On Friday, my quadriceps cramped up after just one set of warm-up squats. I barely managed 275 for one rep before reracking.
Today, the same circumstance arose, except that this session didn't entail building up sets, so instead of only one final, heavy set, there were five. I felt my quads tighten up after the first rep of 285, but rather than stop there, I willed out five ATG reps, albeit shaky ones. The weight felt light and I had exceptional drive from the bottom position, but the pain inhibited stabilization. I did another set, and got 5. On the third, only 3 before I reracked. It's extraordinary the difference that mental capacity makes, evident by Friday and today.
As for the cramps, I can't pinpoint a particular reason. I'll spend tomorrow stretching and walking to keep blood flowing in the region.
I've been coming short of these squat sessions lately, but the first thought to pass through my mind when walking away is, "I can't wait to try that again" (probably moreso now because such a trivial obstacle hindered my performance).