I like this thread.
I understand what Dan says, weight lifting (what we call
body-building, although under that label goes too much stuff) may produce stiffness depending on how you do it and how frequently.
However an athletical preparation is needed in all sports (aikidokas should go running or also do some weight lifting if they enjoy it, or rope skipping which is great to make you jump like a cricket
) - consider 100mt runners, those shoulders many have are not strictly necessary for running, and yet they train legs and arms regularly with weights and although their legs are supposed to be ultrafast, they do squats with incredbly heavy loads that, arguably, should have slowed them down.
If you make weight lifting together with other athletical preparations, more aerobic oriented, they won't really make you physically starchy. Of course it also depends on how much you maximize weight when you lift - you can maximize weight or repetitions, or better a combination of both.
By and large it does not rally matter what you practice, as long as there is an atlethical intensity that produces improved
cenesthesis («the combination of organic sensations that comprise an individual's awareness of bodily existence»). Being aware of your body in motion is important, improves your performance and preparedness for all scenarios (well, at least physically) and induces a feeling of physical wellness that is
bound to be helpful.
Whatever atlethical preparation you may get, transfers its benefits to whatever sport you practice, inclusive of aikido.
A trivial example: a speed bag. You may hit it while holding in your grip half or 1 whole kilo. After half an hour of that you won't be less fast, on the contrary you will be
ultrafast.
Weight lifting, if practiced intensively having another goal in mind, and consequently matched with other preparations, won't stiffen you - but Dan is right insomuch as if you practice weight lifting having in mind only weight lifitng, well then it will slow you down because you're implementing a work out that does not take rapidity into account (arguably, a bodybuilder who goes in competitions does not need that and so does not care).
ps does not need or needs not? English lessons