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I have started riding BMX back in the year 2000, so the bikes were much heavier back then and I was building a lot muscles through that. For me BMX now is just a sport, the time I could have turned pro are long gone, still I love to ride my bike mostly every time, at least as long, as I don't get hurt.
Nowadays almost every rider is trying to get their ride as light as possible. Either most riders nowadays don't know that you are building more muscles with heavier bikes, or they are not as physical strong as the riders back then were. I can understand that the pros are riding as light bikes as possible, but if you are like me, who is riding for the sport and the fun, you don't need to pull of a triple tailwhip or a 1080.
I personally do not look that much into saving weight, I am just happy with a strong build in my ride. Bikes are way more reliable nowadays.
So why lifting weights, when a heavier BMX is doing the same and is even much more fun?
(There is one road fools episode out there, where a guy had welded his own bikes out of steel, he has so much muscles. Watch it, it proves my point)