The effect of different surfaces on biomechanical loading of shoulder and lumbar spine during pushing and pulling of two-wheeled containers

Abstract

Seven waste collectors pushed and pulled a two-wheeled container on three different surfaces: flagstones, paving stones, grass. Net torques at the shoulder joint and the lumbar spine as well as the compression and shear forces in the lumbar spine at the L4/L5 level were calculated for the tilting, initial and sustained phases. The lumbar spine compression force was below 1800 N and the shear force was below 200 N in all situations. The shoulder torque when pulling with one hand was up to 80 N m. The container weight affected the magnitude of the push/pull forces and the load on the shoulders but not the load on the lumbar spine. The type of surface affected the magnitude of the push/pull forces during initial and sustained phases, and affected the load on the shoulder in the sustained phase. However, it did not affect the compression in the lumbar spine.

Introduction

Manual material handling, especially heavy lifting, is known to be a risk factor for disorders in the low back (Bernard, 1997; Bigos et al., 1986; Hoogendoorn et al., 1999). Waste collection often includes manual handling of heavy containers. To minimise the body load during waste collection, wheeled containers have gradually replaced bins, bags and other kinds of containers that must be carried (Schibye et al., 1997). In this way, lifting is replaced by pushing and pulling in such work tasks, and pushing and pulling is subjectively rated as less strenuous than lifting (Straker et al., 1997). From a biomechanical point of view, pushing and pulling tasks are more complex to analyse compared to lifting. Instead of being close to vertical, the force directions are virtually unknown, and may vary during different phases of the task. Because of this, torques in the shoulder may have different directions and magnitudes. For the lumbar spine the situation is even more complex, since in many situations the torque produced by gravity on the upper body counteract the torque produced by the push/pull force. Therefore, an increase in the push/pull force may even diminish the load on the lumbar spine (Schibye et al., 2001b).

The biomechanical load during pushing and pulling is far less studied compared to lifting. Most studies which consider the force direction predicts modest lumbar spine compression forces for pushing, typically in the range of 2000 N (de Looze et al., 1995; Lee et al., 1991; Resnick and Chaffin, 1995; Schibye et al., 1997). For pulling in front of the body, higher compression forces have been found (de Looze et al., 1995; Lee et al., 1991; Troup and Chapman, 1969). Pulling behind the body is only studied in a few cases (Jørgensen et al., 1985; Schibye et al., 1997), showing loads comparable to pushing. The difference between lifting and pushing/pulling is also seen for the maximum acceptable weight, assessed psychophysically. Here, the limits for pushing and pulling are more than twice of those for lifting, lowering and carrying (Straker et al., 1996).

Pushing and pulling of waste containers take place on different surfaces: asphalt, flagstones, paving stones, gravel, grass, and occasionally soil. These different surfaces provide different resistance against the movement of the container. Significant differences in rolling resistance have been found for manually pushed carts, the softer surface causing the largest resistance (Al-Eisawi et al., 1999). Such differences in rolling resistance may result in different magnitudes and directions of the pushing or pulling force, and differences in working postures. The objective of the present study was to study the task demands and loads on the shoulder and the lumbar spine, for different combinations of task, phase, and surface, and relate these demands to the strength of the subjects.