Effects of BMI and task parameters on joint angles during simulated small parts assembly

Functional limitations of persons classified into different obesity categories were evaluated while performing a simulated small parts assembly tasks. Joint angles (measured using electro-goniometers) and maximum forward function reach (MFFR) were used to quantify function limitations, and tasks were completed on three workstations designed for the 5th, 50th, and 95th percentile workers based on anthropometric data tables. Results revealed that BMI category did not significantly affect measured joint angles. Results also indicated that joint angles when working at the 95th percentile workstation configuration were significantly larger than those measured at the 5th percentile workstation configuration. Further, obese class 2 and obese class 3 groups MFFRs were significantly shorter than the normal weight group, which was expected. These results imply that workstation design considerations should include obesity levels, and that work should be placed near the worker and should be elevated to reduce pressure on joint angles while working for larger (obese) individuals.     

Human Factors Requirements for the Development of U.S. Army Materiel∗†

This study investigated the effects of visual cues, muscular fatigue, task performance and experience of working on inclined surfaces on activity of postural muscles in the lower limbs associated with maintaining balance on three inclined surfaces – 0°, 14° and 26°. Normalised electromyographic (NEMG) data were collected in 44 professional roofers bilaterally from the rectus femoris, biceps femoris, tibialii anterior and gastrocnemii medial muscle groups. The 50th and 95th percentile NEMG amplitudes were used as EMG variables. Results showed that inclination angle and task performance caused a significant increase in the NEMG amplitudes of all postural muscles. Visual cues were significantly associated with a decrease in the 95th percentile EMG amplitude for the right gastrocnemius medial and tibialis anterior. Fatigue was related to a significant decrease in the NEMG amplitude for the rectus femoris. Experience of working on inclined surfaces did not have a significant effect on the NEMG amplitude.     

Review of: “Handbook of Psychophysiology. Edited by Norman S. Greenfield and Richard A. Sternbach.” (New York: Holt,Rinehart and Winston, 1972.) [Pp. xii-f-1011.] £14-30.

The purpose of this study was to assess the effects of hip belt use and load placement in a backpack on perceived exertion and postural stability. Thirty participants were instructed to stand on a force plate and walk along a designated route under five conditions: unloaded, high-load placement, low-load placement, hip belt on and hip belt off. The average velocity and sway area from the force plate were measured. Participants also rated their perceived stability and exertion. Compared to the unloaded condition, all loaded conditions significantly increased average velocity, sway area, perceived stability and exertion. Hip belt use did not affect average velocity and sway area; however, participants reported higher levels of stability and lower levels of exertion with hip belt use. Load placement did not affect average velocity, sway area, perceived stability or exertion. This study showed that wearing a backpack loaded to 20% of body weight reduced postural stability, while manipulation of load placement in a backpack did not affect subjective and objective measures of postural stability. Also, hip belt use only improved subjective measures of postural stability.     

Walking on ballast impacts balance

Railroad workers often perform daily work activities on irregular surfaces, specifically on ballast rock. Previous research and injury epidemiology have suggested a relationship between working on irregular surfaces and postural instability. The purpose of this study was to examine the impact of walking on ballast for an extended duration on standing balance. A total of 16 healthy adult males walked on a 7.62 m × 4.57 m (25 ft × 15 ft) walking surface of no ballast (NB) or covered with ballast (B) of an average rock size of about 1 inch for 4 h. Balance was evaluated using dynamic posturography with the NeuroCom^® Equitest System^? prior to experiencing the NB or B surface and again every 30 min during the 4 h of ballast exposure. Dependent variables were the sway velocity and root-mean-square (RMS) sway components in the medial–lateral and anterior–posterior directions. Repeated measures ANOVA revealed statistically significant differences in RMS and sway velocity between ballast surface conditions and across exposure times. Overall, the ballast surface condition induced greater sway in all of the dynamic posturography conditions. Walking on irregular surfaces for extended durations has a deleterious effect on balance compared to walking on a surface without ballast. These findings of changes in balance during ballast exposure suggest that working on an irregular surface may impact postural control.     

Surface slope effects on shingling frequency and postural balance in a simulated roofing task

The purpose of this study was to investigate the influence of surface slopes (18 degrees , 26 degrees , 34 degrees ) on the maximum acceptable roof shingling frequency for males performing a simulated roof-shingling task. The psychophysical roof shingling frequency was also compared to the postural sway and trunk motion values. The maximum acceptable roof shingling frequency and selected trunk motion decreased significantly with an increase in slope. Postural sway however increased significantly with an increase in slope. The study also revealed that workers were experiencing a greater postural sway at the earlier phase of task on the steeper surface. This suggests that there might be an adaptation period associated with working on a slope and the body's ability to compensate for a loss of balance. Therefore, more emphasis should be given to the workers in this adaptation period, with additional monitoring and cautionary measures. Collectively, the findings of the study could be utilized for improving work practices on roofs, while reducing the potential risks of falls in roofing construction.     

Postural implications of obtaining line–of–sight for seated operators of underground mining load-haul-dump vehicles

Operators of load–haul–dump (LHD) vehicles use awkward postures that may be held statically and at extreme ranges of motion for long shift periods to spot hazards in underground mining. This study examined postural variables associated with three amounts of seat rotation intended to maximize line-of-sight during forward driving. Three different models, representing the 1st, 50th and 99th percentile male for height and weight, were positioned with appropriate hand and foot constraints in the virtual LHD cab modelled in Classic JACK v4.0. A total of 15 virtual movement strategies were developed to model the postural behaviour of typical workers and each virtual subject was tested, first with the seat in a neutral 0° position and then with it rotated counter-clockwise to 20° and 45°. Results revealed that reductions in trunk rotation, trunk lateral bend and neck rotation were associated with the seat rotation intervention. The general relationship observed was that as seat rotation increased, view of critical visual attention locations and visible line-of-sight area increased while postural load variables decreased. For the most part, 20° of seat rotation was beneficial but 45° produced significantly greater changes to postural load and visible visual attention locations.     

Postural stability after whole-body vibration exposure

Fall injuries to operators of heavy mobile equipment are more frequent during egress rather than while climbing onto the equipment. One possible contributing factor is a loss of postural stability during egress. Because many of these operators are exposed to prolonged periods of whole-body vibration (WBV) while controlling the equipment, a study was conducted to determine if postural stability is impaired by WBV exposure. Subjects were tested for a difference in standing postural sway before and after seated WBV exposure. The WBV consisted of 40 minutes of vertical vibration generated by a motion platform, which emulated actual vibration levels of an operating under-ground shuttle car. Two independent variables, each at two levels, consisted of vibration and vision. The measured dependent variables were postural sway amplitude and velocity of sway. No significant difference between the vibration and no vibration conditions were found. Based on the results of this study, it cannot be concluded that WBV at the exposed frequencies influences postural stability. The results suggest that other factors may be the primary contributors to fall injuries while exiting the vehicle, such as problems with foot placement accuracy or egress system design.     

Ergonomics and quality in paced assembly lines

The aim of this industrial study was to measure relationships between workstation ergonomics and product quality. Quality on two paced manual assembly lines for disposable cameras was measured by the number of defects per week at each workstation. Two ergonomic variables, the time required for the task and the postural deficiencies, were together able to predict over 50% of the quality variance on each assembly line. This study shows the direct effect of ergonomics variables on quality results. © 2001 John Wiley & Sons, Inc.     

Shoulder posture and localized muscle fatigue and discomfort

In many industries workers perform manual assembly tasks with hands postured above the shoulders. Awkward shoulder and arm postures are often viewed as acceptable given costs of workplace modification, postural exertions which are in compliance with current design recommendations, ready availability of strong workers, and numbers of electromyographic studies which fail to detect significant signs of localized muscle fatigue (LMF). An experiment was conducted to: (a) study the onset and severity of (LMF) in the shoulder when performing a stylus-to-hole Fitts reciprocal movement task under a range of postures, hand loads, ratios of work-to-rest, and task durations, and (b) to evaluate the efficacy of three techniques (i.e., changes in EMG behaviour, postural tremor, and cross-modal matching estimates) in detecting and monitoring posturally-based LMF and discomfort in the shoulder complex. Experimental findings showed that posturing hands above shoulder level significantly increased the risk of LMF and postural discomfort even in light-weight manual assembly environs where postural exertions are small, and that cross-modal matching estimates and postural tremor were more sensitive metrics of LMF in the shoulder complex than EMG RMS voltage and mean spectral power frequency metrics. The basis for experimental findings, as well as potential application of LMF metrics in future postural stress investigations, are discussed. Recommendations for workplace posture are provided for job designers facing work height decisions in manual assembly environs.     

Workstation variables and visual discomfort associated with VDTs

We have investigated the effects of a range of workstation factors upon the visual symptoms experienced by a group of 92 visual display terminal (VDT) users. Subjects in the study kept a diary over five consecutive working days in which they recorded the types of visual and postural symptoms which occurred and the types of work tasks being performed. Each subject's workstation was analysed for screen legibility and stability, discomfort and disability glare, and required head postures. By the use of multiple regression analysis techniques we have considered the relative contribution of these factors to the symptoms reported by the users of these workstations. Screen legibility significantly influenced the occurrence of symptoms of ocular discomfort and vertical head movements significantly affected the incidence of postural/headache symptoms.     

Effects of carried weight on random motion and traditional measures of postural sway

The purpose of the study was to investigate the effects of load weight carried by soldiers upon postural sway. Fourteen US Army enlisted men participated. Postural sway and muscle activity were measured while participants stood on a force plate. The load weight conditions, comprised of Army clothing and load-carriage equipment were 6, 16, and 40 kg. With an increase in load weight, stabilogram-diffusion analysis revealed that random movement of postural sway decreased. Also, with an increase in load weight, center of pressure excursions increased linearly but muscle activity changed minimally. In short, increasing load weight challenged the load carriers' stability, reduced the randomness of postural sway and required the load carriers to exert greater control of the load in order to maintain balance.     

Effectiveness of vertical visual reference for reducing postural instability on inclined and compliant surfaces at elevation

Falls from elevation continue to be the most serious hazard for the workers in construction. Simple and cost effective technical approaches to improve workers' balance on sloped roofs and deformable/unstable platforms have potential to reduce the risk of falls. This study evaluated the effectiveness of simple vertical structures as visual references (cue) for balance improvement. Twenty-four construction workers were tested while standing on sloped and deformable surfaces at elevation and performing undemanding visual tasks with vertical structures positioned at different proximal locations. Workers' balance performance was assessed by sway parameters calculated from the center-of-pressure movement collected with a force platform. The study results indicate increased instability on the sloped and deformable surfaces at elevation, and show that a simple vertical structure, e.g., a narrow bar, can serve as a visual cue and assist workers' balance. Workers' balance improved linearly with cue proximity in the tested distance range both on the sloped and the deformable surfaces. At a moment of instability, workers can redirect their attention to a proximal structure, available in the line of sight, to assist balance control. These findings may be useful in modifying elevated work environments and construction procedures to improve workers' postural balance during various construction phases.     

Postural implications of obtaining line-of-sight for seated operators of underground mining load-haul-dump vehicles

Operators of load-haul-dump (LHD) vehicles use awkward postures that may be held statically and at extreme ranges of motion for long shift periods to spot hazards in underground mining. This study examined postural variables associated with three amounts of seat rotation intended to maximize line-of-sight during forward driving. Three different models, representing the 1st, 50th and 99th percentile male for height and weight, were positioned with appropriate hand and foot constraints in the virtual LHD cab modelled in Classic JACK v4.0. A total of 15 virtual movement strategies were developed to model the postural behaviour of typical workers and each virtual subject was tested, first with the seat in a neutral 0 degrees position and then with it rotated counter-clockwise to 20 degrees and 45 degrees . Results revealed that reductions in trunk rotation, trunk lateral bend and neck rotation were associated with the seat rotation intervention. The general relationship observed was that as seat rotation increased, view of critical visual attention locations and visible line-of-sight area increased while postural load variables decreased. For the most part, 20 degres of seat rotation was beneficial but 45 dgrees produced significantly greater changes to postural load and visible visual attention locations.     

Work activities and musculoskeletal complaints among preschool workers

The potential for musculoskeletal trauma among preschool workers has been largely unexplored in the United States. This case report describes an investigation conducted to identify and evaluate possible causes of back and lower extremity pain among 22 workers at a Montessori day care facility. Investigators met with and distributed a questionnaire to school employees, and made measurements of workstation and furniture dimensions. Investigators also recorded the normal work activities of school employees on videotape, and performed a work sampling study to estimate the percentage of time employees spend performing various tasks and in certain postures.

Questionnaire results from 18 employees indicated that back pain/discomfort was a common musculoskeletal complaint, reported by 61% of respondents. Neck/shoulder pain, lower extremity pain and hand/wrist pain were reported by 33, 33 and 11% of respondents, respectively. Observation and analysis of work activities indicated that employees spend significant periods of time kneeling, sitting on the floor, squatting, or bending at the waist. Furthermore, staff members who work with smaller children (i.e. six weeks to 18 months of age) performed more lifts and assumed more awkward lower extremity postures than employees who work with older children (3–4 years of age). Analysis of two lifting tasks using the revised NIOSH lifting equation indicated that employees who handle small children may be at increased risk of lifting-related low back pain. Investigators concluded that day care employees at this facility are at increased risk of low back pain and lower extremity (i.e. knee) injury due to work activities that require awkward or heavy lifts, and static working postures. Recommendations for reducing or eliminating these risks by modifying the workplace and changing the organization and methods of work are presented.     

Influence of fatigue time and level on increases in postural sway

The purpose of this study was to investigate the influence of fatigue time and fatigue level on the increases in postural sway during quiet standing. Centre of pressure-based measures of postural sway were collected both before and after fatiguing participants using three different fatigue levels and two different fatigue times. Results showed increasing fatigue time increased sway velocity and sway area, and increasing fatigue level increased sway velocity. Fatigue time effects are important to consider when applying laboratory-based findings to the field given that the fatigue time can differ substantially between the two. Fatigue level effects imply a dose – response relationship between localized muscle fatigue and risk of falling that can have important implications in work/rest cycle scheduling for occupations at risk of injurious falls.     

Design and sizing of ergonomic handles for hand tools

In this paper, handles for two commonly used hand tools, the chisel and the off-set pliers, are designed using ergonomic principles. These were sized for both males and females falling in the 5th percentile, 50th percentile and 95th percentile groupings. The stresses developed in the ergonomically designed chisel handle while in use were analysed to verify the validity of the design. This chisel handle was then manufactured, and preliminary evaluation using electromyography was conducted. In these tests, the stresses exerted on the flexor and extensor muscles of the arm were measured and compared with those obtained during the use of a conventional handle. Under similar working conditions, results clearly showed that the ergonomically designed handle allows higher working efficiency than existing handles.     

Stability of sitting postures: the influence of degrees of freedom

Observational studies of sitting have shown that, during spontaneous sitting, people adopt a variety of postures. Various researchers have formulated theories to explain why people adopt their sitting postures. Branton (1969) hypothesized that there is continual need for postural stability while sitting. Dempster (1955) stated that additional stability could be obtained through temporarily closing chains of body segments, or, in other words, through decreasing the number of degrees of freedom of the body. The present study elaborates on Dempster's theory. The aim of this study was to determine the influence of the degrees of freedom of the body on postural stability in sitting postures. For 21 different sitting postures, the total number of degrees of freedom was determined. Postural sway, a measure for postural stability, was determined using a 3D motion and position measurement system with ten healthy subjects. This study shows that the mean path length at the level of the second thoracic vertebra (PL0.05), a measure derived from postural sway, increases significantly (p < 0.0001) with an increase of the number of degrees of freedom of the body (DoFB). Closer examination of the data showed that a model taking into account only the degrees of freedom of the lumbar and thoracic spine and pelvis seems to be a better predictor of postural sway than the total number of degrees of freedom of the body.     

Postural stability effects of random vibration at the feet of construction workers in simulated elevation

The risk of falls from height on a construction site increases under conditions which degrade workers’ postural control. At elevation, workers depend heavily on sensory information from their feet to maintain balance. The study tested two hypotheses: “sensory enhancement” – sub-sensory (undetectable) random mechanical vibrations at the plantar surface of the feet can improve worker’s balance at elevation; and “sensory suppression” – supra-sensory (detectable) random mechanical vibrations can have a degrading effect on balance in the same experimental settings.Six young (age 20–35) and six aging (age 45–60) construction workers were tested while standing in standard and semi-tandem postures on instrumented gel insoles. The insoles applied sub- or supra-sensory levels of random mechanical vibrations to the feet. The tests were conducted in a surround-screen virtual reality system, which simulated a narrow plank at elevation on a construction site. Upper body kinematics was assessed with a motion-measurement system. Postural stability effects were evaluated by conventional and statistical mechanics sway measures, as well as trunk angular displacement parameters.Analysis of variance did not confirm the “sensory enhancement” hypothesis, but provided evidence for the “sensory suppression” hypothesis. The supra-sensory vibration had a destabilizing effect, which was considerably stronger in the semi-tandem posture and affected most of the sway variables.Sensory suppression associated with elevated vibration levels on a construction site may increase the danger of losing balance. Construction workers at elevation, e.g., on a beam or narrow plank might be at increased risk of fall if they can detect vibrations under their feet. To reduce the possibility of losing balance, mechanical vibration to supporting structures used as walking/working surfaces should be minimized when performing construction tasks at elevation.     

Effects of foot placement on postural stability of construction workers on stilts

Stilts are elevated tools that are frequently used by construction workers to raise workers 18-40 inches above the ground. The objective of this laboratory study was to evaluate the potential loss of postural stability associated with the use of stilts in various foot placements. Twenty construction workers with at least 1 year of experience in the use of stilts participated in this study. One Kistler™ force platform was used to collect kinetic data. Participants were tested under six-foot-placement conditions. These 6 experimental conditions were statically tested under all combinations of 3 levels of elevation: 0″ (no stilts), 24″ stilt height and 40″ stilt height. SAS mixed procedure was used to evaluate the effect of different experimental conditions. The results of the multivariate analysis of variance (MANOVA) and repeated measures of univariate analyses of variance (ANOVAs) demonstrated that stilt height, foot-placement direction, and foot-placement width all had significant effects on the whole-body postural stability. This study found that the higher the stilts were elevated, the greater the postural instability. A stance position with one foot placed forward of the other foot produced greater postural instability than a position with the feet parallel and directly beneath the body. This study found that placement of the feet parallel and directly beneath the body, with the feet positioned a half shoulder width apart, caused a greater amount of postural sway and instability than one and one-and-half shoulder width. This study also found that construction workers using the stilts could perceive the likely postural instability due to the change in foot placements.     

The effects of safety handrails and the heights of scaffolds on the subjective and objective evaluation of postural stability and cardiovascular stress in novice and expert construction workers

Work performed on scaffolds carries the risk of falling that disproportionately threatens the safety and health of novice construction workers. Hence, objective measures of the postural stability, cardiovascular stress, and subjective difficulty in maintaining postural balance were evaluated for four expert and four novice construction workers performing a manual task in a standing posture on a scaffold with and without safety handrails at two different elevation heights. Based on a multivariate analysis of variance, the experience, scaffold height, and presence of a handrail were found to significantly affect measures of the postural stability and cardiovascular stress. At a lower level of worker experience, a higher scaffold height, and in the absence of a handrail (which may correspond to higher risk of a fall), postural stability was significantly reduced, while cardiovascular stress and subjective difficulties in maintaining postural balance increased. We emphasize the importance of training and handrails for fall prevention at construction sites.