Sitting incorrectly for periods of time even for as little as 20 minutes can cause strain on the body. It may feel easier to sit slumped in a chair not using your muscles to support you but this can lead to weakening of the spinal ligaments (think of ligaments as the sellotape that holds your bones together). This in turn causes weakness and increased incidence of strain. When you first start to sit correctly you may experience some discomfort from the postural muscles. This will pass as the muscles get stronger. The most common conditions I treat as a result of not sitting correctly are low back pain, shoulder pain and headaches. As well as sitting correctly it is advised that you get up every 20 minutes. Ligaments have an elastic limit, which when exceeded adopt irreversible damage. This elastic limit is reached at roughly 20mins of stretch.