Chairs

Primary considerations:

Is your chair height adjustable?
Does your chair support your lower back?
Is there room between the front edge of the chair seat and the back of your knees?
Can you easily reach your work without interference from the arms of your chair?
When using the keyboard or mouse, are you able to keep your arms in a comfortable position with elbows in at your sides?
Do your feet rest flat on the floor or footrest?
When you sit upright in your chair, are your thighs approximately parallel to the floor?
Does your chair have an upright locking feature?

Other considerations:

Adopting two or more seated postures will allow various muscle groups time to relax and recover.

If your chair backrest is adjustable, raise or lower it so that the contour of the chair provides maximum lumbar (lower back) support.

If your chair has armrests, they should allow you to get close to your work. If you are typing, they should be at a height where they barely contact your elbows when your arms are resting comfortably at your side. Chair armrests should not force you to elevate your shoulders or wing your arms to the side.

What to look for when purchasing a chair, (before purchasing, try out the chair for a few days):

Maintaining a “neutral” lumbar (low back) curve is important for individuals who find they are seated for long periods.
But “lumbar support” is only an asset if the chair backrest provides support in the lumbar region.
Consequently, the chair you select should offer backrest height adjustment.

When writing or keying, many workers prefer to sit upright; however, when speaking to co-workers or performing less intensive work, reclining is recommended as it reduces pressure on the lumbar spine.
Optimally, your chair will have an upright locking mechanism, a reclining backrest, and backrest tension adjustment.
A variable backrest stop is also available on some chair models.
A chair’s seat pan depth should be suited to the worker.
A seat pan that is too short results in increased pressure on the buttocks and thighs.
A seat pan that is too deep can restrict circulation at the knee and makes it difficult to attain proper lumbar support.
Many workers may use a chair over its useful life. A sliding seat pan will provide comfortable seating for a wide range of individuals.
Some chairs have a seat pan tilt option.
By lowering the front of the seat pan, the lumbar spine assumes a posture closer to the neutral standing posture - which reduces pressure on the lumbar spine.
Some people may find this feature less important than other chair adjustment options; however, individuals with intense low back pain may benefit from this feature.

The ergonomist's opinion:

Sitting with several "bad" postures for short periods may be better (or at least no worse) than using one "good" posture over long periods.

Individuals are encouraged to use three or more seated postures throughout the workday to allow various muscle groups time to relax and recuperate.

Recommend postures include:

"perched” on the edge of the chair - with pelvic tilt to maintain good lumbar lordosis (low back curvature);
upright - with seat back locked and the gluteus maximus all the way back in the seat;
reclined; and
other postures you are comfortable with.

Sitting with “bad” postures for short periods will allow routinely used muscle groups to relax. Blood flow patterns will also change and allow recovery of taxed soft tissues. However, if you find yourself sitting “badly” for extended periods, it may be a sign that your workstation requires adjustment or different equipment.

When sitting unsupported it’s a good idea to tilt the pelvis forward to re-establish a “neutral” lumbar spinal posture. Maintaining this posture requires muscle activity and will be easier to sustain if the individual is involved in regular fitness activities

The classic upright seated position with perfect 90o torso angles is perfectly acceptable, but rarely used. “Good” posture is important, but even the best seated posture should not be solely utilized.