My biggest problem with laying on the floor with a bad back is getting up again! If you're struggling to bend anyway, then getting up can be a nightmare! My mum did it once, and ended up having to get an ambulance out to help her up!
Also, when you're back is playing up, forcing it to be completely straight (like when laying on the floor) can cause the muscles to over-stretch - making the whole thing even more painful! (at least, thats what I've found over the years... no medical background here

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And yes, dicolfenec is amazing stuff! It's the only thing that keeps me moving most days!