After reading a Thomas Kurz article, the next workout, I checked out my posture in the squat. I realized yesterday that my shoulders are coming too far forward and experimented with correcting it. Basically, the bar has traveled to where it's over my mid-foot or to my toes rather than being over my heels. Doing a few reps with my torso more upright, I could really feel my hamstrings working. It, of course, felt awkward, but I'm going to concentrate on my form the next few workouts and see whether I feel any difference (in my strength, feeling in my joints, etc.).
Verkhoshansky claims that for building mass one should train the same muscle group(s) two days in a row. Citing research, he claims that a bout of exercise the next day increases catabolism and therefore the resultant rebuilding.