Just read Yahoo news that said...

The longstanding advice to "sit up straight" has been turned on its head by a new study that
suggests leaning back is a much better posture. A 135-degree body-thigh sitting posture with your
knees slightly bent was demonstrated to be the best biomechanical sitting position, as opposed to
a 90-degree posture, which most people consider normal,"
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NOt sure if this is true, but it's what the study suggests...