I think the main point to remember here is about flexibility. Muscles can be made flexible in a relatively short amount of time. Whereas tendons take a very long time to gain flexibility. If one starts early in life and maintains flexibility, it is rather easy. But, when adults start to get into Aikido, most haven't done any joint flexion for quite some time. Most gaijin like myself are very interested in quick results. It took me about 1.5 years to gain flexibility in my knees and other joints. Trying to get this flexibility too quickly will result in small tears in the ligaments which results in scar tissue. This will make it take even longer to get the desired flexibility in the joints. So I say that patience and constant practice 'within' one's limitations (and a little beyond of course) are what is needed. Not listening to your body and making it do what it's not designed to do will result in injury. Aikido is not the cause.