do bmx riders have to be strong?

It kinda depends... Is your bike one of those that weighs 45 pounds? If it does then someone small in strength wont beable to do much referring to tricks. What you really need is seed. I am not that buff myself and I can do most the tricks in the book, on dirt jumping.

sorry i missed spelled speed.. (seed)=(speed) :D

I wouldn't say that bmxers need to be strong as in Football player strong. I don't think that the muscle build is what matters. When I started riding I was purly dirt and I could hit the jumps just fine and I wasn't built or lifting weights. I found though that the more I rode street and grinded ledges that muscles did begin to build and tone themselves. I have also noticed that with more toned muscles your agility is better. The tricks I used to struggle on I can pull with ease (part practice, part muscle?). Anyway, as far as dirt goes I feel that momentome (spelling?) is a big part. I hope I have helped.