Although the office chair industry is certainly a booming one, few people stop to learn what they need to know about fitting their chair to their height, frame and any postural condition they might have. I spoke with several office furniture salespeople while researching this article, and all of them told me most customers base their purchase decisions on color, style and price. Rare is the office worker who is interested in tinkering with the controls located at the bottom of the chair seat.
Come on. The levers and adjustments are there for a reason. If nothing else, learn to adjust the height of your chair. This adjustment is the foundation for all the others. The reason, in my opinion, is that the height adjustment is the primary way you have to change the angle of your hip joint during sitting. This angle, in turn, affects the position of your pelvis and degree of curve in your lower back, possibly altering the normal alignment of your spine. And it affects certain muscles (quadriceps, psoas, hamstrings) that play an important role in posture-related back pain.
So when you sit for long stretches of time, you my be increasing your risk for wear-and-tear conditions of the spine. You also raise the potential for injury while you are away from your desk and doing more active things.
The All Important Hip Angle - How Much Hip Flexion Do You Have While Sitting?Just think, it all starts with something you can control by simply adjusting your chair height - the angle of your hip joint. The hip angle can be understood as how close (lots of hip flexion) or how far away (less hip flexion) your trunk is to the top of your thigh when you are sitting in your chair. When you adjust your height, you adjust the degree of flexion at your hip joint.
A 2011 study in Spine Journal measured the load that sitting has on simulated spinal discs. Researchers concluded that pressure on the spine can be relieved with a more open angle between the trunk and the thigh, i.e. the hip joint angle.
Another small study, this one from 2006, took images using a positional MRI and found that degenerative changes in the spine play a role in determining the optimal forward and back placement of the trunk during sitting. The author, Dr. Francis Smith, told About.com the study was successful in identifying the optimal angle between the trunk and top of thigh for sitting (135 degrees). This angle translates to your hip being higher than your knee, he said.
Of course, the back rest, seat tilt and lumbar support features of your chair can help support a pain-free back, and they should be used. But to set yourself up with ideal alignment from the beginning, nothing beats the getting the correct seat height for your frame.
Seat Too High, Seat Too Low, Seat Just RightAnother way to understand your hip joint angle is to compare the height of your knees to the height of your hips. This is usually the easiest way to assess if the chair height is right while you are in the process of adjusting it.
When your chair seat is the right height, your feet will be flat on the floor. Your feet should reach the floor without causing pressure at the back of your thighs. If you’ve got a case of dangling feet (which may be due to your own height), place a foot rest or thick book under them.
Knees should be approximately level with, or lower than, your hips. Level, in this case, corresponds to a 90-degree angle between the hip and trunk, which is relatively stress-free on the hips and back.
If you can’t reach your feet to the floor, your chair is probably too high. The federal Occupational Safety and Health Administration (OSHA) says this is potentially hazardous because it may lead to your schooching forward, thereby foregoing the support of the back rest. Sitting this way would be considered an awkward posture and a risk factor for work-related musculoskeletal disorders (MSD). Symptoms of an MSD due to awkward sitting posture may include muscle fatigue, swelling, pain, numbness or decreased circulation.
If your knees are higher than your hips, your chair is probably too low. In this case, your hip joints will have an extreme degree of flexion. Most people’s backs can’t handle this well because their hip muscles aren’t flexible enough. If this is you and you sit with your knees higher than your hips, your position may be responsible for your low back pain.
Karadimas EJ, Siddiqui M, Smith FW, Wardlaw D. Positional MRI changes in supine versus sitting postures in patients with degenerative lumbar spine. J Spinal Disord Tech. 2006 Oct 19. http://www.ncbi.nlm.nih.gov/pubmed/17021413
Rohlmann A, Zander T, Graichen F, Dreischarf M, Bergmann G. Measured loads on a vertebral body replacement during sitting. Spine J. 2011 Jul 19. http://www.ncbi.nlm.nih.gov/pubmed/21775218.
US Department of Labor. Computer Workstations. Chairs. http://www.osha.gov/SLTC/etools/computerworkstations/components_chair.html. Accessed August 25, 2011
Washington State Department of Labor and Industries. Office Ergonomics: Computer Workstation & Mobile Computing. http://www.lni.wa.gov