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Objective: The objective was to evaluate the effects of vertical key spacing on a conventional computer keyboard on typing speed, percentage error, usability, forearm muscle activity, and wrist posture for both females with small fingers and males with large fingers. Background: Part 1 evaluated primarily horizontal key spacing and found that for male typists with large fingers, productivity and usability were similar for spacings of 17, 18, and 19 mm but were reduced for spacings of 16 mm. Few other key spacing studies are available, and the international standards that specify the spacing between keys on a keyboard have been mainly guided by design convention. Method: Experienced female typists ( n = 26) with small fingers (middle finger length ≤ 7.71 cm or finger breadth of ≤ 1.93 cm) and male typists ( n = 26) with large fingers (middle finger length ≥ 8.37 cm or finger breadth of ≥ 2.24 cm) typed on five keyboards that differed primarily in vertical key spacing (17 × 18, 17 × 17, 17 × 16, 17 × 15.5, and 18 × 16 mm) while typing speed, error, fatigue, preference, forearm muscle activity, and wrist posture were recorded. Results Productivity and usability ratings were significantly worse for the keyboard with 15.5 mm vertical spacing compared to the other keyboards for both groups. There were few significant differences on usability ratings between the other keyboards. Reducing vertical key spacing, from 18 to 17 to 16 mm, had no significant effect on productivity or usability. Conclusions: The findings support the design of keyboards with vertical key spacings of 16, 17, or 18 mm. Applications: These findings may influence keyboard design and standards.

International standards that specify the spacing between keys on a keyboard have been guided primarily by design convention. Experienced typists (N=37) with large hands typed on five keyboards with different horizontal and vertical key spacing (19x19, 18x19, 17x19, 16x19, and 17x17mm) while productivity, comfort ratings, left and right extensor carpi ulnaris (ECU) and flexor carpi ulnaris (FCU) muscle forces, and right and left wrist extension and ulnar deviation were recorded. Productivity and usability ratings were significantly worse for the 16x19 keyboard. There was a trend for muscle activity to increase in the left forearm and decrease in the right forearm with decreasing horizontal key spacing. There was also a trend for left wrist extension to increase and left ulnar deviation to decrease with decreasing horizontal key spacing. The study findings support key spacing on a keyboard between 17 and 19mm. These findings may influence keyboard standards and design of keyboards.

In comparison with people in other occupations, dentists and dental hygienists are at increased risk of developing work-related musculoskeletal disorders, including carpal tunnel syndrome. An important risk factor in dental practice is forceful pinching, which occurs during dental scaling. Ergonomically designed dental instruments may help reduce the prevalence of MSDs among dental practitioners.In the authors' study, 24 dentists and dental hygienists used 10 custom-designed dental scaling instruments with different handle diameters and weights to perform a simulated scaling task. The authors recorded the muscle activity of two extensors and two flexors in the forearm with electromyography, while thumb pinch force was measured by pressure sensors.Handle designs of periodontal instruments had significant (P.05) effects on hand muscle load and pinch force during a manual scaling task. The instrument with a large diameter (10 millimeters) and a light weight (15 grams) required the least amount of muscle load and pinch force. There was a limit to the effect of handle diameter, with diameters larger than 10 mm having no additional benefit; however, the study did not identify a limit to the effect of reducing the weight of the instrument, and therefore instruments lighter than 15 g may require even less pinch force.The results from this study can guide dentists and dental hygienists in selection of dental scaling instruments.

Work-related upper extremity musculoskeletal disorders, including carpal tunnel syndrome, are prevalent among dentists and dental hygienists. An important risk factor for developing these disorders is forceful pinching which occurs during periodontal work such as dental scaling. Ergonomically designed dental scaling instruments may help reduce the prevalence of carpal tunnel syndrome among dental practitioners. In this study, eight custom-designed dental scaling instruments with different handle shapes were used by 24 dentists and dental hygienists to perform a simulated tooth scaling task. The muscle activity of two extensors and two flexors in the forearm was recorded with electromyography while thumb pinch force was measured by pressure sensors. The results demonstrated that the instrument handle with a tapered, round shape and a 10 mm diameter required the least muscle load and pinch force when performing simulated periodontal work. The results from this study can guide dentists and dental hygienists in selection of dental scaling instruments.