Sitting Posture

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How to modify your chair at work to limit your risk of injury
Subsequent posts will address how sitting posture contributes to injury.
But before addressing this, it is important to understand that our seating surface plays a
large role in our ability to achieve and maintain an efficient posture.
Because of the way that chairs are manufactured we need to consider four aspects of
the chair that will affect how you sit at your desk:
  • The chair height
  • The shape of the seat surface
  • The depth of the seat surface
  • The chair back

The height of the chair is critical. Many people sit too low, which will create a bias into a flexed lumbar, slumped position – particularly if you have tight hamstrings and gluteal muscles. Some people – those who are vertically challenged – may find that most chairs are too high, and their feet cannot rest on the ground. This is also a problem because it means that their weight cannot be distributed into the ground and their legs become long levers, pulling on the hips and lumbar spine. The appropriate height of the seat is, generally, the height at which you feel you have the most equal mobility between slouching and sitting up tall. This will enable you to find a comfortable, middle position, where there is minimal tension on your lower back and hips. You should have an open hip angle, meaning your hips are slightly higher than your knees. If you are one of those people whose feet dangle and can’t reach the ground, find something to use as a foot support. Reams of paper work are great because you can adjust the height by removing paper until it supports your feet at just the right height.

The shape of the seat surface is important as well. If the seat is lower in the middle it
will both create a bias into slumping, and it will also create a bias into hip internal
rotation (the knees will rotate inwards). This will limit mobility at the hips, so that
when you reach for something in front of you, you will reach from your spine
instead of by moving at your hips. If your chair has this quality, use a thin towel to
fill in the gap created by the depression in the middle of the chair.
Petes blog pic
The depth of the seat surface is important, particularly in situations when one is
sitting unsupported. Unsupported sitting simply means you are not using the chair
back. When sitting unsupported, more than half of your femur (upper leg bone)
needs to be off of the end of the seat. You should sit in a position in which your
pubic bone (the front of you pelvis) is about 2-3 inches from the front edge of the seat.
This is because the femur acts like a lever. If more than half of it is off of the chair,
weight will be biased into your feet – as long as your hips are higher than your
knees. If less than half of the femur is off of the chair, weight will be biased into your
pelvis and lumbar spine.
Supported sitting means utilizing the chair back. If you find yourself using the chair
back frequently while sitting, it is important to assess the qualities of the chair back.
  • Is there a space between the seating surface and the chair back? If there is, you need to fill this in. Use a towel, or if the gap is so large that towel will fall through to the ground, you may need to use a sheet to put over the entire chair so that any support you use in to fill in that space will not fall to the ground. This may seem extreme, but this space between the seat surface and chair back is the primary reason that people end up slouching for prolonged periods while sitting, which is terrible for lumbar discs.
  • Is the chair back flat and vertical? Or is it undulating to accommodate the natural curves of the spine? Notice I said ‘the spine’, because your chair was not designed solely with you in mind. Everybody’s natural curve of their spine is unique. You need to adapt your chair back to you – don’t try to adapt to your chair back. This will require that you use a towel or free article of clothing to support any open spaces between your body and the chair back,so that you are supported in a comfortable position.

Hopefully this information will help as a starting point for improving yoursitting posture. Next we will look at how you can better position yourself in anunsupported position.

-Pete Rumford DPT, MS PT, CFMP, FAAMPT

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