Messenger bags...

Buffalo Bill I used a shoulder bag for 14 years as a messenger, and there were times when it hurt me, but I never suffered any upper or lower back problems. I used only single straps and rarely bothered with cross-strapping.

In my view, these types of problems are rarely to do with weight carried externally, more likely to do with muscle imbalances, leading to distortions of posture. Is cycling your only form of exercise? Do you stretch? Have you considered yoga or some other sort of complementary activity?

i am a student of the Shadow Yoga School
Boxe/Kickboxe
I don't have a problem with my back it doesn't hurt or feels uncomfortable.

and Dave, never talk to a girl about something you have an obsessive passion about eg football or cycling, i'm not that fanatic about bicycles and my girl calls me a bike geek but i'm ok with that coz she's my girl already ,you get me? Immagine what a 20 something early 30s girl would think when you start mentioning your custom buil serotta with space age components and matching lycra outfit to go with it.