Reynolds fell to seventh with the drive-through, and then handed the car over to Youlden to cross the line in 13th.
As the defending winners, Reynolds in particular and Youlden have had a busy lead-up to the Supercheap Auto Bathurst 1000, which the former admits took its toll.
"I’ve had a very big lead-up, we’ve had terrible sleep, and I just haven’t looked after myself that well," Reynolds said.
"I’ve got to learn to say ‘no’ to certain things and doing things and we tried [to say no].
"Yesterday I was probably running on 70 percent, this morning I was probably 50 percent, [in] the race I was pretty much at zero.
"[It's] the biggest race of the year, I had the fastest car, I’m just bitterly disappointed, just so bitter.
"We’ve been the best performing team all week, everyone’s put their heart and soul into it, and I failed at the end of the day."
Reynolds said the situation was not one he's encountered before in his training.
"Sometimes you just try and be mentally strong and your body just gives up," he added.
"That’s what gave up; my body just gave up at the end of the day.
"I’ve never had that; I’ve done Half Ironman stuff, run for three or four hours straight, and never had anything like that in my life.
"I was pushing really hard, though, for the whole race. Every time I got in I was giving it everything I had."
Reynolds said the cramp kicked in as he was trying to fight Lowndes, after "my mind started to go".
He and Erebus discussed handing over to Youlden at the final pitstop, but opted against it.
Right-leg cramping was then accompanied by a cramp in the left leg, as Reynolds selected first gear and tried in vain to keep the clutch engaged.
"I thought my screen was foggy, but then they [removed] the tear-off and it didn’t get any better, and that’s when I realised my vision was stuffed," he added.
"The vision was diminishing, so I was in a bad state, but I’m too competitive for my own good and I just didn’t want to give up.
"At the end of the day, if I stayed out, I would have crashed the car, one hundred percent.
"So the best thing we did was to just put Luke in but when he (engineer Alistair McVean) was asking me ‘Do you want to stay in?’, of course I want to stay in, as a racing driver you never want to get out.
"You’re second on the road, you’ve had a drama, but you can’t give up.
"I was in a world of pain; [the cramp] was the worst feeling I’ve ever had in a race car in my life."