اثرات ماوس ها از نقطه نظر ارگونومیک بر عملکرد کار و واکنش های ذهنی

The biomechanical benefits (e.g., muscular activity) of slanted ergonomic mice have been comprehensively identified; however, their effects on task performance and subjective responses have not been fully investigated. The present study examined the effects of two slanted mice (slant angle = 30° and 50°) in comparison with a conventional mouse (slant angle = 0°) in terms of task performance (task completion time and error rate) and subjective responses (perceived discomfort score and overall satisfaction score). Experimental results showed that all of the task and subjective measures worsened as the slant angle of the target mice increases. For example, the task completion time (unit: ms) and overall satisfaction score (unit: point) of the 30° slanted mouse (time = 0.71, satisfaction = −0.09) and 50° slanted mouse (time = 0.73, satisfaction = −0.79) significantly deteriorated than the conventional mouse (time = 0.65, satisfaction = 1.21). The slanted mice seem to compromise biomechanical benefits with task performance and subjective responses.

The computer mouse is commonly used with graphic user interfaces. Using a computer mouse comprises one- to two-thirds of total computer usage time (Cook et al., 2000 and Lee et al., 2008). In addition, the most frequently used input device among computer users is the computer mouse (Cook and Kothiyal, 1998; Jensen et al., 2002 and Muller et al., 2010). The conventional computer mouse has been identified as a risk factor for upper extremity musculoskeletal disorders (WMSDs) and localized pain. The conventional mouse requires a user to pronate the forearm and to extend the wrist during operation (Gustafsson and Hagberg, 2003). The pronation of the forearm may result in the development of WMSDs (Zipp et al., 1983; Hagberg, 1997; Liao and Drury, 2000). The extension of the wrist increases carpal tunnel pressure (CTP), which would be a potential risk factor for carpal tunnel syndrome (CTS) (Keir et al., 1999, Fogleman and Brogmus, 1995, Bower et al., 2006 and Mogk and Keir, 2007). In addition, the conventional mouse may lead to micro lesions in the low-threshold motor units because they have been continuously activated while using a computer mouse (called Cinderella Hypothesis; Crenshaw et al., 2007). Therefore, prolonged awkward posture and monotone movements can induce localized pain and discomfort on upper extremities (Muller et al., 2010, Cook and Kothiyal, 1998 and Hedge et al., 2010). Slanted ergonomic mice have been introduced to reduce the negative effects of the conventional mouse in terms of arm posture, muscular activity, and CTP. The key feature of the ergonomic mice is the slanted angle of the top surface from the left side to the right side. The slant surface, contacted with the palmar side of the hand, can significantly reduce forearm pronation and wrist extension (Muller et al., 2010, Chen and Leung, 2007 and Hedge et al., 2010) as well as reduce demands on muscle recruitments in the upper extremities and CTP at the wrist (Gustafsson and Hagberg, 2003). The slant surface of an ergonomic mouse can restrict performance during mouse usage tasks and can affect the level of subjective preference. Gustafsson and Hagberg (2003) reported that use of a vertical mouse (slant angle = 90°) decreased productivity by 24% in comparison with a conventional mouse. Furthermore, their subjective preference results showed that most of the participants preferred the conventional mouse more than the vertical mouse. Similarly, Scarlett et al. (2005) has revealed that use of a vertical mouse showed worse completion time and error rate than a conventional mouse by 10% and 20%, respectively. Although the slant angle of a computer mouse seems to negatively affect the task performance and subjective preference, its effects haven't been comprehensively studied yet. Chen and Leung (2007) studied the relationship between slant angle and upper extremity muscle use, and suggested that the optimal slant angle is between 20° and 30°. However, they did not consider performance or subjective measures in the determination of optimal slant angle, although the results on physiological measures (e.g., EMG) may be different from performance and subjective measures (Niesen and Levy, 1994 and Gustafsson and Hagberg, 2003). Therefore, to understand the benefits of the slanted ergonomic mouse in comparison with the conventional mouse, the effects of slant angle on task performance and subjective responses should be examined. The present study investigated the effects of two slanted ergonomic mice (slant angle: 30° and 50°) on task performance and subjective responses in comparison with a conventional mouse (slant angle: 0°). To compare the performance and subjective responses among the target mice, an experiment consisting of two mouse-intensive tasks (pointing and dragging) was conducted with 40 participants.

The actual differences among the task performances of the three mice were small, although they were statistically significant. The maximum difference in Time among the three mice was only 0.07 s (% difference = 11). The max difference of the Error was only 0.37 per 15 trials. This small performance decrement for the new type of mice was already reported in previous studies. Gustafsson and Hagberg (2003) found that a vertical mouse (slant angle = 90°) decreased a ∼24% productivity in comparison with a conventional mouse during a document editing task. Muller et al. (2010) showed that a pen-type mouse had about 14% worse performance than a conventional mouse in pointing and dragging tasks. The participants didn't prefer to the slanted mice in comparison with the CM. The Satisfactions on the two slanted mice (SM30 = −0.09; SM50 = −0.77) were all negative, indicating slight dissatisfaction. Contrary, the Satisfaction of the CM (1.21) was a positive value, which indicates slight satisfaction. This trend in user satisfaction for the new type of mice corresponded with previous studies' results for a vertical mouse (Gustafsson and Hagberg, 2003) and a pen-type mouse (Muller et al., 2010). The in-depth debriefing of the present study provided a cogent reason for the decrease in Satisfaction with the slanted mice. Most of the participants emphasized that the slanted mice required excessive use of wrist deviation during operation in comparison with the CM. In addition, some of the participants claimed that wrist deviation motion in the neutral forearm position (0°) was harder than that in the pronated forearm position (90°). This subjective opinion may result from the reduction of the range of motion (ROM) at the wrist joint while forearm pronation (Chaffin et al., 1999). The results of the present study were possibly contaminated by confounding design specification of the target mice. Since the present study used the three mice available in the market, the design specifications such as overall weight and size, which were not focused on this study, could not be perfectly controlled across the target mice. For example, the slanted mice were slightly heavier (SM30 = 120 g, SM50 = 130 g) and bigger (SM30 = 7.3 × 10.6 × 5.2, SM50 = 8.2 × 10.0 × 8.0; unit: cm) than the CM (80 g, 5.5 × 11.0 × 3.5). These differences might affect the experimental results. Although the slanted mice were slightly heavier and bigger than the CM, they still have a biomechanical benefit in terms of muscular activity. As a follow-up study, the effect of the slanted mice on the muscular activity of extensor carpi radialis (ECR), which relates to wrist extension and deviation motions (Chen and Leung, 2007; Agarabi et al., 2004), had been investigated for a single participant. A root mean square (RMS) analysis for that participant showed that mean RMS values of the SM30 (24.1 mV) and SM50 (28.1 mV) were lower than that of the CM (36.6 mV) during the PT and DT. This indicated that the slanted mice required less muscular activities in comparison with the CM, although they were slightly heavier and bigger. The similar tendency was already found in Chen and Leung (2007) who investigates EMG signals on different slanted mice (slant angle = 0°, 10°, 20°, 25°, 30°) with different weights (range = 80.7 g–100.1 g). Long-term use of the slanted mice may improve task performance due to learning effect (Kotani and Horii, 2003), but the slanted mice still have a weakness in comparison with the CM. As a follow-up experiment, the present study conducted the same experiment with a participant who had been using one of the slanted mice (SM50) for three months. The participant began to use the slanted mouse due to neck and shoulder pain caused by extensive use of a conventional mouse. She was totally satisfied with the slanted mouse and believed that it significantly relieved her pain as reported in Aaras et al. (2001). However, the experimental results of that participant showed that the task performance of the slanted mice (mean: 0.83) was still worse than that of the CM (mean: 0.56). This result agreed with what was found by Straker et al. (2000).