First time posting on a forum, dont know how they work, but bare with me lol, Anyways I just reciently got a bmx, parents had to really carve out alot of money for it, its a nice one too, gt bk xl. Anyway's i've bein biking for maby 3 miles back and forth, not steep hills but rather long, like a slow but long incline, which is harder on your body, its all ups and downs and with my small sprocket on my bmx, it doesn't feel bad, it doesn't hurt my back when i stand up and my knees are fine, even when i have a heavy backpack, its more about exercise, dont doubt a bmx because they are made for park, mines fantastic, breaks on them are great, they are ALOT easy'er to control, and i find them alot lighter than other mountain bikes and commuting bikes, yet they are a bit sensitive, and honestly all i wanna say is they are fantastic i find, Only thing that will kill is slow inclines, i can blow past someone wit my small gears