Back in pain

I can load everything in my sig into a lowepro naturetrekker ii and walk all day. It has an adjustable waist to top of the straps velcro thingy that works incredibly well.

If you are carrying a fair amount of weight; get a backpack with waist straps. Go to REI and tell them you're thinking about backpacking for the first time and ask them about backpack fitting.

They will tell you most of the weight actually sits on your hips and NOT on your shoulders.

This should help you a LOT.

bad backs - a hazard of the photographers life
my advice travel as light as you can
use wheels where possible
e.g the lowepro wheeled bags

dont use shoulder bags
do yoga, learn to stretch

see a good chiropractor

take care of your back it has to last you a long time
(I've known several pros who have had to give up work bacause of their backs)

www.ralphhodgson.co.uk

Maybe you have a low pain threshold

It's all about posture .. maybe you need to dry some dummy runs using different stuff to weight the bag to see where the pain kicks in.