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.