I simply refuse to ride my bike unless I wear a back protector. You can damage an arm or a leg, but your spine? No thanks.
I think Knox offer the best protection out there and I currently use their Stowaway protector as it rather conveniently rolls up when not in use:
http://www.planet-knox.com/Knox/index.asp
Am thinking about going for the complete shirt to give comprehensive torso protection.
My jacket has the short Knox back protector built in (so at the moment I use two together!) but it's the lower spine that's essential to protect fully. People that can't be bothered (yet are happy to spend sixty quid on some bauble for their bike) are (in my humble opinion) somewhat brain dead. Surely it's just common sense and a question of personal responsibility?
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