Ergonomic Risk Assessment of Static Postures and Repetitive Tasks Using Rapid Entire Body Assessment and Strain Index

This study aims to assess the risk levels of work-related musculoskeletal disorders (WMSDs) of Vietnamese female workers performing repetitive tasks and static postures. The workers perform the repetitive tasks which are printed circuit board assembly and printing on housing in a factory in Ho Chi Minh City, Vietnam. The risk assessment tools are used consisting of Rapid Entire Body Assessment and Strain Index. The results show that the risk levels of WMSDs of workers are from medium to high levels. Therefore, ergonomic measures are needed to reduce the risk levels and improve the working conditions. The suggestions of ergonomic interventions are ergonomic workstation redesign considering anthropometric data of Vietnamese female workers, appropriate work methods and training for workers .


INTRODUCTION
Productivity is one of the most important factors to increase profit of organizations. Besides, productivity raise standard of living not only individuals but also countries. Many industrial engineering methods are used in order to improve productivity such as methods engineering, quality management, lean six sigma, etc. However, if workers work hard to increase productivity but expose improper ergonomic working conditions such as force, awkward postures, monotonous repetitive tasks, sustained duration and stress/anxiety, their bodies can develop pains and injuries. These pains and injuries are called work-related musculoskeletal disorders (WMSDs). WMSDs decrease the comfort of workers, increase the cost of health care. In the long term, WMSDs result in reducing productivity and thus lower the living standard.
In 2017, an article was shown the WMSDs of Vietnamese female workers in three industrial zones (Bích, et al., 2017). The result was presented that over two-third of workers reporting at least 1 WMSDs. Among different industries, workers in electronic assembly lines reported highest level of WMSDs.
There is a need to assess the risk levels of WMSDs of Vietnamese female workers who working in the electronic assembly lines. The results of risk levels of WMSDs can be used to making decision in terms of ergonomic interventions.
The purpose of this study is measuring the risk level of WMSDs of Vietnamese female workers who assemble PCB and printing on housing in a LED light bulbs factory in Ho Chi Minh City, Vietnam. Rapid Entire Body Assessment (REBA) and Strain Index (SI) are applied to determine the risk level in terms of static postures and repetitive tasks. The solutions are ergonomic workstation redesign taking into account the anthropometry of Vietnamese female workers, work method improvements and training.
This paper consists of the introduction, literature reviews, methods, results and discussions and conclusions.

LITERATURE REVIEWS
There are many ergonomic tools for risk assessment such as material handling assessment tools, psychophysical data tables, upper extremity assessment tools, and entire body assessment tools. The table below shows the summary of several entire body assessment tools for measuring the risks of working postures and monotonous, repetitive tasks. • High risk levels (REBA score = 12.6) • A new toolbox shelf and a chair were designed and implemented so that REBA score was reduced.
• Technological sewing process (Kirin & Šajatović, 2021) • Medium or high risk at all operations • Workstation redesign considering the height of workers and the work methods • The causes of pains are repetitive motions, heavy material handling, work for long time, and awkward posture • Some ergonomic solutions should be taken to improve health, safety and productivity Strain Index

METHODS
The research method is presented as a procedure consisting of six steps as Figure 1 (Amjad Hussain, 2016).
The first step is selecting appropriate tasks and workers in assembly line. The repetitive, monotonous task is usually defined as repeated motions in less than 30 seconds (Tyosuojelu, 2022). When workers perform repetitive tasks in a long time, their bodies may develop musculoskeletal pains and injuries. Hence, workers in the assembly line are chosen for risk assessments. Two workers and their tasks are chosen for assessing. Task 1 is PCB assembly and task 2 is printing on housing.
The following is descriptions of task 1 and task 2 for further understanding of the operations.
• Task 1: Components such as capacitor, diode, resistor, inductor, integrated circuit are assembled into the board. • Task 2: Brand name of company is printed on the plastic housing of LED bulbs. The observations of the repetitive tasks and the worker postures are taken in PCB assembly, printing on housing workstations by video analysis. The video analysis is often used in methods engineering to determine the work methods and cycle time of an operation. In this study, a videotape is analyzed to identify the angles of two groups of postures and the frequency of motions.
REBA and SI are employed to assess the risk levels relating to static postures and repetitive tasks.

Rapid Entire Body Assessment (REBA)
REBA is a tool using for analyzing of human posture in work systems (Hignett & McAtamney, 2000). The purposes of REBA are analysing the risk of musculoskeletal in composition of work elements, scoring the risks, providing indicators for ergonomic interventions. There are group A and group B in REBA scoring tool.

Strain Index (SI)
Strain Index is employed for risk assessment of upper extremity disorder. There are six task variables including intensity of excertion, duration of excertion, efforts per minute, hand/wrist posture, speed of work and duration perday (Moore & Garg, 1995). The steps to calculate the SI of the repetitive task is presented as Figure 3 (Bridger, 2018).
Belowing is the SI value calculation: SI = (intensity of exertion multiplier) × (duration of exertion multiplier) × (efforts per minute multiplier) × (posture multiplier) × (speed of work multiplier) × (duration per day multiplier) There are various scales of SI value. In this study, the scale of SI value and risk of WMSDs is presented as Table 3 (Moore & Garg, 1995).

RESULTS AND DISCUSSIONS
Two operations are examined to compare the risk levels. They are PCB assembly and printing on housing of LED bulbs. Table 4 shows the REBA score after analyzing a videotape about worker posture in a PCB assembly workstation. Both left hand and right hand of worker operate simultaneously, so Group B is calculated for both hands. Table 4. REBA Score calculation for static posture of worker in PCB assembly worstation REBA Score calculation Group A Group B Trunk = 3 (flexion from 20 0 to 60 0 ) Neck = 2 (flexion more than 20 0 ) Leg = 1 (sitting) Load/Force = 0 (loading small parts) Upper arm = 3 (flexion from 45 0 to 90 0 ) Lower arm = 1 (flexion from 60 0 to 100 0 ) Wrists = 1 (flexion from 0 0 to 15 0 ) Coupling = 1 (fair coupling) Score A = 4 Score B = 6 Score C = 6 Adding Activity Score = 1 (repeated motions more than 12 times per minute) REBA Score = 7 REBA Score equals to 7 means that the risk level is medium. Therefore, the actions is necessary. The SI value of repetitive task of worker in a PCB assembly workstation is as follow: SI (right hand) = 6.4 The SI values of the right hand and the left hand are 6.4, thus the risk of work-related musculoskeletal disorders is medium level. Table 6 shows the REBA score after analyzing videotape about worker posture of printing on housing. Right hand Trunk = 2 (flexion 0 0 -20 0 ) Neck = 2 (flexion 10 0 -20 0 , side bending) Leg = 1 (sitting) Load/Force = 0 (loading small parts) Upper arm = 2 (20 0 extension to 20 0 flexion, abducted) Lower arm = 2 (flexion >100 0 ) Wrists = 2 (flexion 0 0 -15 0 , twisted) Coupling = 1 (fair coupling) Upper arm = 2 (20 0 extension to 20 0 flexion, abducted) Lower arm = 2 (flexion >100 0 ) Wrists = 2 (flexion >15 0 , twisted) Coupling = 1 (fair coupling) Score A = 3 Score B = 5 Score C = 4 Adding Activity Score = 1 (repeated motions approximately 20 times per minute) REBA Score = 5 REBA Score equals to 5 means that the risk level is medium. Therefore, the actions is needed. The SI value of repetitiveness of both hands in printing on housing is as below: