displacement curves were generated for Figure I above. Operator Force Applied was the result of averaging the forces actually applied by all subjects for each keyboard. Items 6 and 7 in Figure 2 show the relative and absolute differences, respectively, between the subjects' force applied during the test (Item 5) and the ' force just necessary to move the key to its bottoming point on its force-displacement curve. These are measures of the relative and absolute Overforce applied by keyboard users.
Generally, the overforce applied is two to three times that simply required to depress the key to the bottom. Other studies have found that the overforce ratio is 3-4 times the peak force. But the main finding of the study is that keyboarders apply substantially more key force to keyboards without tactile feedback than to those that do have feedback.
The added force applied to the no-feedback keyboard, a little over 100 grains force per keystroke, would be significant over a day's keyboarding. At a 60 word/minute repetition rate (105,000 keystrokes over a 6-hour day) this extra force would accumulate to an additional ten million grams (or 10,000 Kg) of force applied per day by the keyboarder through their hands and fingers.
The three keyboards tested are representative of best of each style of keyboard, based on the author's detailed testing of around 50 keyboards. But since they differ somewhat in their keyset design, it is necessary to do both a relative and an absolute analysis. And, in this regard, they are similar in their proportion of overforce but markedly different in the absolute amount of overforce. This might, however, be an effect of the rotation between keyboards, so that the keying behavior based on the feel of each keyboard may have carried over to the other keyboards. Were each keyboard used exclusively for a week or so, the keying forces unique to each keyboard may represent greater differences. In any event additional testing is required to explore this area.
ANSI/HFS 100-1988, American National Standard for Human Factors Engineering of Visual Display Terminal Workstations, Human Factors Society, 1988.
Bammer, G. & Blighault, I. "A Review of Research on Repetitive Strain Injuries", in Buckle, P. (Ed); Musculoskeletal Disorders at Work, Proceedings of a Conference at the University of Surry, Guildford, April 13-15, 1987.