Long hours of computer work in the modern workplace are associated with visual and musculoskeletal symptoms and disorders, which are recognized by the World Health Organization as major occupational health concerns. The rising age of the workforce imposes new challenges in office ergonomics due to the changes in vision and musculoskeletal characteristics of these computer users.

To investigate the effects and mechanisms of oculomotor demands on the most common complaints associated with prolonged computer use, blurred vision, eyestrain, and irritation of the eyes, a within-subject design laboratory study with 17 young subjects was conducted. Subjective ratings of symptoms and objective static and dynamic accommodation and convergence responses were recorded during and after a 2-hour visually demanding computer task. Each subject repeated the task over 6 days using different combinations of natural and optically-modified oculomotor demands. While there was little effect of viewing distance on symptoms, an elevated static accommodation response and a reduced dynamic divergence response were found for the natural near viewing distance condition (33 cm) compared to the far viewing distance (100 cm). Optical manipulation had no effect at the far viewing distance (100 cm), whereas artificially decreasing the accommodation demand at the near viewing distance increased eyestrain. Divergence limits was identified as a factor for predicting the symptom of eyestrain.

A second within-subject, full-factorial design laboratory study with 19 young and 7 older, presbyopic subjects investigated the effects of age, font size, and reflective glare on productivity, upper body posture, and visual and upper body discomfort, in a setting where the subjects could freely adjust their posture and chair position while performing visually demanding tasks on a computer. Compared to the larger font size, the smaller fonts had significant negative effects on all outcome measures: productivity (speed and accuracy) was lower, visual and neck symptoms were higher, and viewing distance was decreased. The reduced viewing distance was primarily due to forward torso flexion (78%), followed by moving the chair forward (4%) and forward head movement (3%). Screen glare induced non-neutral upper body postures, including neck flexion, torso forward flexion, and head movement to the side. There was little effect of age and presbyopia on the study findings.

In summary, the first study demonstrated no difference in visual symptoms when viewing a computer monitor for 2 hours at 33 vs. 100 cm distance. However, there was some evidence of persistent oculomotor effects at the 33 cm distance. The second study demonstrated the benefits of a larger font size and eliminating screen glare on productivity, upper body postures, and symptoms when performing visually intensive tasks. The benefits were similarly experienced by both the younger and older computer users. The non-neutral, forward leaning postures, observed in many computer users, are likely due to font sizes that are small relative to visual acuity.

Abstract

Association between wrist angle and carpal tunnel syndrome among workers

by

Doohee You

Doctor of Philosophy in Environmental Health Science

University of California, Berkeley

Professor David Rempel, Chair

Introduction

Carpal Tunnel Syndrome (CTS) is the most common work-related peripheral neuropathy among workers who perform hand-intensive tasks. It has a greater impact on time away from work and disability than almost all other workplace injuries. For prevention, the National Institute of Occupational Safety and Health recommends reducing four primary biomechanical risk factors: hand tool vibration, grip and pinch force, repetitive hand exertions, and awkward wrist posture. However, the findings from studies evaluating the relationship between wrist posture and CTS are inconsistent. Few studies have measured wrist postures of workers and very few studies have been prospective in design. This dissertation presents a meta-analysis evaluating the effects of wrist posture on CTS among workers followed by an analysis of wrist posture risk from a large prospective study of blue-collar workers.

Method

A meta-analysis reviewed the findings from prior studies that investigated the association between wrist posture and CTS and calculated a pooled relative risk.

In addition, data was collected from a 28-month prospective study of 447 blue-collar workers at four industrial sites in the U.S. These workers were followed with symptom surveys and nerve conduction studies at the wrist to identify incident cases of CTS. Video recordings of the workers performing their usual tasks were analyzed for grip type and wrist posture. Inter-observer reliability of wrist posture measurement was examined using Interclass Correlation Coefficients (ICC) and Kappa coefficient. Using each worker's task exposure and estimated time on task, a time weighted average exposure metric was calculated for several measures of wrist posture. A survival analysis assessed the relationship between wrist postural risk factors and incident cases of CTS. The final multivariable models were adjusted for confounding.

Result

Nine epidemiologic studies met the inclusion criteria for the meta-analysis. The pooled relative risk of work-related CTS increased with increasing hours of exposure to wrist deviation, or extension/flexion [RR = 2.01; 95% CI: 1.646-2.43; p < 0.01; Shore adjusted 95% CI 1.32 to 2.97]. The inter-rater reliability for wrist posture measures from video found moderate (Kappa = 0.48) to substantial (Kappa = 0.64) agreement between observers across the five different types of grip. The ICC ranged from 0.57 and 0.67 with the highest correlation for high-force power grip.

Job level measures of wrist posture were merged with other data from the prospective study. During the 28 months of follow-up there were 29 incident cases of dominant hand CTS. Multivariable analysis indicated that the hazard ratio for CTS more than doubled with the combined exposures to wrist extension and forceful (> 10 N) pinch. The HR, after adjusting for age, gender, BMI, and medical history, was 2.83 (95% CI 0.91-8.76; p-value <0.2), where the cutoff between low and high exposure was 6.07 degree mean wrist extension.

Discussion and conclusion

We found that exposure to wrist extension, especially during the application of forceful pinch grip increased the risk of CTS. A new method for estimating posture from videos was developed and found to have good inter-rater reliability; levels of reliability that are higher than prior studies. Recommendations are made for video analysis methods for estimating wrist posture. The analysis of the prospective data suggests that reducing mean wrist extension, if above 6 degrees, or reducing the combination of wrist extension and high-force pinching will reduce CTS risk among blue-collar workers in hand intensive jobs. Workplace interventions should incorporate training and engineering interventions to reduce sustained exposure to these factors. These findings may be useful in the design of workplace safety programs to prevent CTS in order to keep our workers healthy and productive.

The goal of this dissertation was to investigate how mechanical loading affected gene expression in tendons. A better understanding of which genes are affected by load may help identify proteins which are involved in the initial response to tendon overuse. Characterizing the gene expression response to load may help identify threshold limits to avoid overuse injuries.

Gene expression levels for eight matrix molecules were quantified for tendon tissue obtained from rabbits exposed to cyclic load. No significant differences in expression levels were found. Long term repetitive loading may not lead to up-regulation at the transcriptional level of these eight genes or localized changes were missed due to homogenization of the tissue samples. RT-PCR may be an inappropriate technique for measuring localized changes in gene expression.

A novel in vitro tissue loading system was designed capable of simultaneously applying specific loading patterns to six tissues under force or displacement control. Loading parameters such as amplitude, rate and frequency can be controlled while biomechanical factors such as creep, force relaxation and modulus can be affected and monitored.

Employing the in vitro tissue loading system, rabbit flexor tendons were exposed to static loads of 0 (stress-shielded), 2 (low), 4 (moderate) or 6 (high) MPa for 20 hours. Gene expression levels for collagenase-1 (MMP-1), interlukin-1b (IL-1b) and cyclooxegenase-2 (COX-2) were measured. High static loads up-regulated MMP-1 expression compared to moderate loads. Correlations found between creep and the expression of MMP-1 and IL-1B suggests that the pathway for their expression, leading eventually to tendon degeneration, may be regulated by the biomechanical factor creep.

These findings indicate in vitro culture systems can be a useful tool in examining the effects of loading on connective tissues, allowing investigators to measure the initial cellular response to mechanical load. Changes in gene expression may indicate possible threshold limits where loading leads to a remodeling response.

Introduction

Wrist tendinosis is a common distal upper extremity disorder that can be work-related. Workplace cross-sectional (Franzblau, 2005) and prospective studies (Leclerc 2001, Thomsen 2007) have linked distal upper extremity pain and disorders to forceful or repetitive hand exertions, but the associations are not consistent across studies. A study rigorously assessing both workplace physical exposures and wrist tendinosis would be useful in clarifying the dose-response relationships which could provide guidance for prevention. This dissertation presents the results from a prospective study of right wrist tendinosis among blue-collar workers, and includes a comparison of exposure measurement methods, a cross-sectional analysis of the cohort at baseline, and a prospective analysis of the association between exposure to physical risk factors and the subsequent development of right wrist tendinosis during a 28 month follow up period.

Methods

Workers (N=450) at four manufacturing industries participated in this study and were followed up for up to 28 months (n=413) with questionnaires and physical examinations every 4 months to identify incident cases of right wrist tendinosis. Workers were eligible to participate if they performed primarily hand intensive manual (not office) work and were not assigned to more than four tasks. Exposure assessment was based on job evaluations by experienced ergonomists and video analyses of tasks. A variety of self-report, observer-rated, direct measure, and video analysis methods were used to quantify workers exposure to force and repetition at baseline. Detailed video analysis determined percent time the right hand applied a high force pinch (> 1 kg-force) or power grip (>4kg-force) for each task. No load and low force pinch (<= 1 kg-force) and grasp (<=4kg-force) were also determined. Repetition was quantified by observer-rated hand activity level (HAL) scale and video analysis of repetition rate (reps/min) in heavy or light grip or pinch postures. A comparison of each worker's longest duration task was performed using the Pearson correlation. All exposure variables were measured on the individual task level and time weighted averages for overall exposure were calculated for each worker. Logistic regression was used to model the log odds of tendonitis at baseline, as a function of exposure variables and relevant confounders. Survival analysis using the Cox Proportional Hazards Model was used to assess the relationship between individual and workplace factors and wrist tendinosis for all prospective data. Force, repetition, and composite measures with p<0.2 in the univariate analyses were evaluated in multivariate models. Potential covariates with p<0.2 in the univariate analysis or a correlation (r>0.3) with the exposure variable of interest were included in the multivariate model and consecutively removed and replaced to assess for confounding.

Results

There were 37 cases of right wrist tendinosis at baseline and 26 incident cases during the 28-month follow up. Analysis of exposure measures revealed correlations (r=0.63) between the visual analog scale (VAS) for hand fatigue and the rate of perceived exertion (RPE). The HAL repetition scale and the Speed of Work observer-rated scales were moderately correlated (r=0.65). The scales' stronger correlation with the percent time spent in grip (r= 0.41) versus pinch (r=-0.25) could be based on the difficulty discerning distinct repetitions while in pinch.

For the prospective analysis, job satisfaction, smoking status, annual income, BMI, education level, job strain, and general health were not significant predictors in the univariate analysis and were therefore not included in multivariate models. Age, gender (HR= 4.8; 95%CI: 2.01-11.45), ethnicity, medical condition, and shift were all assessed for confounding, though only age and gender remained in the final models. The two most predictive models included the percent time spent in high force pinch adjusting for repetition (HR = 2.27; 95% CI: 0.72-7.19 and HR=3.42; 95%CI: 0.9-13.01 for medium and high groups), and the ACGIH TLV Hand Activity Level (HR = 3.95; 95% CI: 1.52-10.26 and HR=2.8; 95%CI: 0.8-9.87 for medium and high groups).

Discussion

In this prospective study, the workplace factors predicting wrist tendinosis were time weighted average values of the percent time spent performing a high force pinch, the normalized peak force, and the ACGIH TLV Hand Activity Level. The percent time spent in power grip was not a significant workplace predictor, nor were any of the measures of repetition. Individual factors including job satisfaction, general health, and BMI were not significant predictors, but gender was. The most predictive measure of force was the video of the work task quantified for the percent time spent in high force pinch; the relationship followed a dose-response pattern. Based on this study, in order to prevent wrist tendinosis, it is recommended that the tasks of workers performing repetitive, hand intensive tasks be designed so that less than 10% of the task time requires pinch force greater than 1 kg.

Background

Methods

Results

Conclusion