Gary D. Langolf

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.

Effect of occupational exposure to elemental mercury on short term memory.

Previous studies have indicated that exposure to elemental mercury is associated with increased short term memory scanning time. In an effort to provide converging evidence that short term memory is one locus of the neurotoxic effect of mercury, two measures of short term memory capacity were used in this study. The first measure, the Wechsler digit span forward, was too imprecise and unreliable to detect any adverse effects. The second measure, an estimate of the workers 50% threshold for correct serial recall, was more satisfactory and provided evidence of a statistically significant decrease in short term memory capacity associated with increasing exposure to elemental mercury (based on a group of 26 workers, urinary mercury average 0.20 mg/l, range 0.0-0.51 mg/l). A replication study of another group of 60 workers was performed to confirm this apparent mercury related effect. Despite lower urinary mercury concentrations in this second group (0.11 mg/l average), a statistical association was again observed relating urine mercury to reduced short term memory capacity.

Asymptomatic sensorimotor polyneuropathy in workers exposed to elemental mercury

Neurologic and electrodiagnostic evaluations and urine mercury level determinations were performed on 138 chlor-alkali plant workers, some of whom were chronically exposed to inorganic mercury vapor. Eighteen subjects had a mild polyneuropathy on clinical examination. These subjects had significantly (p < 0.05) elevated urine mercury indexes, reduced sensation on quantitative testing, prolonged distal latencies with reduced sensory evoked response amplitudes, and increased likelihood of abnormal needle electromyography compared with the remaining 120 subjects. Similar results were found for subgroups matched by sex and age. We conclude that elemental mercury exposure is associated with a sensorimotor polyneuropathy of the axonal type; the degree of neurologic impairment appears related to the magnitude of exposure.

Elemental mercury exposure: peripheral neurotoxicity.

Nerve conduction tests were performed on the right ulnar nerve of factory workers exposed to elemental mercury vapour. Time integrated urine mercury indices were used to measure the degree of exposure. Workers with prolonged distal latencies had significantly higher urine mercury concentrations when compared with those with normal latencies. Significant correlations between increasing urine mercury concentrations and prolonged motor and sensory distal latencies were established. Elemental mercury can affect both motor and sensory peripheral nerve conduction and the degree of involvement may be related to time-integrated urine mercury concentrations.

Acute neurobehavioural effects of toluene.

An acute inhalation chamber study of 42 college students was performed to investigate the relation between exposure to 0, 75, and 150 ppm of toluene and changes in central nervous system function and symptoms. Paid subjects were exposed for seven hours over three days. Verbal and visual short term memory (Sternberg, digit span, Benton, pattern memory); perception (pattern recognition); psychomotor skill (simple reaction time, continuous performance, digit symbol, hand-eye coordination, finger tapping, and critical tracking); manual dexterity (one hole); mood (profile of mood scales (POMS]; fatigue (fatigue checklist); and verbal ability were evaluated at 0800, 1200, and 1600 hours. Voluntary symptoms and observations of sleep were collected daily. An analysis of variance and test for trend was performed on the difference and score for each concentration reflecting an eight hour workday where each subject was their own control. A 3 x 3 Latin square study design evaluated toluene effects simultaneously, controlling for learning across the three days and the solvent order. Intersubject variation in solvent uptake was monitored in breath and urine. A 5-10% decrement in performance was considered significant if it was consistent with a linear trend at p less than 0.05. Adverse performance at 150 ppm toluene was found at 6.0% for digit span, 12.1% for pattern recognition (latency), 5.0% for pattern memory (number correct), 6.5% for one hole, and 3.0% for critical tracking. The number of headaches and eye irritation also increased in a dose response manner. The greatest effect was found for an increasing number of observations of sleep. Overall, no clear pattern of neurobehavioural effects was found consistent with the type 1 central nervous system as classified by the World Health Organisation. Subtle acute effects, however, were found just below and above the ACGIH TLV of 100 ppm toluene, supporting the position that the guideline be lowered since the biological threshold of behavioural effects may be comparable with the TLV.

A neurological evaluation of workers exposed to mixtures of organic solvents

Workers with long term exposure to mixtures of organic solvents below regulatory limits have been reported to experience mild, but clinically detectable, sensory or sensorimotor polyneuropathies. In conjuction with a cross sectional study of behavioural performance a clinical neurological evaluation was conducted among printers and spray painters to examine dose response relations. All 240 subjects completed an occupational history and symptom questionnaire and underwent a clinical neurological examination. On average, subjects had been employed on their current job for six years. Classification of solvent exposure for each subject was based on exposed versus non-exposed job titles and observations during an industrial hygiene walk-through or on the measured concentration of solvents in full shift personal air samples. The average full shift solvent concentration was 302 ppm for printing plant workers and 6-13 ppm for workers at other plants. Isopropanol and hexane were the major constituents. Neurological abnormalities consistent with mild polyneuropathy were found in 16% of subjects; none was clinically significant. Exposed/non-exposed comparisons showed slightly higher frequency of symptoms in the exposed subjects which was not related to solvent level. Subjects categorised as exposed during the walk- through survey also had poorer vibratory sensation measured at the foot and diminished ankle reflexes. In multiple linear regression models, however, controlling for age, sex, alcohol intake, and examiner, no significant (p less than 0.05) relation was found between solvent concentration and poor neurological function except for two point discrimination measured at the foot. This investigation has not provided evidence for dose related adverse neurological effects from exposure to moderately low levels of solvent mixtures for a relatively short duration, although this may be due to the shortness of exposure duration, the type of solvent exposure, or to selection factors.

Arm Posture and Human Movement Capability

Many workers perform manual assembly tasks or use hand tools while the hands are postured above the shoulders or to the side of the body. Experiments were conducted to study the sensitivity of speed-accuracy movement performance to a wide range of hand locations around the shoulder under various levels of hand loads, ratios of work to rest, and task durations. Subjects performed a spatially constrained stylus-to-hole Fitts reciprocal movement task designed to simulate high-incentive manual assembly operations while providing basic information regarding changes in human move and positioning capabilities. Significant decrements in movement performance occurred when hands were postured above shoulder level. Move and positioning times increased 15.3% and 26.5%, respectively, with elevation of the hand from −15 to 60 deg respective to shoulder level. Posturally based decrements in movement capability were unrelated to differences in subject strength capability found among test postures. The consequences of elevated arm postures on human move and positioning capability are presented along with workplace design and methods recommendations for job designers facing work-height decisions in manual assembly environments.

SHOULDER POSTURAL FATIGUE AND DISCOMFORT A Preliminary Finding of No Relationship with Isometric Strength Capability in a Light-Weight Manual Assembly Task

Does greater strength capacity in the shoulder-complex afford increased protection against regionalized fatigue and discomfort induced by sustained awkward arm postures in light-weight manual assembly environments? This question was addressed by testing the relationship between differences in shoulder complex strength capacity, produced by variations in arm posture within a subject, and among subjects assuming equivalent arm postures, and severity of fatigue and discomfort sensed during a low-exertion manual performance task. Experimental findings showed that: (a) awkward arm postures produced substantial and rapid onset of postural fatigue and discomfort during a light-weight manual performance task where strength demands were low (i.e., less than 15 percent of maximum voluntary contraction (MVC)), (b) variations in strength capability found among arm postures within an individual subject, or 3mong subjects assuming the same arm posture, did not affect onset of substantial fatigue or discomfort when hands are postured near or above shoulder level, and (c) postures which simply appeared to be awkward, or which compromised strength capacity (e.g., working with the arm to the side of the body, or aligned in the coronal plane), did not necessarily increase discomfort of fatigue. Our findings suggest caution against sole reliance upon population or individual worker upper-extremity strength capabilities as predictors of fatigue and discomfort in the shoulder complex when manual exertions are small (e.g., light-weight manual assembly activities involving small parts or small hand-tools) and hands are postured at or above shoulder level.

The Use of Sternberg's Memory-Scanning Paradigm in Assessing Effects of Chemical Exposure

This study evaluated the utility of a binary classification task in assessing the neurotoxic effects of elemental mercury. Twenty-six mercury cell chlor-alkali workers were tested on two occasions. This reaction-time paradigm provided reliable, stage-specific measures of cognitive functioning. In the two stages studied, processing times were found to increase significantly with increasing levels of exposure to mercury. The stage-specific nature of the measures used permits the inference that mercury has a locus of effect in the central nervous system.

Quantification of Human Performance Circadian Rhythms

The quantification of worker performance changes during a shift is critical to establishing worker productivity. This investigation examined the existence of circadian rhythms in response variables that relate most meaningfully to the physiological and neurological state of the body for three subjects maintaining a resting posture for 25 hours on five separate occasions. Significant circadian variation ranging from 3% to 11% of the mean value was detected for elbow flexion strength, physiological tremor, simple reaction time, information processing rate and critical eye-hand tracking capacity.