Re: Knees and Shoulders - avoiding damage
I think you have a couple of misperceptions here. First, you seem to believe that most injuries in aikido are inflicted by one's partner. This isn't true -- as in other physical activities, most injuries are self-inflicted and result from poor technique, poor conditioning, attempting to do something you're not ready to do, etc. A dojo is a place of martial arts practice, and very few dojos of any kind take a cavalier attitude towards injury -- if we did, as my sensei says, we would soon have no one to practice with.
Your other misunderstanding is about the relation between peacefulness and aikido, and what any of that has to do with actions and physical consequences. You say that you want to study aikido because you want "to live in harmony with all of creation", which makes me think that you are one of those who believe that you'll walk into the dojo on your first day of class, and sensei will begin to enlighten you on the cosmic harmony. In fact, what sensei will almost certainly do is show you the basics of how to stand properly, and then show you how to step properly, and how to turn properly, and then how to fall properly...and then you will spend a sweaty hour or two practicing what you are taught. The word "harmony" will not be mentioned, nor will the word "peace". When class is over, students will bow off the mat, probably do some housekeeping chores around the dojo, change in the dressing room, and leave. They will not gather in a circle and discuss how peaceful their practice of aikido is making them and how they're coming with that cosmic harmony thing; they will talk about their kids or what happened at work or going to get a beer.
I don't say that by way of mocking your desire for harmony and peacefulness, although it may sound that way. The (IMO simplistic) label of "the peaceful martial art" is extremely misleading, and burdens aikido with unreasonable expectations. Aikido, like many other pursuits, is one through which a practitioner may improve his or her character in various ways, but the improvement comes from the practitioner's sincere practice more so than what he/she is practicing. In aikido, for example, many of us have come to terms with physical limitations -- and if you don't think that that is the pursuit of peacefulness, consider that the first person you must make peace with is yourself, and that there is no more fundamental way to make peace with yourself than in your physical body. That we happened to accomplish this on the mat of an aikido dojo is happenstance: people find this same peace on the playing field, the hiking trail, the running track. Aikido has no exclusive claim to that magic.
I think you've also got some misunderstandings about martial arts in general. You say "If I am into getting hurt or hurting people I would no doubt join kick-boxing", and then you further cite iaido as a less injurious, more peaceful possibility. There's a wealth of unintended irony here. The purpose of iaido is to draw a sword and kill an opponent, and the fact that you don't actually do this doesn't change iaido into decorative sword-waving. Kickboxers have killed very few people over the years; the same cannot be said of iaido's antecedents.
So, think about exactly what it is you want. No practice is guaranteed to be injury-free. No martial arts practice is going to take you by the hand and lead you to ki or peacefulness or enlightenment. Any martial arts practice is going to be 99% ordinary everyday sweat and effort among people who are not monks or enlightened beings...it's not going to be anything like the movies. Is it worthwhile? For me, sure, or I wouldn't be doing it. But I'm looking for different things than the things you say you are looking for.