Don't sit up straight !

Your mother's advice to sit up straight in your chair could be a prescription for lower back pain. A new study using new imaging technology reveals that sitting upright with a straight back (90 degree) and thighs parallel to the floor increases the strain on the lumbar discs in the lower back.
In fact it's better to lean back a bit in a chair, even if looks like slouching. The best position is what you get in a La-Z-Boy, 135 degree between back and thighs.

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Source: Chicago Tribune, Nov.28, 2006.