Production Systems for Manual Repetitive Jobs. Effects on Autonomy and Variety of Work and Prevalence of Musculoskeletal Disorders in Upper Limbs

Doktorsavhandling, 1997

The aim of this thesis is to improve workers' conditions and it deals particularly with companies in the engineering industry. In many of these companies, production systems are designed to have manual repetitive jobs. Many such jobs are characterized by static work positions and repetitive motions, which are considered to be risk factors for musculoskeletal disorders in the upper limbs. There is a lack of knowledge of how to prevent the development of such disorders, but a variety of tasks and muscular activities are viewed as a means for preventing musculoskeletal disorders in upper limbs. Autonomy can be seen as a prerequisite for variety. In this thesis, the overall objective was to gain better knowledge of how to design production systems for manual repetitive jobs. A first purpose was to analyze how manual repetitive jobs in the engineering industry were in fact designed. A second purpose was to study interrelationships between work roles, work behavior, work-related stress and the prevalence of musculoskeletal disorders in the upper limbs, as well as how individual worker characteristics influence these relationships. The work has thus been explorative. Four case studies were made in two companies where the methodology mainly involved interviews, medical examinations, observations of work, use of questionnaires and video recordings. Three reference case studies were also carried out. All case studies focused on work groups. The thesis work emerged in a number of stages. Initially, a method for detailed analyses of hand and arm movements was developed on the basis of technical and medical methods. The so called Hand-Arm-Movement-Analysis (HAMA) method was used to analyze assembly tasks and illustrate work methods and work positions in the first case study. Using theories and empirical data, three theoretical models were successively developed during the thesis work. The models were used as a frame of reference in the analysis of the production systems studied. One model treats relationships between work role, work behavior, work-related stress and influence of individual worker characteristics, while the two other models treat the design of and factors influencing the design of production systems. Briefly, a production system is described as consisting of a social and a technical system. The results of the case studies show that manual repetitive jobs were designed differently, with different task contents, both within and between the companies studied. This especially concerned the extent to which workers were given autonomy and variety in their work, which differed between the work groups as well as the different work roles. The results show that workers differed greatly as regards work behavior (i.e. work method, work pace, work position and work-rest patterns) even when carrying out the same task. The prevalence of musculoskeletal disorders in the upper limbs was highest in the company in which workers were given higher autonomy as regards the technical system, more limited autonomy as regards the social system, high object variety and limited motor variety. The conclusion arrived at in this thesis is that there are many aspects of autonomy and variety that should be considered when production systems are designed, as autonomy and variety were found not to be one-dimensional concepts. More research is needed to analyze the importance of different aspects of autonomy and variety for the risk of musculoskeletal disorders in the upper limbs.More research is also needed to validate the theoretical models presented.