You hit on it. There are a lot of people here who don't seem to understand that there is a basic difference between association and causation, and between anecdote and evidence. I have little doubt that many have noticed an association between big-muscled people who have lifted a lot of weights and tense, self-resisting movement patterns. It proves nothing and means nothing, other than the fact that one has encountered a few big-muslced guys who were stiff. The true hubris here is from people who may have some experience in "internal skills" that pretend this makes them experts on subjects with which they obviously lack even passing familiarity: exercise science, neurology, physiology... not to mention basics of reasoning or problems of knowledge.

The science shows that there is no basis for the claim that lifting weights interferes with learning relatively dissimilar skills or movement patterns. In fact, the evidence to the contrary is overwhelming. All the world's best athletes in every kind of competitive athltetic activity - from biathalon, to marathon running, to water ballet - now use at least general weight training to some extent - even if it is just to provide a minimal base of stength and injury prevention.

The claim that engaging in a few resistance training exercises a few times per week necessarily interferes with motor learning is beyond false. It is evidently absurd to anyone who has a body, as I have pointed out. I challenge anyone to recount a single instance of one exercise or activity confusing or disrupting a dissimilar one. Extensive running does not interfere with one's ability to dance or walk. Writing does not interfere with one's ability to type, use a fork, play the piano, or even draw.

The only such effects that have ever been recorded are when the resistance training exercises have been devised that are extremely similar to the skill in question, for instance: swinging a weighted tennis racket, or sprinting with stapped-on ankle weights. Training methods like these have been shown to interfere with the higher level skills they mimic. (People who favor swinging around heavy bokken take note.) This effect has only been noticed when the training exercise in question is virtually identical to the movement skill. By contrast, overhead barbell presses, squats, or pullups are nowhere near similar enough to Aikido movements to have any effect on them whatsoever. The only way this kind of exercise could possibly interfere would be if it took up so much time and energy that one's Aikido training suffered directly from the resulting neglect.