Mechanical load on the low back and shoulders during pushing and pulling of two-wheeled waste containers compared with lifting and carrying of bags and bins
Introduction
Low back pain is one of the major musculoskeletal problems among workers performing heavy manual material handling, and an important risk factor is heavy lifting [1], [2]. In order to reduce the mechanical load on the worker many lifting and carrying situations have in Denmark been replaced by pulling and pushing. Among waste collectors the equipment is changed from bags and bins to two- or four-wheeled containers. However, while extensive information exist about risk factors and mechanical load on the body during lifting and carrying, very little is known about risk factors and the mechanical load on the body during push/pull operations. Hoozemans et al. [3] concluded from reviewing various epidemiological studies that 9–18% of low back injuries are associated with push/pull and Hoozemans et al. [4] have reported an increased risk of shoulder complains in connection with regular push/pull work.
Acceptable limits for lumbar compression strength have been derived from several in vitro studies [5], [6], [7], and the American National Institute of Occupational Safety and Health (NIOSH) has proposed limits for manual material handling based on some of these data in combination with epidemiological data. Thus during lifting a disc compression less than 3400 N is regarded acceptable, and the maximum permissible value is around 6400 N. The use of the tissue tolerance level for the lumbar spine may be applicable to the pushing and pulling situation as well as to lifting and carrying. However, in a lifting situation a straightforward relation exists between the weight and position of the lifted object and the torque at the low back, and a rough biomechanical calculation of the internal load can be made from information of the working posture and the weight and position of the lifted object. Thus in these situations it is possible from external exposure information to validate if the recommended limit of internal exposure is exceeded. In contrast in the push/pull situations this kind of simple calculations are not possible since it is impossible to assess the size and direction of the external force even when the weight of the handled object is known. Theoretically, a relation exists between the horizontal part of the push/pull force and the weight of the container, since the size of the horizontal part of the force is determined by the size of the rolling resistance, which is increasing with increasing weight. However, if the total push/pull force is not horizontal, it will be incorrect to use only the horizontal component for calculation of the internal torque in the low back and shoulder.
The aims of the present experiments therefore were to study the load on the low back and shoulders in the same subjects performing two typical methods of waste handling, namely pushing and pulling of two-wheeled containers and lifting and carrying bins and bags with the same amount of waste. Further, to investigate whether a relation exists between the weight of the object and the size of the external forces, and between the external force and the mechanical load on the low back and the shoulders during pushing and pulling of waste containers. And finally, to validate if it is permissible to use only the horizontal part of the push/pull force in a biomechanical calculation of the internal mechanical load on the low back.
Section snippets
Subjects
Seven male, professional waste collectors participated in the study. Their average age, height and weight (range) were 42 (36–46) years, 1.82 (1.76–1.89) m and 77 (70–91) kg, respectively. They all had many years of experience in handling all types of waste containers used in the present study, i.e. two-wheeled containers, bags and bins. None of the subjects suffered from low-back disorders and all received written information about the project and gave their written consent before the
Pushing and pulling
Figs. 3(a) and (b) illustrate the sequence of events in the push and in the pull situation. Numbers below the figures denote time (s) from onset of operation. Numbers in frame show the video fields selected for biomechanical analysis (e.g. Fig. 3(a): 0.3=tilting, 0.8=initial and 3=sustained). Arrows show the direction of force, and the length of the arrow illustrates the size of the force on the handle of the container.
Discussion
From the present experiments, it can be concluded that during pushing and pulling of the two-wheeled container, the compression force at L4/L5 in all situations is rather small (about 400–1600 N) and never exceeds the value of 3400 N proposed as an acceptable limit by NIOSH [7]. This is also the case when the subjects are standing with a full paper bag or bin on the back. However, during lifting of the bag or bin the compression at L4/L5 may exceed the above limit. The main reason for the
Acknowledgements
The present study was part of the 1993–1998 research programme Waste Collection and Recycling, which was supported jointly by the Danish Ministry of the Environment and the Ministry of Labour.
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