Elsevier

Clinical Biomechanics

Volume 16, Issue 7, August 2001, Pages 549-559
Clinical Biomechanics

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

https://doi.org/10.1016/S0268-0033(01)00039-0Get rights and content

Abstract

Objective. Compare the mechanical load on the low back and shoulders during pushing and pulling a two-wheeled container with the load during lifting and carrying the same amount of waste.

Background. Only little is known about risk factors and mechanical loads during push/pull operations.

Design. A complete 23 factor push/pull experiment. A two-wheeled container with 25 or 50 kg was pushed in front of and pulled behind the body by seven waste collectors. Further, the same subjects lifted and carried a paper bag and a dustbin both loaded with 7 and 25 kg.

Method. All operations were video recorded and the push/pull force was measured by means of a three-dimensional force transducer. Peak Motus and Watbak software were used for digitising and calculation of torque at L4/L5 and the shoulder joints and compression and shear forces at L4/L5.

Results. During pushing and pulling the compression at L4/L5 is from 605 to 1445 N. The extension torque at L4/L5 produced by the push/pull force is counteracted by the forward leaning of the upper body. The shear force is below 202 N in all situations. The torque at the shoulders is between 1 and 38 Nm.

Conclusion. In the present experiments the torques at the low back and the shoulders are low during pushing and pulling. No relation exists between the size of the external force and the torque at the low back and the shoulder.
Relevance

Pushing and pulling are common in many workplaces and have often replaced lifting and carrying situations. This has emphasised the need for more knowledge of the internal mechanical load on the body during these activities.

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 In order to reduce the mechanical load on the worker many lifting and carrying situations have in Denmark been replaced by pulling and pushing.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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