You commented: “This strain may not cause injury (or even be noticeable) when you are young, or if you come from a culture which conditions the knee joint to withstand the strain by regularly squatting rather than sitting in a chair. . . .”
You make a good point that conditioning, and to some extent culture, is an important factor in how well the knee accommodates deep squatting. One exercise I learned early in my taijiquan training involves standing as you would at the beginning of the taiji form, raising the arms to shoulder height, then slowly bending the knees and folding at the pelvis into a low squat, remaining in that position for a moment, then unfolding to a full standing posture again with the arms again rising to shoulder height. This is repeated several times, exhaling while lowering, inhaling while rising. While in the squat, and throughout the exercise, the entire sole of the foot should be comfortably on the ground, with no lifting of the heels. Once the practitioner has learned this well, the body should be easily balanced and relaxed in the low squat. This exercise not only conditions for balance, it is an excellent way to stretch and relax muscles in the legs, back, and abdomen. I have long been a big fan of squatting. There’s a sort of “bird on the beach” feeling about this posture, and one can rapidly go from a squat to a standing position if the occasion calls for it. As you note, in some cultures it’s a very common posture while taking a break, waiting for a bus, socializing, etc.
This being said, if one hasn’t learned this from an early age, or unless one can facilitate conditioning that makes a comfortable squat achievable, I think it would be best to avoid bending the knee any lower than feels comfortable and strong.
[This message has been edited by Louis Swaim (edited 01-31-2007).]