When characterizing work and defining the exposed loads on workers one needs to concentrate on
• force requirements,
• amounts of static loads,
• rest periods,
• variability
inside and between work-tasks and workdays.

Muscle activity (electromyography, EMG) measurements have been conducted to define workloads in several industries and several studies have looked connection to experienced pain, reported sick leaves and/or diagnosed injuries. Nordander et al. (2016) collected data from a number of cross-sectional epidemiological studies with many different occupational groups and working situations. They concluded that shoulder complaints and diagnoses were associated with the activity of trapezius and forearm extensor muscles. Another study (Nordander et al. 2013) pointed out that the prevalence of complaints during workdays increased when muscle activity (% from maximal voluntary contraction, MVC) during work tasks increased and decreased with % increase in muscular rest time. Jakobsen et al. (2014) investigated associations between perceived exertion and objectively assessed muscular and cardiovascular load during a full working day among blue-collar workers with manual lifting tasks. They found that high perceived exertion (Borg scale ≥ 4) was related to high activity of the trapezius muscle. The probability for experiencing high exertion during work increased 18-fold for each percentage increase in time above 60 % MVC.

From an ergonomic point of view, in addition to tasks with high force demands, it is especially important to get information about the static load levels, resting pauses and variation in muscle usage during work tasks. Hanvold et al. (2013) aimed to evaluate if sustained trapezius muscle activity predicts neck and shoulder pain over a 2.5-year period in female hairdressers, male electricians and a group of students. They divided the amount of sustained activity (meaning constant low-level muscle contraction without proper breaks) during a work day to low (0-29 %), moderate (30-49 %) and high (50-100 % of sustained muscle activity). They concluded that subjects in the high-level group had 3-times more neck and shoulder pains during a 2.5-year period than those in the low-level group. This is in accordance with Veiersted et al. (1990) that reported that less short pauses / gaps in muscle activity predicted future patient status in female chocolate manufacturing workers. Those workers with previous episodes of complaints were shown to have fewer gaps in muscle activity than were healthy subjects. In addition, Nordander et al. (2000) concluded that cleaners that had a higher prevalence of neck / shoulder myalgia as compared to office workers showed also much less muscular rest during whole workday EMG registrations. Wide interindividual variation in muscular rest among cleaners was to some extent explained by body mass index (BMI) and age, with lower values of rest for older subjects with a high BMI.

Ostensvik et al. 2009_01 concluded that large amounts of long sustained low-level muscle activity periods (10 and 20 min duration) in the right upper trapezius muscle were associated with musculoskeletal discomfort / pain. On the other hand, the total numbers of short sustained activity periods (1.6, 5 and 10 s durations) were negatively correlated to discomfort / pain after a working day. The interpretation of these results may be that short, sustained activity periods have a salutary effect on muscle use, and long sustained activity periods over 10 or 20 min without breaks have a deleterious effect. Østensvik, et al. (2009_02) further evaluated the relationship between long, sustained activity periods in the trapezius muscle and neck pain after 1 year in forest machine operators. The results showed that exposure to several long, sustained activity periods (> 8 min) in the right upper trapezius muscle increased the risk of pain in the neck and shoulder.

In addition to resting pauses and gaps in EMG, the more forceful dynamic muscle contractions in between static workloads may be beneficial. Falla & Farina (2007) demonstrated less fatigue and a greater change in the spatial distribution of upper trapezius EMG amplitude over time when a sustained contraction was interrupted by periodic increases in force compared to a constant force contraction at lower average load. This result suggests that redistribution of activity within a muscle during a sustained contraction may reduce fatigue by decreasing muscle fibre overload with more uniform changes in the extra-cellular environment in different muscle regions. High variation in workload is beneficial. Balogh et al. (2016) collected information of workloads in all the tasks carried out in grocery stores and examined if variations in workload implied a decreased risk for work-related musculoskeletal disorders. Cashier work showed substantially lower physical workloads but significantly more neck / shoulder complaints than picking work and mixed work. Mixed work involved more variation than the other tasks, with high between-minute-variation and the highest between-minute-variation to within-minute-variation -ratio. It is believed that the higher prevalence of complaints among subjects performing cashier work than those with picking work or mixed work can be explained by a higher variation in the latter tasks. By combining work tasks with different physical exposure levels, an increased variation in workload can be achieved.

Arviddson et al. (2012) measured variation in muscle activation in meat cutters and hypothesized that a high within-minute-variation indicates a high physical workload in terms of repeated wide movements with high velocities, while a high between-minute-variation corresponds to more variable work during the workday. They found the highest within-minute-variation in the sixth-part carcass system with the highest physical workloads and musculoskeletal disorders prevalence, which was well in accordance to the hypothesis. Madeleine et al. (2008) studied newly employed workers in fish / poultry industries and followed them prospectively for 6 months, during which some developed pain in the neck / shoulder region. Additionally, motor variability among butchers with extensive work experience was compared to that of novices. Larger variability in both arm and trunk range of movement was seen for the butchers with long-term experience compared with the novice group. This may imply developed motor strategies over time among workers staying in the job, towards a more efficient use of available degrees of freedom. This may, in turn, act as a protective factor with regard to work-related musculoskeletal disorders. Pain developed in half of the subjects during the first six months of work, in general, decreased the size of motor variability.

When work-task related loading profiles are known and considered in work-development, work-related musculoskeletal disorder and pain incidences can be lowered. Voerman et al. (2007) investigated the effects of ergonomic counselling on work-related neck-shoulder pain and disability. Pain intensity and disability had significantly decreased immediately after four weeks of counseling and the effects remained at follow up.

Author:
Merja Hoffrén-Mikkola, PhD (biomechanics), Content Developer, Myontec

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