The effect of keyboard keyswitch make force on applied force and finger flexor muscle activity

Finger Posture and Finger Load are Perceived Independently

The ability to track the time-varying postures of our hands and the forces they exert plays a key role in our ability to dexterously interact with objects. However, how precisely and accurately we sense hand kinematics and kinetics has not been completely characterized. Furthermore, the dominant source of information about hand postures stems from muscle spindles, whose responses can also signal isometric force and are modulated by fusimotor input. As such, one might expect that changing the state of the muscles – for example, by applying a load – would influence perceived finger posture. To address these questions, we measure the acuity of human hand proprioception, investigate the interplay between kinematic and kinetic signals, and determine the extent to which actively and passively achieved postures are perceived differently. We find that angle and torque perception are highly precise; that loads imposed on the finger do not affect perceived joint angle; that joint angle does not affect perceived load; and that hand postures are perceived similarly whether they are achieved actively or passively. The independence of finger posture and load perception contrasts with their interdependence in the upper arm, likely reflecting the special functional importance of the hand.

Going Short: The Effects of Short-Travel Key Switches on Typing Performance, Typing Force, Forearm Muscle Activity, and User Experience

This study examined the effects of 4 micro-travel keyboards on forearm muscle activity, typing force, typing performance, and self-reported discomfort and difficulty. A total of 20 participants completed typing tasks on 4 commercially available devices with different key switch characteristics (dome, scissors, and butterfly) and key travels (0.55, 1.3, and 1.6 mm). The device with short-travel (0.55 mm) and a dome-type key switch mechanism was associated with higher muscle activities (6%-8%, P < .01), higher typing force (12%, P < .01), slower typing speeds (8%, P < .01), and twice as much discomfort (P < .05), compared with the other 3 devices: short-travel (0.55 mm) and butterfly switch design and long travel (1.3 and 1.6 mm) with scissor key switches. Participants rated the devices with larger travels (1.3 and 1.6 mm) with least discomfort (P = .02) and difficulty (P < .01). When stratified by sex/gender, these observed associations were larger and more significant in the female participants compared with male participants. The devices with similar travel but different key switch designs had difference in outcomes and devices with different travel were sometimes not different. The results suggest that key travel alone does not predict typing force or muscle activity.

Differences in typing forces, muscle activity, wrist posture, typing performance, and self-reported comfort among conventional and ultra-low travel keyboards

This study investigated the relative impact of ultra-low travel keyboards on typing force, muscle activity, wrist posture, typing performance, and self-reported comfort/preference as compared to a conventional keyboard. In a repeated-measures laboratory-based study, 20 subjects were invited to type for 10 min on each of five keyboards with different travel distances of 0.5, 0.7, 1.2, 1.6 (ultra-low travel keyboards), and 2.0 mm (a conventional keyboard). During the typing sessions, we measured typing force; muscle activity in extrinsic finger muscles (flexor digitorum superficialis and extensor digitorum communis), shoulder (trapezius) and neck (splenius capitis); wrist posture; typing performance; and self-reported comfort/preference. While using the ultra-low travel keyboards, subjects typed with less force and wrist extension, and had more ulnar deviation (p's < 0.0001) compared with conventional keyboard. However, these differences in typing forces were less than 0.5 N and less than 4° for both wrist extension and ulnar deviation. The general trend of data did not show any consistent or substantial differences in muscle activity (less than 2 %MVC) and typing performance (<5 WPM in speed; < 3% in accuracy), despite the observed statistical difference in the finger flexors and extensors muscle activity (p's < 0.19) and typing performance (p < 0.0001). However, the subjects preferred using conventional keyboards in most of the investigated self-reported comfort and preference criteria (p's < 0.4). In conclusion, these small differences indicate that using ultra-low travel keyboards may not have substantial differences in biomechanical exposures and typing performance compared to conventional keyboard; however, the subjective responses indicated that the ultra-low keyboards with the shortest key travel tended to be the least preferred.

Comparison of Surface to Indwelling Extrinsic Finger Muscle EMG during use of Computer Pointing Devices

Differences between indwelling and surface electromyography (EMG) were quantified by simultaneously recording extrinsic muscle activity of the index finger (flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), extensor indicis proprius (EIP), and extensor digitorum communis (EDC)) in a laboratory-based study. Eleven subjects performed computer-related pointing and dragging tasks using a mouse, pen and tablet, touchpad, and trackball. Surface electrodes poorly estimated FDP activity as demonstrated by low and variable cross-correlation values (0.47–0.64) and variable, high estimated error (RMS difference=11–46%MVC). Surface electrodes estimated FDS activity well (cross-correlation=0.79; RMS difference=11%MVC) when hand and forearm posture changes were minimized. Differences in forearm supination and pronation reduced the quality of FDS muscle activity estimation. Surface electrodes estimated EIP and EDC activity well with high cross-correlation values (0.77–0.84) and low RMS differences (8–12%MVC). Surface EMG can be used to assess EIP and EDC activity, and FDS activity when posture changes are minimized, during pointing device tasks. Surface EMG does not accurately assess FDP activity.

Wearable Keyboard Using Conducting Polymer Electrodes on Textiles

A wearable keyboard is demonstrated in which conducting polymer electrodes on a knitted textile sense tactile input as changes in capacitance. The use of a knitted textile as a substrate endows stretchability and compatibility to large-area formats, paving the way for a new type of wearable human-machine interface.

Mobile input device type, texting style and screen size influence upper extremity and trapezius muscle activity, and cervical posture while texting

This study aimed to determine the effects of input device type, texting style, and screen size on upper extremity and trapezius muscle activity and cervical posture during a short texting task in college students. Users of a physical keypad produced greater thumb, finger flexor, and wrist extensor muscle activity than when texting with a touch screen device of similar dimensions. Texting on either device produced greater wrist extensor muscle activity when texting with 1 hand/thumb compared with both hands/thumbs. As touch screen size increased, more participants held the device on their lap, and chose to use both thumbs less. There was also a trend for greater finger flexor, wrist extensor, and trapezius muscle activity as touch screen size increased, and for greater cervical flexion, although mean differences for cervical flexion were small. Future research can help inform whether the ergonomic stressors observed during texting are associated with musculoskeletal disorder risk.

Fatigue development in the finger flexor muscle differs between keyboard and mouse use

PURPOSE

The aim of the present study was to determine whether there were any physiological changes in the muscle as a result of intensive computer use.

METHODS

Using a repeated measures experimental design, eighteen subjects participated in four different 8-h conditions: a control (no exposure) condition and three exposure conditions comprised of 6 h of computer use (keyboard, mouse, and combined keyboard and mouse use) followed by 2 h of recovery. In each condition, using 2 Hz neuromuscular electrical stimulation, eight temporal measurements were collected to evaluate the fatigue state (twitch force, contraction time, and ½ relaxation time) of the right middle finger Flexor Digitorum Superficialis (FDS) muscle before, during, and after computer use.

RESULTS

The results indicated that 6 h of keyboard, mouse, and combined mouse and keyboard use all caused temporal fatigue-related changes in physiological state of the FDS muscle. Keyboard use resulted in muscle potentiation, which was characterized by approximately 30% increase in twitch force (p < 0.0001) and 3% decrease (p = 0.04) in twitch durations. Mouse use resulted in a combined state of potentiation and fatigue, which was characterized by an increase in twitch forces (p = 0.002) but a prolonging (11 %) rather than a shortening of twitch durations (p < 0.0001).

CONCLUSIONS

When comparing mouse and keyboard use, the more substantial change in the physiological state of the muscle with mouse use (potentiation and fatigue compared to just potentiation with keyboard use) provides some physiological evidence which may explain why mouse use has a greater association with computer-related injuries.

The effect of key size of touch screen virtual keyboards on productivity, usability, and typing biomechanics

OBJECTIVE

We investigated whether different virtual keyboard key sizes affected typing force exposures, muscle activity, wrist posture, comfort, and typing productivity.

BACKGROUND

Virtual keyboard use is increasing and the physical exposures associated with virtual keyboard key sizes are not well documented.

METHOD

Typing forces, forearm/shoulder muscle activity, wrist posture, subjective comfort, and typing productivity were measured from 21 subjects while they were typing on four different virtual keyboards with square key sizes, which were 13, 16, 19, and 22 mm on each side with 2-mm between-key spacing.

RESULTS

The results showed that virtual keyboard key size had little effect on typing force, forearm muscle activity, and ulnar/radial deviation. However, the virtual keyboard with the 13-mm keys had a 15% slower typing speed (p < .0001), slightly higher static (10th percentile) shoulder muscle activity (2% maximum voluntary contractions, p = .0 I), slightly greater wrist extension in both hands (2 degrees to 3 degrees, p <.01), and the lowest subjective comfort and preference ratings (p < .1).

CONCLUSIONS

The study findings indicate that virtual keyboards with a key size less than 16 mm may be too small for touch typing given the slower typing speed, higher static shoulder muscle activity, greater wrist extension, and lowest subjective preferences.

APPLICATIONS

We evaluated the effects of virtual keyboard key sizes on typing force exposures, muscle activity, comfort, and typing productivity.

Differences in typing forces, muscle activity, comfort, and typing performance among virtual, notebook, and desktop keyboards

The present study investigated whether there were physical exposure and typing productivity differences between a virtual keyboard with no tactile feedback and two conventional keyboards where key travel and tactile feedback are provided by mechanical switches under the keys. The key size and layout were same across all the keyboards. Typing forces; finger and shoulder muscle activity; self-reported comfort; and typing productivity were measured from 19 subjects while typing on a virtual (0 mm key travel), notebook (1.8 mm key travel), and desktop keyboard (4 mm key travel). When typing on the virtual keyboard, subjects typed with less force (p's < 0.0001) and had lower finger flexor/extensor muscle activity (p's < 0.05). However, the lower typing forces and finger muscle activity came at the expense of a 60% reduction in typing productivity (p < 0.0001), decreased self-reported comfort (p's < 0.0001), and a trend indicating an increase in shoulder muscle activity (p's < 0.10). Therefore, for long typing sessions or when typing productivity is at a premium, conventional keyboards with tactile feedback may be more suitable interface.

Relation between perceived effort and the electromyographic signal in localized low-effort activities

Hand-based human–machine interfaces are complex tasks that involve repetitive or sustained movements and postures of the hands that can lead to overuse syndromes of the musculoskeletal system. Consequently, it is important to minimize the physical effort that occurs at these interfaces. The evaluation of physical effort can be performed either by subjective evaluation of the relative perceived effort (e.g., Borg scale) or by objective physiological measurements (e.g., electromyography – EMG). However, the relation between these two measures has not been sufficiently studied for localized low-effort activities. This study investigated the relation between EMG and Borg ratings, as well as the issue of gender differences during low-effort activity of forearm muscles. Nine females and nine males performed eight different hand gestures (localized low-effort activity), during which EMG signals were recorded from six forearm muscles and Borg ratings were obtained. On average, the female subjects rated the gestures as less effortful than the male subjects, and also demonstrated a higher positive correlation between the EMG and Borg ratings. Furthermore, the linear model that was fitted for predicting the Borg ratings based on gender and the combined activity of muscles provided an R-squared value of approximately 0.3.

Performance and touch characteristics of disabled and non-disabled participants during a reciprocal tapping task using touch screen technology

Touch screens are becoming more prevalent in everyday environments. Therefore, it is important that this technology is accessible to those with varying disabilities. The objective of the current study was to evaluate performance and touch characteristics (forces, impulses, and dwell times) of individuals with and without a movement disorder during a reciprocal tapping touch screen task. Thirty-seven participants with a motor control disability and 15 non-disabled participants participated. Outcome measures include number of correct taps, dwell time, exerted force, and impulse. Results indicate non-disabled participants had 1.8 more taps than participants with fine motor control disabilities and 2.8 times more than those with gross motor impairments (p<0.05). Additionally, people with gross motor control disabilities demonstrated longer dwell times and greater impulses (p<0.05). The average force used to activate the buttons was 6.2 N, although the button activation force was 0.98 N. Differences in reciprocal tapping and touch characteristics exist between those with and without motor control disabilities. Understanding how people (including those with disabilities) interact with touch screens may allow designers and engineers to ultimately improve usability of touch screen technology.

Are there Differences in Force Exposures and Typing Productivity between Touchscreen and Conventional Keyboard?

As the use of tablets is becoming increasingly prevalent, it is important to understand how using a touchscreen (virtual) keyboard affects typing forces, productivity and comfort. Thus, the objective of this study was to investigate whether there were differences in typing forces, typing productivity and users’ discomfort between virtual and conventional keyboards. A total of 19 subjects (10 males and 9 females) typed for 10 minutes on a virtual keyboard and two conventional keyboards. The results showed that virtual keyboard use resulted in lower typing forces (p < 0.0001), lower typing performance (p < 0.0001), and higher subjective discomfort at the hand/wrist and the neck/shoulder (p < 0.0001). The results indicate that using a virtual keyboard may not cause any detrimental effect on physical exposures, but may increase musculoskeletal discomfort on the upper extremities and neck/shoulder regions; therefore, appropriate interventions should be considered for the prolonged use of a virtual keyboard.

Effect of touch screen button size and spacing on touch characteristics of users with and without disabilities

OBJECTIVE

The aim of this study was to investigate the effect of button size and spacing on touch characteristics (forces, impulses, and dwell times) during a digit entry touch screen task. A secondary objective was to investigate the effect of disability on touch characteristics.

BACKGROUND

Touch screens are common in public settings and workplaces. Although research has examined the effect of button size and spacing on performance, the effect on touch characteristics is unknown.

METHOD

A total of 52 participants (n = 23, fine motor control disability; n = 14, gross motor control disability; n = 15, no disability) completed a digit entry task. Button sizes varied from 10 mm to 30 mm, and button spacing was 1 mm or 3 mm.

RESULTS

Touch characteristics were significantly affected by button size. The exerted peak forces increased 17% between the largest and the smallest buttons, whereas impulses decreased 28%. Compared with the fine motor and nondisabled groups, the gross motor group had greater impulses (98% and 167%, respectively) and dwell times (60% and 129%, respectively). Peak forces were similar for all groups.

CONCLUSION

Button size but not spacing influenced touch characteristics during a digit entry task. The gross motor group had significantly greater dwell times and impulses than did the fine motor and nondisabled groups.

APPLICATION

Research on touch characteristics, in conjunction with that on user performance, can be used to guide human computer interface design strategies to improve accessibility of touch screen interfaces. Further research is needed to evaluate the effect of the exerted peak forces and impulses on user performance and fatigue.

An inexpensive and accurate method of measuring the force of responses in reaction time research

Together with reaction time (RT), the force with which people respond to stimuli can provide important clues about cognitive and affective processes. We discuss some of the issues surrounding the accurate measurement and interpretation of response force, and present a response key by which response force can be measured regularly and unobtrusively in RT research. The advantage of the response key described is that it operates like a standard response key of the type used regularly in classic RT experiments. The construction of the response key is described in detail and its potential assessed by way of an experiment examining response force in a simple reaction task to visual stimuli of increasing brightness and size.

Alternative computer mouse design and testing to reduce finger extensor muscle activity during mouse use

OBJECTIVE

The purpose of this study was to design and test alternative computer mouse designs that attempted to reduce extensor muscle loading of the index and middle fingers by altering the orientation of the button switch direction and the force of the switch.

BACKGROUND

Computer users of two-button mouse designs exhibit sustained lifted finger behaviors above the buttons, which may contribute to hand and forearm musculoskeletal pain associated with intensive mouse use.

METHODS

In a repeated-measures laboratory experiment, 20 participants completed point-and-click, steering, and drag tasks with four alternative mouse designs and a reference mouse. Intramuscular and surface electromyography (EMG) measured muscle loading, and movement times recorded by software provided a measure of performance.

RESULTS

Changing the direction of the switch from a conventional downward to a forward design reduced (up to 2.5% maximum voluntary contraction [MVC]) sustained muscle activity (10th percentile EMG amplitude distribution) in the finger extensors but increased (up to 0.6% MVC) flexor EMG and increased movement times (up to 31%) compared with the reference mouse (p < .001). Implementing a high switch force design also increased flexor EMG but did not differ in movement times compared with the reference mouse (p < .001).

CONCLUSION

The alternative mouse designs with altered switch direction reduced sustained extensor muscle loading; however, trade-offs with higher flexor muscle loading and lower performance existed.

APPLICATION

Potential applications of this study include ergonomic and human computer interface design strategies in reducing the exposure to risk factors that may lead to upper extremity musculoskeletal disorders.

Effects of a vertical keyboard design on typing performance, user comfort and muscle tension

To circumvent the awkward pronated hand position inherent to conventional horizontal keyboards, a vertical, split keyboard was designed with flexible cushions supporting the wrists, allowing relaxed hand and arm postures. During eight twice-weekly 30-min training sessions, the performance and subjective comfort of nine experienced typists were tested. Typing speed and error percentage, and surface electromyographic activity of six forearm muscles and two postural muscles were recorded in separate sessions at the end of each week. Typing speed rapidly recovered to the preset rate of 300 keystrokes/min and error percentages were similar for the two keyboards. The vertical keyboard caused lower muscular activity in especially finger extensor muscles, did not increase postural muscle activity, and self-reported comfort was higher. Thus, the vertical keyboard was easily mastered, was experienced as comfortable, and caused less stress on muscles sensitive to repetitive strain injuries.

The effect of alternate style keyboards on severity of symptoms and functional status of individuals with work related upper extremity disorders

There is evidence that performing job tasks involving repetition, vibration, sustained posture or forceful movement may contribute to symptoms of work related upper extremity disorders. Typing is one such activity; symptoms that develop as a result of this activity can affect performance of work, self-care and leisure occupations. Studies investigating the impact of ergonomic keyboards on symptom reduction are limited, and little research exists regarding the reduction of key activation force as an intervention.

METHODS

This randomized, prospective study used a sample of 68 symptomatic workers employed by a single company. One group received a commercially available ergonomic keyboard, a second group used a modified version of the same keyboard designed to reduce activation force, vibration and key travel. We measured symptoms and clinical signs, functional status, and device satisfaction in both groups over a six-month study period.

CONCLUSIONS

Between-groups analyses indicated that the groups performed similarly on the outcomes of interest. Repeated-measure analysis identified a reduction of symptoms, an improvement in functional status, preference for and increased satisfaction with the intervention keyboards, and maintenance of typing speed and accuracy for both groups.

Effect of VDT keyboard height and inclination on musculoskeletal discomfort for wheelchair users

This study investigated the effect of keyboard height and inclination on musculoskeletal discomfort for wheelchair users. Eight Taiwanese male wheelchair users (28.75 +/- 8.75 years) were recruited as participants to perform nine experimental combinations of data entry tasks. Three keyboard heights and three inclinations were evaluated. Musculoskeletal discomfort was estimated by Rating of Perceived Exertion and Subjective Preference Ranking. Each subject performed a data entry task for all nine experimental combinations in a random order. The seated posture of all participants during the data entry operation was the upright posture. The height of the screen's center was adjusted according to the eye level of each subject. Analysis showed the keyboard height and keyboard inclination significantly affected rating of musculoskeletal discomfort. It is suggested that the optimum keyboard height choice is elbow-level height or 5 cm below elbow level with the keyboard inclination horizontal to the seat of the wheelchair.

Electromyographical assessment on muscular fatigue—an elaboration upon repetitive typing activity

The objectives of this study were to quantify the electromyographic activities (EMG) of finger muscles during prolonged, low-forces, and repetitive typing with an ergonomically designed VDT workstation, as well as to analyze the occurrence and the possible mechanisms of muscular fatigue in touched typists. Thirty healthy female typists were recruited to type consecutively for 2 h. The surface EMG of extensor digitorum communis (EDC) and flexor digitorum superficialis (FDS) of both hands was recorded throughout the entire test. Electrical activity (EA) and median frequency (MDF) were calculated, and then regressed against the time courses to obtain the slopes of progress. Further analysis of the EMG parameters was done by the joint analysis of spectra and amplitudes (JASA). The results indicated that maximum voluntary electrical activation (MVE) decreased after 2-h typing, and did not recover to the initial values even after a 10-min break. Besides, there was a trend of decrement in frequency throughout the entire trail, and the MDF reduced by 25% in comparison with the initial values. With the JASA plot, 74% of the muscles manifested fatigue after 2-h typing activity. Furthermore, we observed that the EDC muscles were more susceptible to muscular fatigue than the FDS muscles. In conclusion, prolonged consecutive typing may induce muscular fatigue in the healthy typists even in an ergonomic typing environment.

Development of a monitoring system for keyboard users' performance

To precisely evaluate the typing performance of a VDT user, a program named KBlog was developed to record the typing activity under the Microsoft Windows operating system. This program is small and simply framed in order to avoid overloading of the operating system. Without interfering with the typing task, this program can record the time of every pressing and releasing movement of each keystroke at the millisecond level. The accuracy and reliability of KBlog was tested by comparing time intervals recorded by KBlog to time intervals of keyboard output electrical signals recorded by an oscilloscope. In the regression analysis on these two indicators, results of high correlation coefficients of almost 1.000 and intercepts within acceptable levels of around 1 ms indicated sufficient accuracy and reliability of this program. Further applications are discussed in this paper concerning both laboratory and field researches.

Effects of keyswitch design and finger posture on finger joint kinematics and dynamics during tapping on computer keyswitches

OBJECTIVE

To examine the effects of postural and keyswitch characteristics on musculoskeletal tissue loading during tapping on computer keyswitches.

DESIGN

We hypothesized that joint torques, stiffness and work parameters differ across keyswitch designs and finger postures typical of those observed during computer keyboard typing. We experimentally measured joint kinematics and calculated joint torques while tapping on different keyswitches in different postures, and analyzed the data using mechanical impedance models.

METHODS

Sixteen human subjects tapped with the index finger on computer keyswitches mounted on a sensor which measured vertical and horizontal forces. Miniature electro-optical goniometers mounted dorsally across each finger joint measured joint kinematics. Joint torques were calculated from endpoint forces and joint kinematics using an inverse dynamics algorithm. A linear spring-damper impedance model was fitted to joint torque, position, and velocity during the contact period of each tap. Subjects tapped in three postures approximating those employed during tapping on three rows of a computer keyboard, on four different keyswitches, resulting in 12 conditions.

RESULTS

More extended finger posture was associated with greater joint torques, energies, and stiffnesses, despite minimal differences in endpoint forces across posture. Greater keyswitch make forces were associated with increased forces, joint torques and joint stiffnesses, however this relationship was not monotonic.

CONCLUSIONS

Joint torques and stiffness parameters differed across keyswitch designs and finger postures. Estimates of joint impedance and work provided a unique perspective into finger dynamics.

RELEVANCE

Determining the causes of work-related musculoskeletal disorders is facilitated by characterizing workplace task biomechanics, which can be linked to specific injury mechanisms.

Effect of office ergonomics intervention on reducing musculoskeletal symptoms

STUDY DESIGN

Office workers invited and agreeing to participate were assigned to one of three study groups: a group receiving a highly adjustable chair with office ergonomics training, a training-only group, and a control group receiving the training at the end of the study.

OBJECTIVE

To examine the effect of office ergonomics intervention in reducing musculoskeletal symptom growth over the workday and, secondarily, pain levels throughout the day.

MATERIALS AND METHODS

Data collection occurred 2 months and 1 month before the intervention and 2, 6, and 12 months postintervention. During each round, a short daily symptom survey was completed at the beginning, middle, and end of the workday for 5 days during a workweek to measure total bodily pain growth over the workday. Multilevel statistical models were used to test hypotheses.

RESULTS

The chair-with-training intervention lowered symptom growth over the workday (P = 0.012) after 12 months of follow-up. No evidence suggested that training alone lowered symptom growth over the workday (P = 0.461); however, average pain levels in both intervention groups were reduced over the workday.

CONCLUSION

Workers who received a highly adjustable chair and office ergonomics training had reduced symptom growth over the workday. The lack of a training-only group effect supports implementing training in conjunction with highly adjustable office furniture and equipment to reduce symptom growth. The ability to reduce symptom growth has implications for understanding how to prevent musculoskeletal injuries in knowledge workers.

Forearm EMG response activity during motor performance in individuals prone to increased stress reactivity

BACKGROUND

Work-related Upper Extremity Disorders (WRUEDs) are conceived of as a multifactorial syndrome caused by the effects of excessive repetitive motions, sustained static postures, and muscular stiffness. Our aim is to test an etiological model derived from a theory by Van Galen and Van Huygevoort [2000] Biol Psychol 51:151-171. The theory holds that physical, emotional, and psychosocial stressors enhance muscular stiffness as a compensatory filtering of impoverished signal-to-noise ratios in the motor system. High individual levels of arousal, as measured by Spielberger et al. [1970], State and Trait Anxiety Test would further enhance a subject's predisposition to react with stiffness responses in conditions of stress.

METHODS

Ten participants with a high- and 10 with a low trait-anxiety score performed a computer task involving series of fast but well-dosed accelerations of the forearm along the surface of a digitizer. To induce cognitive stress a tone had to be remembered simultaneously with the aiming task. Pen-tip displacements and surface electromyographic (EMG) signals were recorded from four forearm muscles.

RESULTS

Memory load did not affect error rates but produced shorter reaction times and prolonged movement times. EMG data show that under stress overall levels of neuromotor activation were enhanced. High-anxious participants exhibited higher cocontraction levels than low-anxious participants.

CONCLUSIONS

The findings support the view that stress and muscular tension are closely related and may provide a clue to the origin of WRUEDs.

Epidemiological study to investigate potential interaction between physical and psychosocial factors at work that may increase the risk of symptoms of musculoskeletal disorder of the neck and upper limb

OBJECTIVES

To investigate potential interactions between physical and psychosocial risk factors in the workplace that may be associated with symptoms of musculoskeletal disorder of the neck and upper limb.

METHODS

891 of 1514 manual handlers, delivery drivers, technicians, customer services computer operators, and general office staff reported on physical and psychosocial working conditions and symptoms of neck and upper limb disorders using a self administered questionnaire (59% return rate). Of the 869 valid questionnaire respondents, 564 workers were classified in to one of four exposure groups: high physical and high psychosocial, high physical and low psychosocial, low physical and high psychosocial, and low physical and low psychosocial. Low physical and low psychosocial was used as an internal reference group. The exposure criteria were derived from the existing epidemiological literature and models for physical and psychosocial work factors. The frequency and amplitude of lifting and the duration spent sitting while experiencing vibration were used as physical exposure criteria. Ordinal values of mental demands, job control, and social support with managers and coworkers were used as psychosocial exposure criteria.

RESULTS

In the multivariate analyses, the highest and significant increase in risk was found in the high physical and high psychosocial exposure group for symptoms of hand or wrist and upper limb disorders after adjusting for years at the job, age, and sex. A potential interaction effect was found for the symptoms of the hand or wrist and upper limb disorders but not for the neck symptoms.

CONCLUSION

This study showed that workers highly exposed to both physical and psychosocial workplace risk factors were more likely to report symptoms of musculoskeletal disorders than workers highly exposed to one or the other. The results suggest an interaction between physical and psychosocial risk factors in the workplace that increased the risk of reporting symptoms in the upper limbs. Psychosocial risk factors at work were more important when exposure to physical risk factors at work were high than when physical exposure was low. Ergonomic intervention strategies that aim to minimise the risks of work related musculoskeletal disorders of the upper limb should not only focus on physical work factors but also psychosocial work factors.

Short term and long term effects of enhanced auditory feedback on typing force, EMG, and comfort while typing

Two studies were conducted to determine the effects of enhanced auditory feedback on typing force, electromyography (EMG) and subjective discomfort. The introduction of enhanced auditory feedback caused a 10-20% reduction in 90th percentile typing force, finger flexor EMG, and finger extensor EMG. Adaptation to the enhanced auditory feedback occurred in <3 min. After 1 week of intermittent enhanced auditory feedback there were no differences in typing force or EMG while subjects were typing with or without the enhanced auditory feedback. The continued use of auditory feedback did not further reduce the levels of typing force or EMG after 1 or 2 weeks of exposure.

Effect of computer keyboard slope and height on wrist extension angle

The goal of this study was to determine the systematic effect that varying the slope angle of a computer keyboard along with varying keyboard height (relative to elbow height) have on wrist extension angle while typing. Thirty participants typed on a keyboard whose slope was adjusted to +15 degrees, +7.5 degrees, 0 degrees, -7.5 degrees, and -15 degrees. The height of the keyboard was set up such that participants' wrists were at the same height as their elbows, above their elbows, and four cm below their elbows. Results showed that as keyboard slope angle moved downward from +15 degrees to -15 degrees, mean wrist extension decreased approximately 13 degrees (22 degrees at +15 degrees slope to 9 degrees at -15 degrees slope). Keyboard height had a similar effect with mean wrist extension decreasing from 21.8 degrees when the keyboard was lower than elbow height, to 7.3 degrees when the keyboard was higher than elbow height. Potential application of this research includes the downward sloping of computer keyboards, which could possibly be beneficial in the prevention of musculoskeletal disorders affecting the wrist.

Effect of keyboard keyswitch design on hand pain

This randomized clinical trial evaluated the effects of keyboard keyswitch design on computer users with hand paresthesias. Twenty computer users were matched and randomly assigned to keyboard A (n = 10) or B (n = 10). The keyboards were of conventional layout and differed in keyswitch design. Various outcome measures were assessed during the 12 weeks of use. Subjects assigned keyboard A experienced a decrease in hand pain between weeks 6 and 12 when compared with keyboard B subjects (P = 0.05) and demonstrated an improvement in the Phalen test time (right hand, P = 0.006; left hand, P = 0.06). Keyboard assignment had no significant effect on change in hand function or median nerve latency. We conclude that use of keyboard A for 12 weeks led to a reduction in hand pain and an improved physical examination finding when compared with keyboard B. There was no corresponding improvement in hand function or median nerve latency.