I did 20 rep squats today for the second time (first time where it was actually bloody hard) and I'm quite the same as you Shrek, after the first 12 I found it alot harder to make myself take the 3 deep breaths between each rep and I would put it down to cardiovascular fitness. I started to get a really fatigued cramping feeling in my lower back and my legs were shaking uncontrolably when i got to my 17th rep, after i racked the weight, my legs gave in and collapsed, mind you this was only 70kg, wasn't sure how much i should have been doing but going to go for 75kg next time.