Assessment of physical work load in epidemiologic studies: concepts,issues and operational considerations

Ergonomic epidemiology is a rapidly increasing field of research providing data on the occurrence of musculoskeletal disorders and possible risk factors. The present paper states, on the basis of a literature overview, that physical work load (mechanical exposure) is poorly denned and measured in most studies on ergonomic epidemiology. On this background the paper: (1) suggests adjustments of mechanical exposure concepts and terminology; (2) concludes that invalid exposure assessment may, to a large extent, explain the lack of quantitative data on relationships between mechanical exposures and musculoskeletal disorders; and (3) suggests some guidelines for future quantitative assessments of mechanical exposure in large populations.     

Effect of block weight on work demands and physical workload during masonry work

The effect of block weight on work demands and physical workload was determined for masons who laid sandstone building blocks over the course of a full work day. Three groups of five sandstone block masons participated. Each group worked with a different block weight: 11 kg, 14 kg or 16 kg. Productivity and durations of tasks and activities were assessed through real time observations at the work site. Energetic workload was also assessed through monitoring the heart rate and oxygen consumption at the work site. Spinal load of the low back was estimated by calculating the cumulated elastic energy stored in the lumbar spine using durations of activities and previous data on corresponding compression forces. Block weight had no effect on productivity, duration or frequency of tasks and activities, energetic workload or cumulative spinal load. Working with any of the block weights exceeded exposure guidelines for work demands and physical workload. This implies that, regardless of block weight in the range of 11 to 16 kg, mechanical lifting equipment or devices to adjust work height should be implemented to substantially lower the risk of low back injuries.     

Job demands, physical fitness, work ability, and age of vehicle inspectors

The aim of this field, laboratory, and questionnaire study was to quantify and compare job demands, physical fitness, and work ability of aging (over 45 years) and young male vehicle inspectors. Dynamic and perceived job demands were moderately low, whereas static load on the neck-shoulder region was high due to bent and/or twisted head and neck postures which occurred at an average of 42% of working hours. Physical job demands were about equal for aging and young subjects. Aging subjects had lower aerobic and muscular fitness than young ones. A reduced work ability was detected in 14% of the aging inspectors. Technical and organizational measures are necessary to reduce static load and fixed repetitiveness of the job. Furthermore, individual physical training is recommended to maintain work ability of both the aging and young vehicle inspectors.     

Influence of material handling devices on the physical load during the end assembly of cars

The effect of using a mechanical device on physical load was analysed during the end assembly of cars. Five tasks, further sub-divided into lifting and positioning, were performed by someone experienced in working with and without the mechanical tool. A practical methodology was used: the subjective walk and talk through method, the NIOSH equation (1991), a three-dimensional goniometer (Back Tracker) and surface electromyography (EMG) of the relevant muscle groups. According to the NIOSH equation, 8 out of 10 of the tasks should only be performed by trained workers and preferably with tools. The Back Tracker revealed that the mechanical tools reduced posture angles significantly (p0.05) for installing the windows, but not for left lateroflexion during pre-assembly of windows. It was clear from the EMG analysis that using tools did not always decrease muscular activity significantly. In general, the appropriateness of each tool needs to be evaluated separately taking the subjective criteria into account.

Relevance to industry

In this study, the relevance of using a mechanical device to reduce the physical load during car assembly, is evaluated. As the tools are expensive, it is important to the industry whether they are being used and how efficient they are.     

Efficacy of adjusting working height and mechanizing of transport on physical work demands and local discomfort in construction work

The efficacy of ergonomics measures to reduce physical work demands in a real working situation is often assumed, but seldom studied. In this study, the effect of adjusting working height and mechanization of transport on physical work demands and local discomfort of bricklayers' work was evaluated during a field experiment in the construction industry. In a within-subjects controlled experiment, 10 bricklayers and 10 bricklayers' assistants worked in two different conditions. Working height of bricks and mortar, and transport of materials were manipulated. The physical work demands were assessed through real time observations at the work site. Local discomfort of the lower back and of the shoulder region was measured by means of a visual analogue scale. Working with a scaffolding console to adjust the working height of the storage of materials resulted in a significant reduction of the frequency and duration of trunk flexion (?>?60°) by 79% and 52% respectively, compared with bricks set out on the ground floor. Mechanization of transport of materials resulted in a significant reduction of the frequency and duration of trunk flexion (?>?60°) by 94% and 92% respectively, compared with the condition of manual handling. The frequency of handling objects (?>?4?kg) reduced significantly by 86%. Local discomfort of the lower back was significantly less in the ergonomic conditions, while no significant difference was found for local discomfort of the shoulder between both conditions in bricklayers' assistants.     

Life-time occupational exposure to heavy work and individual physical capacity

The aim of the present study was to retrospectively analyze the effect of occupational physical activity on maximal isometric hand grip strength and maximal oxygen consumption among males and females between 19 and 64 years of age in different occupations. A life-time occupational physical activity index was formed from questions in a questionnaire. The maximal isometric hand grip strength was measured with a dynamometer and maximal oxygen consumption was estimated from a submaximal bicycle ergometer test. The results showed a negative correlation between physical activity and estimated maximal oxygen consumption among males but no other statistically significant associations between life-time physical activity and the present physical capacity was found. The present results suggest that a high level of occupational physical activity does not maintain individual physical capacity.     

Reliable exposure assessment strategies for physical ergonomics stressors in construction and other non-routinized work

The objective of this research was to provide guidelines for the reliable assessment of ergonomics exposures in non-routinized work. Using a discrete-interval observational sampling approach, two or three observers collected a total of 5852 observations on tasks performed by three construction trades (iron workers, carpenters and labourers) for periods of several weeks. For each observation, nine exposure variables associated with awkward body postures, tool use and load handling were recorded. The frequency of exposure to each variable was calculated for each worker during each of the tasks on each of the days. ANOVA was used to assess the importance of task in explaining between-worker and within-worker variability in exposures across days. A statistical re-sampling method (bootstrap) was used to evaluate the reliability of exposure estimates for groups of workers performing the same task for different sampling periods. Most exposures were found to vary significantly across construction tasks within trade, and between-worker exposure variability was generally smaller than within-worker exposure variability within task. Bootstrapping showed that the reliability of the group estimates exposure for the most variable exposures within task tended to improve as the assessment periods approached 5 – 6 d, with marginal improvements for longer assessment periods. Reliable group estimates of exposure for the least variable exposures within task were obtained with 1 or 2 d of observation. The results of this study demonstrate that an initial estimate of the important environmental or task sources of exposure variability can be used to develop an efficient sampling strategy that provides reliable estimates of ergonomics exposures during non-routinized work.     

The relationship between work ability and oxidative stress in Japanese workers

Work ability is based on the balance between personal resources and work demand. This study focused on the personal resources component of work ability. The aims of this study were to elucidate the association between work ability and cardiovascular (CV) risk factors, particularly oxidative stress, and to estimate the effect of a community-implemented lifestyle modification programme on work ability and CV risk factors. Urinary 8-iso-prostaglandin F_2α (PGF_2α), a biomarker of oxidative stress, was negatively correlated with psychological resources, as measured by the Work Ability Index (WAI). Overall WAI score was unchanged following the programme, while CV risk factors and antioxidative activity improved. A reduction in PGF_2α levels was correlated with an improvement in subjective work ability relative to job demands, as assessed by a WAI item. Taken together, the results suggest that lifestyle modification programmes enhance the personal resources component of work ability and are associated with a reduction in oxidative stress.     

A work measurement method for application in Indian agriculture

The usual scales of rating in work measurement practices are based on subjective assessment of effort and walking speed of workers. In the present paper an attempt was made to correlate walking speed and stride length with selected anthropometric and physiological measurements (heart rates), using three groups of farm workers. Linear simple and multiple regressions were obtained to identify the best possible correlation between the above parameters. Accordingly, a rating scale was developed and applied in ploughing, transplanting, reaping and sheafing operation in rice cultivation.

The result showed that the preferred speed of walking of the farm workers was 1.39 m/s and step frequency at that speed was 103 steps/min. On application of the suggested scale on the abovee four agricultural operations, it was observed that the majority of the workers' working loads and efforts were more than they should be.     

A gender approach to work ability and its relationship to professional and domestic work hours among nursing personnel

The association between working hours and work ability was examined in a cross-sectional study of male (N=156) and female (N=1092) nurses in three public hospitals. Working hours were considered in terms of their professional and domestic hours per week and their combined impact; total work load. Logistic regression analysis showed a significant association between total work load and inadequate work ability index (WAI) for females only. Females reported a higher proportion of inadequate WAI, fewer professional work hours but longer domestic work hours. There were no significant differences in total work load by gender. The combination of professional and domestic work hours in females seemed to best explain their lower work ability. The findings suggest that investigations into female well-being need to consider their total work load. Our male sample may have lacked sufficient power to detect a relationship between working hours and work ability.     

Participatory ergonomics applied in installation work

This paper evaluates a step-by-step participatory approach to better work, applied in reducing the musculoskeletal workload in installation work. To arrive at a reduction in workload, a steering group led the project through the following steps: 1. Introduction: defining the goal and informing all 7000 employees. 2. Analysis: finding major loading tasks. 3. Solution phase: creating solutions in group sessions and prioritise. 4. Testing: promising solutions were tested during real operations. 5. Implementing: spreading the knowledge through the whole company and asking for additional solutions. 6. Evaluation: the effect and process were evaluated. The result was that 138 devices were bought. Seven out of nine devices were used daily. Users reported a good or very good reduction in musculoskeletal loading and were satisfied. The project was cost-effective within 1 year. Adding organizational measures or system solutions could have increased the effect, and more direct participation could have increased the impact. The first steps (introduction, analysis of work, solution generation and user tests) were well organized and contributed to the success. The processes in the last steps were unstructured and the evaluation was not representative. Sixty additional devices were suggested by employees of which 12 were selected. These devices seemed a success, but no data could be obtained on the use and experience with these solutions.     

Relationship between maximum acceptable work time and physical workload

For safe job design, it is necessary to know the maximum acceptable work time (MAWT) for a given workload. The aim was to establish the relationship between MAWT and physical workload. Cycling tests at six different work rates relative to personal maximum working capacity were performed by 12 young adults in the laboratory. The oxygen uptake (VO₂) in the per cent maximum aerobic capacity (%VO_2max), relative heart rate (RHR) and relative oxygen uptake (RVO₂) were collected throughout the test. MAWT was determined by observing the heart rate data during the test. The results showed that MAWT was negatively correlated with %VO_2max, RHR and RVO₂(p<0.01). Three exponential regression models were obtained and all their R² >0.80. These models suggest that long-hour shifts (>10 h) should assign a lower work intensity than for an 8-h workday. It is also logical that the workload limit for a 4-h work shift could be set at about 10%VO_2max higher than the suggested limit for an 8-h workday.     

Assessment of back load in assemblyline work using electromyography

Abstract

When the back is loaded, the amplitude of the myoelectric signals in the lumbar region of the back has been found to correlate well to the compression force in the spine measured by means of disc pressure. The purpose of this paper is to use surface electromyography to quantify the load on the back in several working situations.

Thirteen male workers participated in a study of three strenuous workstations along the assembly line of a car factory; mounting of a side panel and a sound insulator in the front compartment, mounting of floor mats, and mounting of the left front seat. Electrodes were placed at the level T8, LI and L3 on both sides of the spine.

The average load during a work cycle is given as mean values of signal amplitude. Standard deviations are calculated to indicate the average load variations. Amplitude histograms are also presented to illustrate the loading pattern in more detail.

The results show that it is possible to record myoelectric signals at the workplace without serious disturbance. The amplitude levels were high at all three workstations. The actual way in which each task was carried out resulted in differences in activity level between the thoracic and lumbar regions of the back which could be used to identify incorrect body postures or work activities. The use of a lifting aid at one workstation was found to give a significant decrease in high amplitude levels. Analysis of myoelectric activity patterns give useful guidance about how to reduce body loading in heavy work situations and also permits quantitative evaluation of such improvements.     

Reducing the physical work load and strain of personal helpers through clothing redesign

This study assessed the effects of redesigning clients’ clothing on the physical work load and strain of personal helpers. Five women, aged 18–54 years, who helped persons with physical disabilities were measured at their worksites before and after development of the clothes worn by clients. The physical work load and strain of the helpers’ dressing/undressing of clients were determined from their hand and back movements, work time, muscular activity, heart rate (HR), percentage of heart rate range (%HRR), and rating of perceived exertion (RPE). The muscular activity of the right (p=0.05) and left (p=0.02) trapezius muscles, HR (p=0.03), and %HRR (p=0.03) of the helpers were lower when the new outerwear was used in place of traditional outerwear. Four helpers reported lower perceived exertion, and three had shorter work time with the new outerwear. This study showed that redesigning clients’ clothing can help reduce the physical work load and strain of personal helpers.     

Responsive upper limb and cognitive fatigue measures during light precision work: an 8-hour simulated micro-pipetting study

Abstract

Many contemporary occupations are characterised by long periods of low loads. These lower force levels, which are relevant to the development of work-related musculoskeletal disorders, are usually not the focus of fatigue studies. In studies that did measure fatigue in light manual or precision work, within and between measurement responses were inconsistent. The aim of this study was to identify fatigue measures that were responsive at lower force levels (<10% MVC) over the course of an 8-h period. A complementary set of fatigue measures, reflecting both neuromuscular and cognitive mechanisms, was measured during a light precision micro-pipetting task performed by 11 participants. Nine measures were found to be significantly responsive over the 8-h period, including: ratings of perceived fatigue, postural tremor, blink frequency and critical flicker fusion frequency threshold. Common field measures, specifically electromyography RMS amplitude and maximum voluntary contractions, did not lead to extraordinary time effects.

Practitioner summary: The findings provide insight towards the responsiveness of a complementary set of field usable fatigue measures at low work intensities Although commonly used measures did not reveal significant increases in fatigue, nine alternative measures were significantly responsive over the 8-h period.     

Perceived physical stress at work and musculoskeletal discomfort in X-ray technologists

A structured questionnaire/interview was designed to explore demographic, personal, occupational and occupational health factors as well as recreational physical activities which can affect X-ray technologists' musculoskeletal symptoms. This questionnaire was piloted for clarity and validity. Subsequently, a random sample of 20 volunteer participants (18 female, 2 male) from two University hospitals were administered the questionnaire in the presence of the investigators to ensure that questions were correctly understood. The data obtained were analysed for magnitude, duration and frequency of activities and for severity, duration and recurrence of morbidity. The X-ray technologists in the sample were found to be a young group of professionals ranging from between 20?–?54 years of age. Eighty-nine per cent of the technologists were physically active and 44% indulged in physical recreational activities. Despite the young age and active life style, the X-ray technologists had significant and diverse musculoskeletal problems; 83% of technologists had backache and 39% of the female technologists had neck pain and 28% shoulder pain. The majority of technologists had suffered multiple episodes of pain. Fifty per cent of the female sample and both male volunteers suffered from upper extremity pain.     

Perceived physical stress at work and musculoskeletal discomfort in X-ray technologists

A structured questionnaire/interview was designed to explore demographic, personal, occupational and occupational health factors as well as recreational physical activities which can affect X-ray technologists' musculoskeletal symptoms. This questionnaire was piloted for clarity and validity. Subsequently, a random sample of 20 volunteer participants (18 female, 2 male) from two University hospitals were administered the questionnaire in the presence of the investigators to ensure that questions were correctly understood. The data obtained were analysed for magnitude, duration and frequency of activities and for severity, duration and recurrence of morbidity. The X-ray technologists in the sample were found to be a young group of professionals ranging from between 20 - 54 years of age. Eighty-nine per cent of the technologists were physically active and 44% indulged in physical recreational activities. Despite the young age and active life style, the X-ray technologists had significant and diverse musculoskeletal problems; 83% of technologists had backache and 39% of the female technologists had neck pain and 28% shoulder pain. The majority of technologists had suffered multiple episodes of pain. Fifty per cent of the female sample and both male volunteers suffered from upper extremity pain.