This study investigated how differently ultra-low key travel (< 2.0 mm) keyboards affect typing force, muscle activity, and typing productivity as compared to a ...
Proceedings of the Human Factors and Ergonomics Society 2017 Annual Meeting
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EFFECTS OF KEY TRAVEL DISTANCES ON BIOMECHANICAL EXPOSURES AND TYPING PERFORMANCE DURING ULTRALOW KEY TRAVEL KEYBOARDS Jonathan Sisley1, Kiana Kia2, Peter W Johnson3, and Jeong Ho Kim1* 1
School of Biological and Population Health Sciences , Oregon State University, Corvallis, OR School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR 3 Department of Environmental and Occupational Health Sciences, University of Washington, Seattle, WA
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This study investigated how differently ultra-low key travel (< 2.0 mm) keyboards affect typing force, muscle activity, and typing productivity as compared to a conventional keyboard. In a repeated-measures laboratory-based study with 20 subjects, we collected and compared typing forces, muscle activity in extrinsic finger muscles (flexor digitorum superficialis (FDS) and extensor digitorum communis(EDC)), and typing performance among five keyboards with different key travel distances (0.5, 0.7, 1.2, 1.6, and 2.0 mm). The results showed that there were differences between ultra-low key travel keyboards (0.5, 0.7, 1.2 and 1.6 mm) and a conventional keyboard (2.0 mm) in typing force (p < 0.001), muscle activity (p > 0.07) expect for FDS (p < 0.01), and typing speed (p < 0.001). However, in general, the differences appears to be practically small: muscle activity (less than 1.3%) and typing force (less than 0.5 newton). The study findings indicates that the ultra-low key travel keyboards may not increase or decrease physical risk factors and typing performances as compared to conventional keyboards.
Copyright 2017 by Human Factors and Ergonomics Society. DOI 10.1177/1541931213601727
INTRODUCTION Previous studies have shown that keyboard characteristics including key activation force, travel distance, force-displacement characteristics, and tactile feedback can affect physical risk factors during computer use (Rempel et al., 1999; Lee et al., 2009; Kim et al., 2014). These studies showed that keyboard key force-displacement characteristics may play a fundamental role in developing upper extremity MSDs. With notebook computers and detachable desktop keyboards gravitating towards thinner designs, key travel distances have significantly decreased. Ultra-low key travel keyboards which key travel is less than 2.0 millimeters (mm) differ from the conventional keyboards which key travel is longer than 2.0 mm since the adaptation for a thinner laptop has created shorter key travel distances and different tactile feedback when keys are activated. Previous studies have shown that changes in key travel distance can alter force-displacement characteristics that affect biomechanical risk factors and usability (Radwin & Ruffalo, 1999; Lee et al., 2009; Kim et al., 2014). These studies showed that the longer key travel was associated with decreased typing force
(Radwin & Ruffalo, 1999; Lee et al., 2009). A few recent studies with 2.0-mm keyboards showed that key travel was associated with decreased typing force (Kim et al., 2014). Increased typing forces due to inadequate key travel distance and tactile feedback may increase muscle activity levels (Martin, et al., 1996; Rempel, et al., 1997) and users’ discomfort (Rempel, et al., 1999). While most of these previous studies on the key travel distance were mainly on traditional keyboards with longer key travel distance (≥ 2.0 mm), there has been little research to investigate the effects of such ultra-low key travel distances (< 2.0 mm) on the biomechanical exposure measures and usability. Given the recent prevalence of ultra-low key travel keyboards and their differences in keyboards characteristics including key travel distance and tactile feedback, it is imperative to investigate the effects of ultra-low key travel keyboard on biomechanical exposures and usability. Therefore, the goal of this study was to determine whether there were differences between ultra-low key travel (< 2.0 mm) keyboard and conventional keyboard (2.0 mm) in typing force, muscle activity in the extrinsic finger muscles and shoulder muscles, and typing performance.
Proceedings of the Human Factors and Ergonomics Society 2017 Annual Meeting
METHODS Subjects A total of 20 subjects (10 male and 10 female) were recruited to participate in the study via e-mail solicitations and printed flyers. All subjects were considered touch typists who could type faster than 40 WPM and had no history of upper extremity musculoskeletal diseases. Eighteen subjects were righthanded and two subjects were right handed. The average age (SD) was 29.5 (7.5) ranging from 19 to 47 years. The experimental protocol was approved by the University’s Institutional Review Board.
Experimental design Prior to their participation in the study, experimentation process, all subjects gave their written consent prior to their participation in the study the heights of chair, desk, and monitor were adjusted based on each subject’s anthropomorphic measures in accordance with ANSI/HFES 100-2007. Briefly, the chair height was adjusted so that the subject’s thighs were parallel to the ground and feet rested firmly on the floor. Then, the height of the desk was set to be 2 cm below the elbow height in order to allow for the subject to comfortably rest their arm by forming a right angle at elbow. The monitor height was adjusted so that the top of the monitor is at eye level. The keyboards were positioned such that the spacebar centered on the subject’s body and 10 cm from the front edge of the desk surface. Lastly, subjects were then allowed to type on a non-testing keyboard to become familiar with a typing program (Mavis Beacon Teaches Typing Platinum - 25th Anniversary Edition, Broderbund Software Inc., USA). During the repeated-measures laboratory experiments, subjects typed for 10 minutes on each of five keyboards that has relatively similar key activation force (0.5 – 0.6 Newtons): - a notebook keyboard with 0.5 mm of key travel (MacBook; Apple; Cupertino, CA); - a notebook keyboard with 0.7 mm of key travel (Thin Touch; Synaptics; San Jose, CA); - a detachable keyboard with 1.2 mm of key travel (Magic Keyboard; Apple; Cupertino, CA); - a detachable keyboard with 1.6 mm of key travel (Surface Typecover, Microsoft, Redmond, WA) - a detachable keyboard with 2.0 mm of key travel (A1234; Apple; Cupertino, CA).
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The order in which the keyboards were used was randomized and counterbalanced to minimize any potential confounding due to keyboard testing order. Between typing sessions, a 5-minute break was given to minimize residual fatigue effects of the previous condition. During the typing session, typing speed (words per minute) and accuracy (% key correctly typed) were measured by the typing program. Simultaneously, we also measured typing forces validated and used in previous studies (Kim et al., 2014). The keyboards were placed on the force platform such that the “H” key of each keyboard aligned with the center of the force platform. Only downward forces (z-axis) associated with alphabetical keys (A-Z) were included for data analyses. A custom-built LabVIEW program (V2016; National Instruments; Austin, TX) was used to record force applied to the keyboard. Muscle activity from the right extensor digitorum communis (EDC), flexor digitorum superficialis (FDS), and the right trapezius (TRAP) was measured at a sample rate of 1000 Hz using Ag/AgCl surface electrodes (Blue Sensor N-00-S; Ambu; Ballerup, Denmark) and a data logger (Mega ME6000; Mega Electronics; Kupio, Finland) (Figure 1).
Figure 1. Experimental setup. The skin preparation, muscle identification and electrode placement were done per the European Recommendation for Surface Electromyography (seniam.org). Briefly, prior to applying EMG electrodes to the skin, the electrode contact area was prepared by
Proceedings of the Human Factors and Ergonomics Society 2017 Annual Meeting
shaving hair with a razor and cleaning skin surface with Alcohol Prep Pads to reduce skin impedance. Then, disposable Ag/AgCl surface electrodes with an 8-mm diameter pick up area (Blue Sensor N-00-S; Ambu; Ballerup, Denmark) were placed with a 20-mm interelectrode spacing over the muscles. The raw EMG data were collected during the entire duration of typing sessions. At the end of the experiment, three Maximum Voluntary Contractions (MVCs) were recorded, each of which lasted for three seconds in duration with a 2minute rest between each MVC. The raw EMG data were processed with a band pass filter of 10-350 Hz. The processed EMG data was normalized as a percentage of the Maximum Voluntary Contraction (%MVC) for each muscle. Then, the 10th (static), 50th (median) and 90th percentile (peak) amplitude probability density function (APDF) of muscle activity will be calculated for subsequent statistical analyses.
Data analysis A mixed model with restricted maximum likelihood estimation (REML) in JMP (Version 12.0.1; SAS Institute Inc., Cary, SC, USA) was used to determine the differences between keyboards in typing speed and accuracy as the data followed a normal distribution. Due to non-normality, Friedman test in R (R 3.3.2, Development Core Team) was used to determine the difference between keyboards in muscle activity and typing force. Any statistically significance when Type 1 error was less than 0.05 was followed up with post-hoc multiple comparison.
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Table 1. Mean (SE) of typing speed (WPM) and accuracy (%). Columns with different superscripts indicate significant difference in typing performance between keyboards at α=0.05. [n =20] Keyboard
Typing productivity The results showed that the typing speed were significantly different between keyboards (p’s < 0.001) as shown in Table 1. The 1.2 mm keyboard had the highest speed among the keyboards. The typing speed tends to be slower with less-than-1.0-mm keyboard as compared to the other longer key travel keyboards expect for 1.6 mm keyboard. However, there were no differences in typing accuracy between keyboards (p = 0.29).
Accuracy (%)
0.5 mm
61.7 (3.1)
a
94.7 (4.8) a
0.7 mm
60.3 (2.8) a
93.9 (0.6) a
1.2 mm
67.4 (2.9) b
95.4 (0.5) a
1.6 mm
60.6 (2.8) a
94.9 (0.4) a
2.0 mm
64.3 (2.8) ab
95.9 (0.4) a
p-value
< 0.001
0.29
Muscle activity (Electromyography) The electromyography data showed that there were no differences between the keyboards in muscle activity in extensor digitorum communis (EDC) and trapezius (TRAP) muscle whereas muscle activity in flexor digitorum superficialis (FDS) differed among the keyboards tested in this study (Table 2). The 50th and 90th percentile values of the FDS muscle activity showed that 1.6 mm keyboard tended to have slightly lower muscle activity as compared to other keyboards (p’s < 0.01). Table 2. Mean (SE) of muscle activity (%MVC) compared across the five keyboards. Rows with different superscripts indicate significant difference in typing performance between keyboards at α=0.05. [n=20]. APDF 10th
RESULTS
Typing speed (WPM)
EDC
FDS
Key Travel 0.5 mm
0.7 mm
1.2 mm
1.6 mm
2.0 mm
8.0 (0.5)
8.3 (0.5)
8.3 (0.5)
7.9 (0.5)
P-value
8.2 (0.5)
0.07
50th 13.3 (1.1) 13.3 (0.9) 13.1 (0.9) 12.9 (1.0) 13.5 (0.9)
0.26
90th 21.8 (1.7) 21.6 (1.4) 21.3 (1.4) 21.0 (1.6) 22.2 (1.4)
0.16
10th
2.7 (0.4)
0.12
50th
7.9 (1.3)a 7.8 (1.3)a 7.5 (1.3)ab 7.0 (1.3)b 7.5 (1.4)ab
