Sitting is something that we do every day, either as part of our job, commuting or after a long day of work when relaxing on the sofa. The way we sit can however lead to long-term spinal health problems.
Let’s first take a look at the science behind sitting and then discuss how to do it properly. Nachemson and Elfstrom (1970) studied what happens to the intervertebral discs (IVDs) during different seating positions. The pressure that occurs within your IVDs when you sit (especially between the 3rd and 4th lumbar vertebrae) is at its least when the spine and hips are at 100 and 110 degrees. Sitting with your hips at 80 or 90 degrees results in IVD pressure increasing upwards of nearly 200%. The diagram below demonstrates IVD pressure when sitting in various positions according to the study done by Nachemson and Elfstrom.
Image 1. Disc pressure during different sitting positions
(Nachemson and Elfstrom,1970)
Another study by Andersson et al (1974) found similar findings when they assessed how the spine is affected when sitting in a chair, at a desk, in a wheelchair and in a car. They also concluded that the highest level of pressure occurred in the flexed or forward sitting posture with the hips at 80 degrees and the lowest when the hips are at 100 degrees.
Not only is the low back significantly affected during poor sitting postures, but also the neck and upper spine as well. A study by Dvorak et al (1991) found that sitting in a flexed position increased cervical disc pressure and muscle activation, however they noted that the most important effects where that of tension on the cervical spinal cord, brain stem and nerve roots. Several other authors also noted the physical effects that occur from poor long-term sitting posture such as a forward head carriage, increased upper cervical curvature, decreased lower cervical curvature, increased upper thoracic kyphosis (hyperkyphosis), shoulder dysfunction and elevation of the first and second ribs (Lafferty-Braun and Amundson, 1989; Ayub, Glasheen-Wray and Kraus, 1984; Saunders and Saunders, 1993; Darnell, 1983).
The images below demonstrate common mistakes when sitting at a desk. Not only is the sitting positon not adequate but the screen is also too low.
Image 2. Sitting in a flexed position
How to sit properly to avoid long-term spinal problems
It’s important to note that the back angle is not the most efficient way to address your siting posture – the key is in the angle of your hips relevant to your spine. For example, if you had your back-rest set at 110 degrees (image 1) then you are essentially leaning back away from your desk and this will make you stretch your arms and head out further to reach the desk and screen. It also does not support your spinal curvatures well and will lead to spinal strain. It is therefore recommended that the back rest be set at 90 degrees and sit on a seat wedge. This will allow the hips to sit at 100 degrees and therefore accomplishing the same effect without the need to lean or stretch forward and allow your spine to sit upright with minimum effort. Your spinal curvatures will also be in a neutral position.
The image below demonstrates correct desk posture. The back-rest is at 90 degrees and the hips are at 100 degrees. The chair is as close to the desk it can be so your arms are not stretched out to reach the keyboard and mouse and the screen is at eye level. Feet are flat on the floor (no angled foot rests)
Image 3. Correct desk posture with seat wedge
Poor sitting posture at work is not the only time your spine is at risk; sitting at home on the sofa can also lead to long-term spinal problems if it is too deep, too low or too soft.
Image 4. Sitting correctly at home
From left to right: the first image demonstrates the worst type of sitting posture, which most commonly occurs when sitting on your sofa (190% IVD pressure), which will lead to chronic repetitive spinal injury, neck and shoulder dysfunction. The second image demonstrates better sitting posture, but still not perfect (140% IVD pressure), this will lead to stress and discomfort in the low back, neck and shoulders. The third and fourth image demonstrates perfect sitting posture (normal disc pressure – 110%).
Poor sitting posture is one of the hardest bad habits to break and many people tell me that they love to sit on their sofa as it’s so comfortable or that they always find themselves slouching. With a little effort and dedication, you can improve your spinal health, reduce the risk of injury and long-term spinal problems and still be comfortable by addressing a simple daily habit.
- Sitting with a 90 degree back rest and your hips at 100 degrees is the optimal sitting position and will allow your spine to sit upright with minimal effort
- Sitting flexed or bent over your desk increases the risk of long-term spinal health problems
- Sitting with the back rest at 90 degrees and your hips at 90 degrees is not ideal and can still lead to spinal problems.
- Sitting on a sofa creates enormous pressure in your low back (nearly 200%) and will also lead to neck and shoulder problems
- A seat wedge is a very simple remedy for most poor sitting postures
- Dvorak, J., Panjabi, M.M., Novotny, J.E. and Antinnes. J.A. (1991). In vivo flexion/extension of the normal cervical spine. Journal of Orthopaedic Research, 9, 828-834.
- Lafferty-Braun, B. and Amundson, L.R. (1989). Quantitative assessment of head and shoulder posture. Archive of Physical Medicine and Rehabilitation, 70, 322-329
- Ayub, E., Glasheen-Wray, M. and Kraus, S. (1984). Head posture: a case study of the effects of the rest position of the mandible. Journal of Orthopaedic and sports Physical therapy, 8, 179-183
- Darnell, M. W. (1983). A proposed chronology of events for forward head posture. Journal of Cranio-mandibular Practice, 1, 49-54
- Saunders, H.D. and Saunders, R. (1993). Evaluation, treatment and prevention of musculoskeletal disorders, Vol 1, Educational Opportunities, Minnesota.
- Nachemson, A. and Elfstrom, G. (1970). Intravital dynamic pressure measurements in lumbar discs. A study of common movements, maneuvers and exercises. Scandinavian Journal of Rehabilitation Medicine, 1, 1-40.