David Brown's work in health and safety is used throughout Australia and New Zealand. If you want to know more, here is his formal resume and here is an informal one.
Free articles on stress, ergonomics and psychology
How a wide backrest changes the way people key
Why some "ergonomic" chairs are flawed

The first sequence of images is taken from our "Fresh Muscles" video.

(Above) The chair shown has a backrest which is about 450 mm wide. The typist, who is of average shoulder width is working on the "home row" of keys. She is sitting forwards a little to allow enough elbow clearance. Note that the elbow is slightly behind the vertical line of the shoulder joint; this is correct, in fact it should be a little further back but the backrest doesn't allow it. 

Typist moves hands to use the bottom row, but elbow hits backrest

Typist sticks elbows out so that her forearms can move freely. 
Note that she has leaned back spontaneously once she changed her elbow position. As long as she tried to key with good technique, the wide backrest prevented her from leaning back. The result is that she has to choose between two problems - she doesn't have any option of good all-round work technique.

This is a CoDesign chair (1985) with a backrest width of 300 mm. That might sound narrow, but it is still wider than the standard typist chair which was used for decades, and which I find very comfortable! You can see from the photo that this woman's waist is considerably narrower than the chair (300 mm). The backrest is not the ideal shape - it interferes a little with the shoulderblades - but it is far better than the chair shown earlier. 

 

Typist on manual typewriter with free elbow movement, backrest width 300 mm. This woman is above average in shoulder and waist width, but the 300 mm backrest suits her very well. 

(Below) This woman is also above-average width. The chair backrest width is unknown, but looks as if it is between 320 and 350 mm. Her elbows clearly don't hit the backrest. 

Conclusion? Backrest width is clearly important, particularly for women with narrow shoulders, but has been misunderstood by some international Standards Committees. It would seem that they have not spent enough time watching people work. 

Here is a representative range of standards and guidelines about backrest width.