I have been reading your book and trying to apply it for a couple of months now. I have had off & on lower back pain for years, but not any more! I can really feel a difference in the strength of my back.(I just turned 50) The only problem is that now I am having difficulty with my knees, which I never have before. They feel achey and stiff most of the time, especially whenever I am on my feet for any length of time. If I have to bend my knees or kneel at all I feel like they are very vulnerable. I know my hamstrings are probably quite tight - I'm trying to do the exercises for that. By the way, I checked to see if I walk on a line.and I do using my natural gait. (I walked on a freshly raked baseball diamond with chalked lines.) Not sure what to do about this. Any ideas?