I think we get vocal activity in certain "popular" fitness themes. When it gets too focused then the other guys lose interest and stop posting so much. The power lifters and strength guys were way more talkative when I first came here.
Perhaps we as mods should try and broaden the discussion by asking more questions to the members about their particular interests and goals in their specific forums.
I think a lot of people like to lurk and read the posts and draw information from them. I prefer it like this compared to other "health" boards that are constantly flooded with the same old newbie questions.
I think people drift in and out of fitness, which accounts for how many members consistently post at any given time. Very few people make a long term commitment to fitness.
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Train the body as it truly is: one, flexible piece!