Investigation of applied forces in alphanumeric keyboard work

Changes in Neck and Shoulder Muscles Fatigue Threshold When Using a Passive Head/Neck Supporting Exoskeleton During Repetitive Overhead Tasks

OBJECTIVE

This study aimed to investigate the effects of a head/neck supporting exoskeleton (HNSE) on the electromyographic fatigue threshold (EMGFT) of the neck and shoulder muscles during a simulated overhead work task.

BACKGROUND

Overhead work is a well-known risk factor for neck and shoulder musculoskeletal disorders due to the excessive strain imposed on the muscles and joints in these regions.

METHOD

Fourteen healthy males performed a repetitive overhead nut fastening/unfastening task to exhaustion while wearing and not wearing the HNSE at two neck extension angles (40% and 80% of neck maximum range of motion). Electromyographic signals were continuously recorded from the right and left sternocleidomastoid (SCMR, SCML), splenius capitis (SCR, SCL), upper trapezius (UTR, UTL), and anterior deltoid (ADR, ADL) muscles. The normalized electromyographic amplitude (nEMG) data was time normalized, and a bisegmental linear regression was applied to determine the muscle fatigue break point.

RESULTS

The results showed a significant increase in fatigue threshold time in the SCMR (p < .001), SCML (p = .002), and UTR (p = .037) muscles when the HNSE was used. However, the EMGFT times for the right and left deltoid and left trapezius muscles showed a nonsignificant reduction due to the head/neck support exoskeleton use. In addition, the neck extension angle did not reveal a significant effect on muscles' EMGFT time.

CONCLUSION

Overall, the findings confirmed a significant delay in fatigue onset in sternocleidomastoid muscles, as measured by the electromyographic fatigue threshold. This finding suggests that the HNSE can be an effective ergonomic intervention for reducing the risk of musculoskeletal disorders in overhead workers. However, further studies are needed to investigate the effect of the HNSE at other neck extension angles and more realistic tasks to ensure the generalizability of our results.

APPLICATION

The present findings emphasize the application of the fatigue onset time to evaluate the effectiveness of ergonomic interventions, including exoskeletons, which can subsequently be utilized to alleviate postural demands and reduce the risk of musculoskeletal disorders.

A PRELIMINARY ASSESSMENT OF PHYSICAL WORK EXPOSURES AMONG ELECTRONIC WASTE WORKERS AT AGBOGBLOSHIE, ACCRA GHANA

Occupational exposure associated with unstructured, informal e-waste recycling has received very limited attention. This study aimed to quantify the occupational physical exposures among informal e-waste workers at the largest e-waste site in Africa. A cross-sectional field survey of 163 male e-waste workers was conducted using a self-report occupational physical activity questionnaire, along with direct work observations, and pedometer estimates of walking activity for a subset of workers (n = 42). Results indicated significant differences in self-reported 7-day work exposures among the three main e-waste job categories, namely, collectors (n = 70), dismantlers (n = 73) and burners (n = 20). Prolonged walking, sitting and standing on five or more days in the workweek was frequently reported by collectors (87%), dismantlers (82%) and burners (60%), respectively. Nearly 90% of collectors and burners and 60% of dismantlers reported lifting and carrying on five or more days in the workweek. The exposure combinations identified suggest a risk for musculoskeletal disorders (MSDs). Findings call attention to the need for research examining potential associations between physical exposures and MSDs affecting e-waste workers in Agbogbloshie. The high exposure variability both between and within workers has implications for future exposure assessments conducted in unregulated, informal work settings.

Hand rest and wrist support are effective in preventing fatigue during prolonged typing

STUDY DESIGN

Case series (longitudinal).

INTRODUCTION

Only few reports concerning the efficacy of commonly used strategies for preventing upper limb occupational disorders associated with prolonged typing exist.

PURPOSE OF THE STUDY

We aimed to investigate whether the duration of typing and the use of 2 strategies (hand rest and wrist support) changes muscle physiological response and therefore the electromyography records.

METHODS

We enrolled 25 volunteers, who were unfamiliar with the task and did not have musculoskeletal disorders. The subjects underwent 3 prolonged typing protocols to investigate the efficacy of the 2 adopted strategies in reducing the trapezius, biceps brachii, and extensor digitorum communis fatigue.

RESULTS

Typing for 1 hour induced muscular fatigue (60%-67% of the subjects). The extensor digitorum communis muscle exhibited the highest percentage of fatigue (72%-84%) after 1 and 4 hours of typing (1 hour, P = .04; 4 hours, P = .02). Fatigue levels in this muscle were significantly reduced (24%) with the use of pause typing (4 hours, P = .045), whereas biceps brachii muscle fatigue was reduced (32%) only with the use of wrist supports (P = .02, after 4 hours). Trapezius muscle fatigue was unaffected by the tested occupational strategies (1 hour, P = .62; 4 hours, P = .85).

DISCUSSION

Despite presenting an overall tendency for fatigue detected during the application of the protocols, the assessed muscles exhibited different behavior patterns, depending on both the preventive strategy applied and the muscle mechanical role during the task.

CONCLUSION

Hand rest and wrist support can successfully reduce muscle fatigue in specific upper limb muscles during prolonged typing, leading to a muscle-selective reduction in the occurrence of fatigue and thus provide direct evidence that they may prevent work-related musculoskeletal disorders.

LEVEL OF EVIDENCE

N/A.

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.

Musculoskeletal Symptoms Related to Video Display Terminal Use

The occupational use of video display terminals (VDTs) has been associated with the increasing incidence of upper extremity musculoskeletal disorders, often called cumulative trauma disorders. To guide clinical and policy decisions about the prevention and treatment of these VDT related disorders, valid and economic measures of total daily VDT use and VDT related job tasks such as data entry or editing will be important. In this study of newspaper reporters and copy editors (n=83), VDT use was measured with employee self reports and by sampling the work behaviors of a subsample of employees. Behavioral sampling estimated VDT use as a characteristic of the job as opposed to a characteristic of individual employee performance. Overall, the two techniques of measuring occupational VDT use compared favorably, with the exception that self reported hours of VDT use tended to exceed the hours of use estimated by behavioral observation for employees who were younger and those who reported greater job demands. The findings suggest that behavioral sampling is a valid technique for estimating VDT use as a job characteristic.

Key Strike Forces and Their Relation to High Level of Musculoskeletal Symptoms

Background

This study aimed to investigate the relation between key strike forces and musculoskeletal symptoms (MSS). Moreover, this study presents a key strike force measurement method to be used in a workplace setting. The correlation between key strike force characteristics and MSS was previously studied, but the measurement methods used either a single-key switch or force platforms applied under the keyboard. Most of the studies were conducted in a laboratory setting. The uniqueness of measurement methods in the current study is their ability to measure forces applied to a specific key in a workplace setting and to provide more information about specific key strike forces during typing.

Methods

Twenty-four healthy computer workers were recruited for the study. The demographic questionnaire, and self-reported questionnaires for psychosocial status (General Nordic Questionnaire for Psychological and Social Factors at Work) and for detecting MSS were filled up, which later helped in dividing the participants into two groups (12 participants with pain and 12 without pain). Participants typed a predetermined text that utilized the instrumented keys multiple times. The dynamic forces applied to the keys were recorded and collected, using four thin and flexible force sensors attached to the preselected keys according to their location.

Results

The results demonstrated that participants with high levels of MSS, specifically in the back and neck, in the last year exerted significantly higher key strike forces than those with lower levels of symptoms (p < 0.005).

Conclusion

The key strike force exerted while typing on a keyboard may be a risk factor for MSS, and should therefore be considered in ergonomic evaluations and interventional programs.

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.

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.

A standardized method for measuring the force required to join wire harnesses and sparkplugs

Understanding the forces required to insert a sparkplug wire (wire) onto a sparkplug (plug), independent of worker variation, is important for ergonomists, engineers, and designers. This paper describes a methodology for measuring the forces required to seat a wire onto a plug. A three-axis programmable mill was used to insert wires onto plugs mounted on a force transducer. Inflection points and slopes of the force-displacement curves were found to correspond to mechanical events as the plug and wire were joined. These events were further isolated by dissecting the wires to better understand the force contribution of each wire component. Liner superposition was then used to show that each of these force elements may be added to estimate the total force required to seat a wire onto a plug. This methodology may be used to quantify the effects of design choices, lubricants (wet and dry), and pre-working on axial insertion forces associated with sparkplugs and other insertions. This paper does not address worker abilities or variation, however, the methodology and equipment described may provide a foundation for the exploration of worker ability, variation and work techniques.

Inertia artefacts and their effect on the parameterisation of keyboard reaction forces

Reaction force measurements collected during typing on keyboard trays contain inertia artefacts due to dynamic movements of the supporting work surface. To evaluate the effect of these artefacts, vertical forces and accelerations were measured while nine volunteers touch-typed on a rigid desk and a compliant keyboard tray. Two signal processing methods were evaluated: 1) low pass filtering with 20 Hz cut-off; 2) inertial force cancellation by subtracting the accelerometer signal. High frequency artefacts in the force signal, present on both surfaces, were eliminated by low pass filtering. Low frequency artefacts, present only when subjects typed on the keyboard tray, were attenuated by subtracting the accelerometer signal. Attenuation of these artefacts altered the descriptive statistics of the force signal by as much as 7%. For field measurements of typing force, reduction of low frequency artefacts should be considered for making more accurate comparisons across groups using work surfaces with different compliances. Direct measures of physical risk factors in the workplace can improve understanding of the aetiology of musculoskeletal disorders. Findings from this study characterise inertia artefacts in typing force measures and provide a method for eliminating them. These artefacts can add variability to measures, masking possible differences between subject groups.

Force and Impulse Production during the Use of a Touch Screen by Individuals with Motor Control Disabilities

People are increasingly required to interact with touch screens at places ranging from grocery stores to airport kiosks. To date, most of the usability research related to touch screens has included young, healthy subjects. Using novel instrumentation consisting of a force plate and a touch screen, a number entry study examined finger-touch screen interaction by participants with Cerebral Palsy, Multiple Sclerosis, and non-disabled controls. Timing data as well as peak forces and impulses in three dimensions were collected for each touch. The results indicate that, although average peak force vector magnitudes, impulses, and dwell times are similar between the groups, there are significant differences within the same three variables by button size. Average peak force vector magnitude increased by 11 percent while the average vector impulse decreased by 29 percent from the smallest to the largest button size. The average dwell time also decreased 23 percent from the smallest to the largest button size.

Effects of psychosocial and individual factors on physiological risk factors for upper extremity musculoskeletal disorders while typing

Psychosocial factors are hypothesized to contribute to work-related musculoskeletal disorder (WMSD) development, although previous research has been largely epidemiological or has focused primarily on the shoulders, back and neck. The objective of this study was to quantify the effects of mental workload and time pressure on perceived workload and physiological responses of the distal upper extremity. A total of 18 typists completed nine 5-min typing sessions representing three levels of time pressure and mental workload. Levels were manipulated by adjusting typing speed and by requiring participants to perform arithmetic tasks while typing. Outcomes were measured in muscle activation levels, wrist postures and movements, key strike force and subjective assessments of workload. In general, increased time pressure increased muscle activation, key strike force and wrist deviations; and increased mental workload increased key strike force. Mental workload and time pressure mediated physical risk factors during typing to increase WMSD risk for the distal upper extremity.

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 keyswitch design of desktop and notebook keyboards related to key stiffness and typing force

This study aimed to compare and analyse rubber-dome desktop, spring-column desktop and notebook keyboards in terms of key stiffness and fingertip typing force. The spring-column keyboard resulted in the highest mean peak contact force (0.86N), followed by the rubber dome desktop (0.68N) and the notebook (0.59N). All these differences were statistically significant. Likewise, the spring-column keyboard registered the highest fingertip typing force and the notebook keyboard the lowest. A comparison of forces showed the notebook (rubber dome) keyboard had the highest fingertip-to-peak contact force ratio (overstrike force), and the spring-column generated the least excess force (as a ratio of peak contact force). The results of this study could aid in optimizing computer key design that could possibly reduce subject discomfort and fatigue.

Effects of negatively sloped keyboard wedges on risk factors for upper extremity work-related musculoskeletal disorders and user performance

Several changes to computer peripherals have been developed to reduce exposure to identified risk factors for musculoskeletal injury, notably in keyboard designs. Negative keyboard angles and their resulting effects on objective physiological measures, subjective measures and performance have been studied, although few angles have been investigated despite the benefits associated with their use. The objective of this study was to quantify the effects of negative keyboard angles on forearm muscle activity, wrist posture, key strike force, perceived discomfort and performance and to identify a negative keyboard angle or range of keyboard angles that minimizes exposure to risk factors for hand/wrist injuries. Ten experienced typists (four males and six females) participated in a laboratory study to compare keyboard angles ranging from 0 degrees to -30 degrees , at 10 degrees increments, and a keyboard with a 7 degrees slope, using a wedge designed for use with standard QWERTY keyboards. Repeatability of exposures was examined by requiring participants to complete two test sessions 1 week apart. Dependent variable data were collected during 10 min basic data entry tasks. Wrist posture data favoured negative keyboard angles of 0 degrees (horizontal) or greater, compared to a positive keyboard angle of 7 degrees , especially for the flexion/extension direction. In general, the percentage of wrist movements within a neutral zone and the percentages of wrist movements within +/-5 degrees and +/-10 degrees increased as keyboard angle became more negative. Electromyography results were mixed, with some variables supporting negative keyboard angles whilst other results favoured the standard keyboard configuration. Net typing speed supported the -10 degrees keyboard angle, whilst other negative typing angles were comparable, if not better than, with the standard keyboard. Therefore, angles ranging from 0 degrees to -30 degrees in general provide significant reductions in exposure to deviated wrist postures and muscle activity and comparable performance.

Development of a short form of the Workstyle measure

BACKGROUND

'Workstyle', or how a worker behaviourally, cognitively and physiologically responds to increased or stressful work demands, has been proposed to help explain the link between ergonomic and psychosocial factors in work-related upper limb disorder symptoms and disorders (WRULD).

AIM

To describe the psychometric properties of a shortened version of the original Workstyle measure.

METHODS

Factor analyses of the Workstyle measure items were conducted to reduce the number of total items. Each of the subscales was then further reduced by randomly selecting half of the items within each subscale. Additionally, two subscales from the original survey (Pain/Tension and Numbness/Tingling) were eliminated because they were not used to calculate the original workstyle total score in order to reduce the influence of current symptoms on an individual's total score.

RESULTS

The Workstyle Short Form was reduced to 32 items. Cronbach's alpha was 0.89 and the test-retest reliability was r = 0.88, P < 0.01, for the total score. The short form score was significantly correlated with the full workstyle total score, r = 0.98, P < 0.01. Higher total workstyle scores were significantly associated with pain, functional limitations and adverse mental and physical health.

CONCLUSION

The Workstyle Short Form demonstrated acceptable psychometric properties. These findings indicate its potential utility in research on WRULD.

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.

Finger joint impedance during tapping on a computer keyswitch

We studied the dynamic behavior of finger joints during the contact period of tapping on a computer keyswitch, to characterize and parameterize joint function with a lumped-parameter impedance model. We tested the hypothesis that the metacarpophalangeal (MCP) and interphalangeal (IP) joints act similarly in terms of kinematics, torque, and energy production when tapping. Fifteen human subjects tapped with the index finger of the right hand on a computer keyswitch mounted on a two-axis force sensor, which measured forces in the vertical and sagittal planes. Miniature fiber-optic goniometers mounted across the dorsal side of each joint measured joint kinematics. Joint torques were calculated from endpoint forces and joint kinematics using an inverse dynamic algorithm. For each joint, a linear spring and damper model was fitted to joint torque, position, and velocity during the contact period of each tap (22 per subject on average). The spring-damper model could account for over 90% of the variance in torque when loading and unloading portions of the contact were separated, with model parameters comparable to those previously measured during isometric loading of the finger. The finger joints functioned differently, as illustrated by energy production during the contact period. During the loading phase of contact the MCP joint flexed and produced energy, whereas the proximal and distal IP joints extended and absorbed energy. These results suggest that the MCP joint does work on the interphalangeal joints as well as on the keyswitch.

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.

From confounders to suspected risk factors: psychosocial factors and work-related upper extremity disorders

Psychosocial variables have recently been more prominent among epidemiologic risk factors for work-related upper extremity disorders (WRUEDs), but bio-behavioral mechanisms underlying these associations have been elusive. One reason is that the psychosocial domain has included many broad and disparate variables (e.g. mood, coping skills, job control, job satisfaction, job stress, social support), and this lack of specificity in the conceptualization of psychosocial factors has produced limited hypothesis testing opportunities. Therefore, recent research efforts have focused on identifying and conceptualizing specific psychosocial factors that might more clearly delineate plausible bio-behavioral mechanisms linking psychosocial factors to WRUEDs. One such factor is workstyle, a strategy that workers may employ for completing, responding to, or coping with job demands that might affect musculoskeletal health. Preliminary studies have provided support for measurable differences in workstyle among individual workers and an association with upper extremity pain and discomfort. An initial self-report measure of workstyle has been pilot tested among office workers and shown acceptable reliability and validity. Future studies are needed to study this construct among other working populations and to determine its relationship with other clinical endpoints. Nevertheless, early findings suggest workstyle may be a potential focus of WRUED prevention efforts.

Developing ergonomic solutions for prevention of musculoskeletal disorder disability

Numerous models have been proposed to help understand the relationship between personal and work factors that affect participation in work. The authors use a generic version of these models as a framework for identifying gaps between job demands and worker capacities. They describe metrics for assessing factors associated with causing or aggravating musculoskeletal disorders. Two case examples are presented to illustrate the hierarchical assessment of jobs and the evaluation of gaps between job demands and worker capacities. Finally, the authors describe the development of a job database to facilitate future evaluations.

Changes in postural risk and general health associated with a participatory ergonomics education program used by heavy video display terminal users: a pilot study

To determine if a brief, participatory ergonomics education program was associated with changes in work posture and general health of heavy video display terminal (VDT) users, 23 full-time VDT users participated in an on-site, small-group, 60-minute ergonomics education session and 1 week later an individual 15-minute follow-up session at their workstation. Posture was assessed by a blinded tester who scored videotape records to complete the Postural and Repetitiveness Risk Factors Index (PRRI), and general health status was assessed via self-administered Short-Form 36 Health Survey (SF-36) questionnaire before the intervention and again 5 weeks later. Five weeks after the 60-minute session, PRRI scores were 19% lower than were preintervention scores (p < 0.01), indicating lower postural risk. SF-36 physical (2% higher) and mental (4% higher) component scores were not statistically different, however, before and after intervention (p > 0.05). Although the participatory ergonomics education program was associated with improved work posture (PRRI scores) after 5 weeks, general physical and mental health (SF-36 scores) did not change within this time period. These results suggest that a participatory ergonomics program, which is of short duration and minimally disruptive to the normal workplace routine, may have a rapid effect on improving work posture. Although awkward posture is thought to be a risk factor for work-related musculoskeletal disorders, multigroup and long-term randomized trials are required to establish the effectiveness of participatory ergonomics programs in reducing the incidence and severity of musculoskeletal disorders associated with heavy VDT use.

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.

A case study in iterative keyboard design using participatory design techniques

Replacing a keyboard can be stressful for operators who take calls all day, particularly when special functions must be sacrificed. This study applied several participatory design techniques to involve over 3,000 operators in the redistribution of function keys on their replacement keyboard. Video analysis and manual data logging methods were surprisingly accurate when measured against software logs. User acceptance was excellent. In the initial cutover program, operator performance levels returned to normal very quickly, resulting in over-confidence and unrealistic expectations of post-implementation performance for the remaining call centres because the focus was on the technology rather than on the impact of technology on business benefits. As a consequence, business plan requirements for future projects have now been changed to accommodate predictions of post-implementation performance and taking into account other changes happening at the same time.

Effect of four computer keyboards in computer users with upper extremity musculoskeletal disorders

Eighty computer users with musculoskeletal disorders participated in a 6-month, randomized, placebo-controlled trial evaluating the effects of four computer keyboards on clinical findings, pain severity, functional hand status, and comfort. The alternative geometry keyboards tested were: the Apple Adjustable Keyboard [kb1], Comfort Keyboard System [kb2], Microsoft Natural Keyboard [kb3], and placebo. Compared to placebo, kb3 and to a lesser extent kb1 groups demonstrated an improving trend in pain severity and hand function following 6 months of keyboard use. However, there was no corresponding consistent improvement in clinical findings in the alternative geometry keyboard groups compared to the placebo group. Overall, there was a significant correlation between improvement of pain severity and greater satisfaction with the keyboards. These results provide evidence that keyboard users may experience a reduction in hand pain after several months of use of some alternative geometry keyboards.

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.

Computer key switch force-displacement characteristics and short-term effects on localized fatigue

This study investigates the effects of key switch design parameters on short-term localized muscle fatigue in the forearm and hand. An experimental apparatus was utilized for simulating and controlling key switch make force and travel using leaf spring mechanisms, and provided direct measurement of applied key strike force using strain gauge load cells. Repetitive key tapping was performed as fast as possible using the dominant index finger for 500 s per condition (8.3 min) and a work-rest schedule consisting of 15 s of key tapping alternating with 10 s of rest. One combination of two make force levels (0.31 and 0.71 N) and two over travel distances (0.5 and 4.5 mm) was presented randomly on four different days. Nine subjects participated. Localized muscle fatigue in the hand and forearm was assessed subjectively using a 10 cm visual analogue scale, and objectively using surface electromyography (EMG). Average peak key strike force exerted was 0.35 N less for the smaller make force and 0.59 N less for the longer over travel distance. Fatigue occurred in all cases but no significant differences were observed between key switch parameters based on RMS EMG. Subjective reports of localized fatigue after 500 s were less when the key switch make force was less; however, a corresponding over travel effect was not observed despite the greatly reduced key strike force for the longer over travel distance. This discrepancy may be explained by the greater finger movement that was observed with increased over travel. Although there was no apparent improvement in short-term discomfort from fatigue when over travel was increased, this study did not consider the potential long-term health benefits from reduced key strike force.

Computer keyboard force and upper extremity symptoms

This case-control study assessed whether office workers who report more severe levels of musculoskeletal symptoms of the upper extremities demonstrate higher levels of keyforce in comparison to controls with less severe symptoms. Office workers reporting working on computer keyboards for four hours per day were classified as cases or controls based upon a median split on a Composite Symptom Severity score (cases = 23, controls = 25). Keyboard force and keying rate were measured during a 15-minute keyboarding task. Measures of task-related discomfort, muscular fatigue, pain, upper extremity symptoms, psychological distress and force were collected at baseline, post-keyboard task, and recovery. Ratings of perceived effort and task credibility were also obtained. Measures of work demands, perceived job stress, and upper extremity strength and flexibility were also collected. The results indicated group equivalence on reported work demands and upper extremity strength. Cases were more likely to receive a medical diagnosis of upper extremity cumulative trauma disorder, awaken from sleep due to symptoms, report higher levels of pain during work, experience greater impact of pain on function, and report higher workload pressure and lower support. Cases generated significantly higher keyboarding forces than controls, although both groups produced forces well above that required to operate the keyboard (4-5 times activation force). Cases reported higher levels of upper extremity symptoms and discomfort than controls, and these measures were highest after the keyboarding task for both groups. No significant correlation between keyforce and key rate was observed in either group. Results suggest that generation of excessive force while working on a computer keyboard may contribute to the severity of upper extremity symptoms. Clinically, the findings suggest that evaluating how an individual worker performs keyboarding tasks, or his or her workstyle, may be helpful in the management of these symptoms and disorders.

Force response of the fingertip pulp to repeated compression--effects of loading rate, loading angle and anthropometry

Repeated loading of the fingertips has been postulated to contribute to tendon and nerve disorders at the wrist during activities associated with prolonged fingertip loading such as typing. To fully understand the pathomechanics of these soft tissue disorders, the role of the fingertip pulp in attenuating the applied dynamic forces must be known. An experiment was conducted to characterize the response of the in vivo fingertip pulp under repeated, dynamic, compressive loadings, to identify factors that influence pulp dynamics, and to better understand the force modulation by the pulp. Twenty subjects tapped repeatedly on a flat plate with their left index finger, while the contact force and pulp displacement were measured simultaneously. Tapping trials were conducted at three fingertip contact angles from the horizontal plane (0 degree, 45 degrees, and 90 degrees) and five tapping rates (0.25, 0.5, 1, 2, and 3 Hz). The fingertip pulp responds as a viscoelastic material, exhibiting rate-dependence, hysteresis, and a nonlinear force-displacement relationship. The pulp was relatively compliant at forces less than 1 N, but stiffened rapidly with displacement at higher forces for all loading conditions. This suggests that high-frequency forces of a small magnitude (< 1 N) are attenuated by the nonlinearly stiffening pulp while these forces of larger magnitude are transmitted to the bone. Pulp response was significantly influenced by the angle of loading. Fingertip dimensions, gender, and subject age had little to no influence on pulp parameters.

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

The design of the force-displacement characteristics or 'feel' of keyboard keyswitches has been guided by preference and performance data; there has been very little information on how switch 'feel' alters muscle activity or applied force. This is a laboratory-based repeated measures design experiment to evaluate the effect of computer keyboard keyswitch design on applied finger force and muscle activity during a typing task. Ten experienced typists typed on three keyboards which differed in keyswitch make force (0.34, 0.47 and 1.02 N) while applied fingertip force and finger flexor electromyograms were recorded. The keyboard testing order was randomized and subjects typed on each keyboard for three trials, while data was collected for a minimum of 80 keystrokes per trial. No differences in applied fingertip force or finger flexor EMG were observed during typing on keyboards with switch make force of 0.34 or 0.47 N. However, applied fingertip force increased by approximately 40% (p < 0.05) and EMG activity increased by approximately 20% (p < 0.05) when the keyswitch make force was increased from 0.47 to 1.02 N. These results suggest that, in order to minimize the biomechanical loads to forearm tendons and muscles of keyboard users, keyswitches with a make force of 0.47 N or less should be considered over switches with a make force of 1.02 N.

Activation force and travel effects on overexertion in repetitive key tapping

Key switch design parameters, including make force, make travel, and over travel, were investigated for minimizing operator-exerted force while maximizing key-tapping speed. A mechanical apparatus was designed, constructed, and used for independently controlling key switch parameters and for directly measuring finger exertions during repetitive key tapping using strain gauge load cells. The task for the 25 participants involved using the index finger of the dominant hand to repeatedly depress a single key as rapidly as possible. Participants received visual and auditory feedback upon a successful keystroke. Peak force exerted decreased 24% and key-tapping rate increased 2% when over travel was distended from 0.0 to 3.0 mm. Although peak force exerted was not significantly affected by make point travel, key-tapping rate increased 2% when make point travel was reduced from 4.0 to 1.0 mm. These results indicate that key switch mechanisms that provide adequate over travel might enable operators to exert less force during repetitive key tapping without inhibiting performance.

Keyboard reaction force and finger flexor electromyograms during computer keyboard work

This study examines the relationship between forearm EMGs and keyboard reaction forces in 10 people during keyboard tasks performed at a comfortable speed. A linear fit of EMG force data for each person and finger was calculated during static fingertip loading. An average r2 of .71 was observed for forces below 50% of the maximal voluntary contraction (MVC). These regressions were used to characterize EMG data in force units during the typing task. Averaged peak reaction forces measured during typing ranged from 3.33 N (thumb) to 1.84 N (little finger), with an overall average of 2.54 N, which represents about 10% MVC and 5.4 times the key switch make force (0.47 N). Individual peak or mean finger forces obtained from EMG were greater (1.2 to 3.2 times) than force measurements; hence the range of r2 for EMG force was .10 to .46. A closer correspondence between EMG and peak force was obtained using EMG averaged across all fingers. For 5 of the participants the force computed from EMG was within +/-20% of the reaction force. For the other 5 participants forces were overestimated. For 9 participants the difference between EMG estimated force and the reaction force was less than 13% MVC. It is suggested that the difference between EMG and finger force partly results from the amount of muscle load not captured by the measured applied force.

An exploratory study of the relationship between biomechanical factors and right-arm musculoskeletal discomfort and fatigue in a VDT data-entry task

The relationship of key force and keystroke rate with right-arm musculoskeletal discomfort and fatigue was explored in a video-display-terminal (VDT) data-entry task. Forty-three data transcribers entered bogus data from tax forms at a VDT for one workday with their right hand. Peak key force and keystroke rate were monitored on a continuous basis. Self-ratings of right-arm discomfort and fatigue were assessed at periodic intervals. Stepwise regression analyses indicated that both lower key forces and lower keystroke rates were associated with higher ratings of right-elbow discomfort. In addition, lower key forces were associated with higher ratings of right-hand discomfort and lower keystroke rates were associated with higher ratings of right-shoulder discomfort and fatigue. The amount of variance accounted for by these models ranged from 7 to 24%. These results appear to be contrary to conventional biomechanical models that postulate a positive association between key force, keystroke rate and musculoskeletal discomfort in VDT work. Further laboratory and field research under controlled conditions is needed to clarify the direction and extent of the cause-and-effect relationship between biomechanical factors and musculoskeletal discomfort in VDT data-entry work.

A quantitative description of typing biomechanics

One approach to combating work- related upper extremity musculoskeletal disorders (UEMSDs) is to improve understanding of MSD risk factors through quantitative biomechanical characterization of manual tasks, including joint postures, joint dynamics, and force exertion. This paper describes a methodology employed to quantitatively assess professional typing in a workplace setting, and the results of that assessment. Results are compared between different kinds of typing tasks, and between typing and hand- intensive industrial tasks, based on nontask- specific biomechanical terms. Quantitative, biomechanical characterizations of manual tasks will lead to identification of appropriate ranges for joint kinematics and force exertion, which will, in turn, facilitate proper design of manual tasks. Additionally, the methodology could be used to assess manual performance of skilled tasks for proper (healthy) technique, or be used to evaluate progress through a course of rehabilitation (when do an impaired worker's motions begin to resemble motions of healthy workers?).

Measurement of creep strain of flexor tendons during low-force high-frequency activities such as computer keyboard use

The aim of this study was to measure tendon strain during low-force, high-frequency activities such as computer keyboard use. Prior to creep strain testing an estimate of flexor tendon force during keyboard use was made. Tendon force was measured indirectly by comparing electromyographic activity of the flexor and extensor digitorum muscles in five human volunteers for various hand activities. Results of the electromyographic study showed that flexor tendon forces during keyboard use may be as high as 60 N. Sixty eight flexor digitorum tendons from 17 fresh-frozen cadaver hands were used for the creep strain tests. Three loading conditions (static, 1 Hz cyclic, (1/4) Hz cyclic) and four load levels (10, 20, 50, 100 N) were used. Results of the creep study showed that for a flexor tendon force of 60 N the total strain of the tendon would be approximately 1.8%. This does not appear to be enough strain to cause permanent damage to the tendon according to current cumulative strain models. RELEVANCE: Recent studies have shown an increase in hand and wrist tendinitis among computer users. At the present time the aetiology is unknown, but risk factors commonly associated with tendinitis are hand forces, wrist postures, and frequency of finger movements. This research looks at tendon creep due to repeated force as a possible aetiology.