Started getting serious into squats a few weeks ago doing rippetoe's, and I notice that when im doing my 3x5 working set of back squats, I get fast,sharp, shooting pains or contractions in my hamstrings and gluts (i expect the fatigue). Im curious...is that a combination of my fast twitch and slow-twitch muscle fibers coming into play? Or is that the action of my fast-twitch fibers working? Hopefully someone with knowledge of exercise physiology might shed some light on this. I'm an exercise geek in college that likes to know everything about everything when it comes to this kinda stuff.
Also, when I'm doing my overhead presses, I find that my lower back hurts with max vertical extention. Am I supposed to make a straight line with the bar and my body? Or do I angle the bar slightly past vertical? In crossfit video's, it seems like rippetoe likes it slightly past vertical. Here's a video of what I'm talking about: http://www.youtube.com/watch?v=sebbhlKhs2E