drive home. I experienced no muscle soreness.
My guess based on limited knowledge:
1. While I may have been bumping up against LT I would guess that I would have experienced greater muscle soreness ride and post-ride if this were the cause of my late ride crash.
2. Electrolytes - Maybe something better than Nuun out there. Might eat a salty snack at the rest stops or enroute?
3. Glycogen Depletion - seems the most likely cause and the one most easily dealt with. I carried a bottle of Gatorade but preferentially drank the water/Nuun (for no particular reason). I have no problem eating at rest stops, but frankly didn't find much appetizing. I can be much more scientific next ride about dealing with this aspect of the problem.
Many out there have actually knowledge in this area and I'd welcome any and all observations and comments.