Feel free to copy by cutting and pasting.

90 - 90 - 90 and All That

by Dan MacLeod, CPE

There's an old rule of thumb that suggests that the correct sitting posture is composed of right angles at the knees, hips, and elbows — the so-called 90 - 90 - 90 rule.  This posture was conceived in the 1880s by a Prussian orthopedic surgeon named Staffel.¹  Apparently, there was no scientific basis for the posture, rather it was rooted in the militaristic culture of the time, with its stiff, erect bearings.

Modern studies suggest something different:

1. When sitting, the posture that creates the least pressure on the discs in the lower back is when the angle at the hips and knees is about 120°.  Note that this is the general posture when sitting in a car, in contrast to that of the traditional office chair.
2. There is no one correct posture for an eight-hour day — the body needs to move and change.

The following graphs and comments summarize the basis for these statements.

Pressure on Lower Back in Various Postures ²
(Numbers indicate percent of standing posture)

Lying	Standing	Bending	Sitting	Lifting

Note the following:

• The least stress on the spinal column (of these five positions) is when lying down.
• Bending over slightly increases the load on the lower back by 40%.  The study confirms what we would expect and provides some quantification.
• Sitting creates a load on the lower back that is 50% greater than standing. The reason is that sitting tends to flatten the natural S-curve of the back.
• Lifting a load creates high force on the spine.  This is a trivial finding of the study, but it helps confirm that the overall picture is sound, that they were measuring what they were trying to measure.

Pressure on Lower Back in Various Sitting Postures ³
(Numbers indicate percent of standing posture)

Relaxed	Writing (arms supported)	Arms hanging	Typing (arms unsupported)

Note the following:

• Leaning back on a chair backrest reduces pressure on the lower back.
• Arm rests that adequately support the weight of the arm reduce lower back stress (not merely shoulder stress).
• The classic typing posture creates the most pressure on the discs in the lower back (of these four sitting positions).

Benefits of Reclining and Lumbar Support ⁴
(Numbers indicate percent of upright sitting posture)

90o upright (no lumbar support)	Upright & lumbar support	Recline (no lumbar support)	Recline & lumbar support

Note the following:

• Good lumbar support on chair backrests reduces disc pressure.
• Reclining the backrest reduces the pressure even more.
• Reclining while using good lumbar support yields the least stress on the lower back (of these four postures).

Contrary to traditional admonitions, leaning back on a chair is not bad for your back. Quite the opposite, especially if there is proper lumbar support.  Note that the reduction in force on the lower back occurs because some weight of the upper torso is supported by the back rest, in addition to putting the spine in its best alignment.

X-Ray Evaluation of Various Postures ^{5
(Study subjects lying on their sides, on an x-ray table)}

180o (standing straight) Vertibrae distorted Muscles unbalanced

135o (semi-crouch) Vertibrae aligned Muscles balanced

90o (sitting erect) Vertibrae distorted Muscles unbalanced

As shown in the previous studies, the data suggest that the neutral posture is a semi-crouch position, not right angles.  If you think about it, that's the posture the astronauts assume when they are weightless.

 

Static Work Postures and MSDs ^{6
(Numbers indicate the risk of injury compared to the lowest group)}

Arm Rests             Extensive             Limited                                Rest Breaks

 

No Exposure and No Problems	Static Posture and Neck/ Shoulder Problems	Static Posture and Cervical Disorders	Bent Wrist and Hand/Arm Discomfort

This study was evaluated a group of computer users for problems related to working in the same position all the time.  Thus, the issues here are different from the strictly posture issues of the previously mentioned studies.

Note the following:

• In the upper graph, limited ability to take rest breaks or to use arm rests was associated with a ten-fold increase in problems.
• Static postures affect various parts of the body.

There is no single posture that is correct for an eight-hour day.  You must move and change positions.

The implication is that there is no one single posture that is correct for a sustained period of time — to stay healthy you need to shift positions and change postures, as well as stretch and have proper furniture and equipment to keep muscles from being constantly tense.  (This is only one sample study; there are many more on this topic of static loading of muscles and musculoskeletal injuries.)

Conclusion

Science does not support the recommendation of sitting at right angles.  Rather, a semi-crouched position is better.  Furthermore, instead of trying to find the perfect posture for yourself, you should shift and change.

Personally, I would not conclude that the 90 - 90 - 90 is wrong.  There is still some value in this rule of thumb — it is an easy rule to remember and it can help you to avoid some extreme postures.  The way to think about it is that the rule can help you get in a starting position from which you can vary. 

You can optimize your position if you tilt the chair seat or by angling the back rest so that you maintain a semi-crouch position.  Clearly there is nothing wrong with leaning back, as long as you are not slouching, that is, rounding out the back and having the curve the wrong way.

Technically, it is even better for your back to work almost lying down in a recliner (see the very first graph again) as long as you have good lumbar support and keep the correct curves.

In all events, the primary goal is to keep from having the furniture force you to sit in any one awkward posture.

References:

¹Mandal, A.C.,  1982. The seated man: theories and realities.  Proceedings of the Humans Factors Society, 520-524.

²Nachemson, A. and Morris, J.M., 1964. In vivo measurements of intradiscal pressure. J. Bone Joint. Surg. 46A: 1077.

^3,4Andersson, G.B.J. and Ortengren, R., 1974. Myoelectric back muscle activity during sitting, Scand. J. Rehab. Med. 3, 104-114, & Suppl. 3, 73-133.

⁵Keegan, J.J., 1953. Alterations of the lumbar curve related to posture and seating. J. Bone Joint Surg. 35A(3) 592- 601.

⁶Bergkvist, U., Wolgast, E., Nilsson, B., and Voss, M., 1995. Musculoskeletal disorders among visual display terminal workers.

There are more summaries of key studies like this in my published books.