Time to onset of pain: effects of magnitude and location for static pressures applied to the plantar foot.

Mechanisms that cause foot discomfort during prolonged standing are poorly understood. There is currently no method for evaluating discomfort associated with low levels of static pressure that are typical during standing. Pain thresholds were measured for 20 healthy participants by applying five levels of static pressure at different plantar foot locations. A survival analysis was performed to determine the effects of pressure magnitude and foot location on the time until pain onset. Time to pain onset was significantly affected by pressure magnitude (P < 0.001); time decreased as pressure increased. Foot location was also significant (P < 0.001); greatest times to pain onset (least sensitive) were observed under the heel and fifth metatarsal head, shortest times (most sensitive) were found under the midfoot. This research presents a novel methodology for evaluating static pressure that may be applicable to product design.

Effects of anti-fatigue mats on perceived discomfort and weight-shifting during prolonged standing.

OBJECTIVE: The aim of this experiment was to investigate the effects of anti-fatigue mats on perceived discomfort and behavioral responses (weight-shifting between the feet) during prolonged standing. BACKGROUND: Prolonged standing is a common requirement in the workplace and is a well-known cause of discomfort. Anti-fatigue mats have been shown to reduce discomfort resulting from standing, but no study has identified a particular mat that performs better than others or examined the relationship between discomfort and weight-shifting. METHODS: Participants stood for 4 hours on four commercially available "anti-fatigue" mats and a hard surface (control condition). Subjective ratings of discomfort were measured, and in-shoe pressure was recorded and used to evaluate weight-shifting during standing. RESULTS: Compared to the control condition, after 4 hours of standing discomfort was reduced by three of the four mats, but discomfort ratings did not significantly differ among mats. However, significant differences among mats were found in the frequency of weight-shifting, and weight-shifting was positively correlated to discomfort. CONCLUSION: These results suggest that subjective reports of discomfort were not sufficiently sensitive to detect differences among mats for the experimental conditions tested. Behavioral responses, specifically weight-shifting between feet, may provide a more sensitive alternative to subjective reports.

The effect of prolonged standing on touch sensitivity threshold of the foot: a pilot study.

OBJECTIVE: To determine the effect of prolonged standing on touch sensitivity of the foot. DESIGN: An observational study with replications. SETTING: University laboratory. PARTICIPANTS: Ten healthy college students (5 men and 5 women), with a mean ± SD age of 23.5 ± 4.1 years and body mass of 67.4 ± 12.6 kg. METHODS: Semmes-Weinstein monofilament tests were administered to 12 locations on the dorsal and plantar surfaces of the foot before and after 4 hours of standing. These locations were formed into several groupings (toes, metatarsal heads, midfoot, heel, all plantar sites, all dorsal sites), and paired t-tests were used to test for significant changes in sensitivity threshold after standing. MAIN OUTCOME MEASUREMENT: The difference between sensitivity thresholds measured before and after standing for different locations on the foot. RESULTS: The average of all sensitivity thresholds on the plantar surface of the foot decreased (indicating increased sensitivity) from 0.56 to 0.36 g (P < .01) after 4 hours of prolonged standing. This change in threshold equated to a difference of 1 Semmes-Weinstein monofilament level. Changes in the sensitivity threshold of the dorsal aspect of the foot were not significant. CONCLUSIONS: Analysis of the results suggests that the plantar foot has greater sensitivity to touch after prolonged standing. These findings may be useful for identifying potential unintended bias in clinical touch sensitivity testing. Future research is necessary to understand the underlying mechanisms for this sensitivity change and to determine the onset and recovery times for sensitivity changes.

Risk factors for lower extremity fatigue among assembly plant workers.

BACKGROUND: Work-related fatigue of the lower extremities is a known cause of lost productivity and significant employer costs. Common workplace solutions to reduce fatigue levels include anti-fatigue matting, shoe orthoses, or sit/stand work stations. However, assessment of these anti-fatigue measures within the workplace has been limited. METHODS: This was a cross sectional study in an automotive assembly plant on employees with at least 6 months tenure. Subject data were collected via questionnaires including Likert-scale questions to define fatigue severity. Jobs were evaluated for lower extremity ergonomic exposures via videotaping, pedometers, interviews, and industrial engineering records. RESULTS: Lower extremity fatigue at the end of the work day was associated with a higher prevalence of smoking, rheumatoid arthritis, job dissatisfaction, use of shoes with firmer outsoles, and increased time on the job spent standing or walking. Supervisor support and increased time spent on carpet were protective. Lower extremity fatigue that interfered with activities outside of work had additional risk factors including higher BMI, prior diagnosis of osteoarthritis, and increased hours per week spent working. CONCLUSIONS: While these results identify carpet as being protective against lower extremity fatigue, no similar relationship was identified for anti-fatigue mats. No adverse relationship was found between hard surfaces such as concrete and lower extremity fatigue. Given the high costs associated with work-related fatigue, future areas for potential intervention include smoking cessation, specific shoe recommendations, and enhancing psychosocial aspects of work such as supervisor support.

Risk factors for foot and ankle disorders among assembly plant workers.

BACKGROUND: Jobs that necessitate prolonged standing and walking activities are commonly associated with worker's complaints of foot and ankle pain. The objective of this study was to determine the relative contributions of work activity (time spent standing, walking, or sitting), floor surface characteristics, weight, BMI, age, foot biomechanics, and other demographic and medical history factors to the prevalence of foot and ankle disorders. METHODS: A cross-sectional observational study design was used to evaluate employees of an engine manufacturing plant. The main outcome variable was foot or ankle disorders defined by pain and a positive physical examination. The independent variables included baseline demographics, medical history, ergonomic exposures, psychosocial factors, shoe characteristics and foot biomechanics. RESULTS: Twenty-four percent of the cohort met the case definition of foot/ankle disorder with 10% defined as new cases. Fifty-two percent had symptoms of foot/ankle. An increased risk of presenting with foot/ankle disorders was associated with high metatarsal pressure on gait assessment, increased time spent walking, female gender, reported high job dissatisfaction, a history of rheumatoid arthritis, osteoporosis or vascular disorder. For the truck/forklift drivers, an increased number of times getting in and out of the vehicle was associated with a higher prevalence of ankle/foot problems. CONCLUSIONS: An increased risk is associated with higher metatarsal pressure and increased time spent walking. These findings suggest several options for primary and secondary prevention strategies. The use of shoe orthoses with a medial longitudinal arch and metatarsal pad as well as including optional sit/stand workstations may be helpful.

Inter-worker variability in lower body postures during assembly line work: implications for exposure assessment.

This study evaluated inter-worker variability in lower body posture and work activity during highly-structured assembly line work. Data were collected from 79 unique assembly line workstations in an engine manufacturing plant. Because the plant utilized work teams, 4-8 workers rotated through each workstation. At least 30 min of videotape was collected from at least three workers at each workstation. A computer-assisted work sampling procedure randomly selected 200 video "freeze-frames" for each worker. Lower body posture/movement (e.g., sit, stand, walk, etc.) was determined for each frame and used to estimate the percentage of time the worker spent in various postures and activities. Chi-square analyses were performed for each workstation to assess the significance of inter-worker differences. Due to variations in individual work methods, significant differences (p <.05) were found at 57 out of 79 workstations (72%). The greatest differences occurred when workers had the option to choose between standing and sitting (significant in 8 of 8 cases; in extreme examples, sit time ranged between 0-100% on one job, and 6.5-98% on another). Studying a single worker (or "proxy") can contribute to substantial error when estimating exposures in workplace studies of ergonomic stressors, since the proxy may not be representative of all workers who perform the job. Individual measurements are preferable, particularly for jobs where workers have substantial latitude to develop individualized work methods.

The effects of insertion method and force on hand clearance envelopes for rubber hose insertion tasks.

OBJECTIVE: The aim of this study was to determine how hand space for manual insertion of flexible hoses is affected by insertion method and force. BACKGROUND: Adequate space is needed during assembly tasks in which workers join parts together with their hands. Hose installations are an example of such a task. Hand clearance envelopes for insertion tasks that involve cylindrical objects, such as a hose, are currently unavailable in the literature. METHODS: Participants inserted a flexible 25-mm rubber hose onto a stationary flange using simulated methods similar to those observed in field studies of automotive assembly tasks. Markers placed on the back of the hand and wrists were used to measure postures during the insertion task. RESULTS: Hand clearance envelopes for high-force insertions were significantly larger across methods by an average of 15% for both male (p < .05) and female (p < .01) participants. Rocking insertions resulted in the largest hand clearance envelopes compared with other insertion methods. Rocking and twisting the hose resulted in mean increases in the cross-sectional area of the hand clearance envelopes of 35% and 24%, respectively, compared with the straight push. Differences were significant (p < .05) for male and female participants for both bead height conditions. CONCLUSION: Both required insertion force and method affect hand clearance envelopes during simulated insertions. APPLICATION: These methods can be used by engineers to determine if there is adequate clearance for the hand to grip selected objects.

Analysis of the Third National Health and Nutrition Examination Survey (NHANES III) using expert ratings of job categories.

BACKGROUND: Few epidemiologic studies have addressed the exposure-response relationships between work activities and symptomatic knee osteoarthritis (OA). This analysis used data from a national survey and ergonomists' ratings to address this issue. METHODS: Interview and knee X-ray data were obtained from the Third National Health and Nutrition Examination Survey. Occupational ratings were obtained using ergonomists. A weighted logistic regression was used. RESULTS: Among men, a significant exposure-response relationship was found between symptomatic knee OA and kneeling. In both genders, there was a significant trend in heavy lifting and severe symptomatic knee OA. Approximately 20.7% of knee OA can be attributed to kneeling >14% of the workday among men. CONCLUSIONS: The significant exposure-response relationships suggest that modest reductions in certain occupational activities can reduce the burden of knee OA. The study was limited by unvalidated expert ratings. Research is needed to identify hazardous characteristics of work activities and to clarify exposure-response relationships.

Expert consensus ratings of job categories from the Third National Health and Nutrition Examination Survey (NHANES III).

BACKGROUND: A method of occupational physical exposure assessment is needed to improve analyses using large data sets (e.g., national surveys) that provide only job title/category information as a proxy for exposure. METHODS: Five ergonomic experts rated and arrived at consensus ratings for job categories used in the Third National Health and Nutrition Examination Survey. Interrater agreement was examined for initial (pre-consensus) ratings. Correlation between consensus ratings and an independent source of ratings (US Department of Labor (DOL)) was used as a basis of comparison. RESULTS: Interrater agreements for the initial ratings were weak. Highest interrater agreement was for sitting (weighted kappa (kappa(w)) = 0.56). Lowest agreement was for standing (kappa(w) = 0.07). Consensus ratings were well correlated with DOL ratings (correlation coefficients >or=0.6). CONCLUSIONS: The correlation between consensus ratings and DOL ratings support the use of expert consensus to assess physical exposures in national data sets. However, validation of expert consensus ratings is required.

Using process control concepts to model conditions required for sudden-onset occupational injuries.

Sudden-onset injury results from a momentary energy exchange between an agent and host, producing immediately discernible tissue damage. These injuries are common in both occupational and nonoccupational settings; typical causes include falls, mechanical contact/crushing, exposure to temperature extremes, and exposure to electrical current. We review epidemiologic and engineering approaches to injury prevention and propose a process control model for describing risk-of-exposure to injury agents during the Pre-event phase of sudden-onset injury. Process control is a proactive approach to quality engineering that is based on the premise of preventing defective products from being manufactured in the first place, instead of relying on reactive inspections to detect defects at the end of the manufacturing process. Principles of process control can be applied by occupational health and safety professionals to prevent workplace injury. The proposed model describes how work activities (process inputs) cause risk-of-exposure to injury agents to fluctuate over the course of a work shift. Risk-of-exposure is a complex function with many input factors including: the nature/magnitude of hazards, the presence and effectiveness of engineering controls, safety climate, management attitudes and practices, the surrounding work environment, the physical and mental states of the worker, and the quality and quantity of supervision and training. Injury can occur only when this function crosses a certain threshold and the host is exposed to injurious energy via physical contact. Certain factors that contribute to risk-of-exposure are stable for extended time periods (weeks, months, years), whereas other factors are transient (durations of minutes or seconds). The model extends classical work by Haddon and others, provides preliminary insights to designing epidemiologic studies and developing fault-tolerant work systems, and illustrates how interdisciplinary approaches can improve our understanding of the etiology of sudden-onset injuries and advancing injury research.

Effects of low back disability status on lower back discomfort during sustained and cyclical trunk flexion.

A laboratory study was conducted to determine the effects of back disability status on endurance time and perceived discomfort during trunk flexion. Eighty participants (40 with chronic or recurrent low back pain (CRLBP), 40 pain-free) were tested. The trunk was flexed to 15 degrees, 30 degrees, 45 degrees and 60 degrees under three conditions: 1) continuous static flexion; 2) cyclical flexion with 20% rest; and 3) cyclical flexion with 40% rest. Each condition was performed for up to 600 s or until the participant reached his/her pain tolerance limit. Dependent variables included time to distracting discomfort (TDD), total endurance time (TET) and perceived discomfort. For continuous exertions, CRLBP participants had lower TDD (p < 0.001), lower TET (p < 0.001) and greater discomfort (p < 0.001) compared to pain-free controls. In both groups, TDD and TET decreased and perceived discomfort increased as the flexion angle increased. For intermittent exertions, CRLBP participants reported greater discomfort than pain-free participants (p < 0.001). Increasing rest from 20 to 40% reduced discomfort in CRLBP participants, but produced no consistent benefit in pain-free participants. To accommodate persons with CRLBP, consideration should be given to reducing both the magnitude (angle) and duration of trunk flexion required by their jobs.

Case studies of ergonomic interventions in automotive parts distribution operations.

Ergonomic job analysis, intervention design, and intervention implementation are essential components of an ergonomics program designed to reduce worker exposure to risk factors associated with musculoskeletal disorders. As part of a 4-year study to reduce overexertion injuries in the service parts division of a major automaker, intervention case studies were identified that could be used at multiple facilities across the division. Interventions were developed and implemented. The three case studies include 1) self-elevating powered vehicle for transporting parts throughout the facility and for reaching to high bin locations; 2) lift and tilt pallet jacks for packing small parts into large bin-like containers; and 3) single-level telescoping conveyor used for delivering hand-held totes for subsequent sorting operations. Several analysis methods were used to assess worker exposure before and after intervention implementation (biomechanical analysis, posture analysis, worker interviews, and activity analysis). Following implementation, a decrease in exposure to risk factors for musculoskeletal disorders was documented. Worker interviews revealed acceptance and agreement that risk factors associated with the targeted tasks were reduced. Each case study includes a description of the implementation hurdles and can serve as both primary and secondary prevention of musculoskeletal disorders. Future work should document worker health and/or symptom changes as well as changes in risk factor exposure.