On Site Analysis of the Muscular Load during Three Car Assembly Tasks

Abstract

This study describes the on site analysis of the muscular load during car assembly operations which involve opening or closing the bonnet. Car assembly involves 21 of these operations but three were selected as representative for analysis of muscular load of the lower back and shoulder muscles. Each cycle of a task lasted about 50 seconds, but was subdivided into a number of subtasks. Subjects were asked to report the most annoying and most strenuous subtasks.

The first task was mounting the support bar for the bonnet of the car, and was subdivided into eight smaller operations. The second task was mounting the windscreen wipers and was divided into 6 subtasks. The third task, mainly involving adjusting the lights, was divided into five parts.

Two subjective screening methods (NIOSH 1991, RULA) and two objective methods (EMG processing and video digitalisation) were used for data analysis. Due to the limitations of the NIOSH 1991 equation in these situations, we determined a Recommended Weight Limit (RWL) of 12 kg. Although the weight of the bonnet did not exceed the RWL, varying between 10.5 and 11.5 kg depending on the quantity of noise-insulation material, this did not guarantee acceptable postures or movements.

The results from RULA indicated that work postures had a harmful effect on the musculoskeletal system. Electromyographic registrations indicated that the shoulder muscles (M. Trapezius and M. Deltoideus) work more dynamically relative to the back muscles (Erector Spinae bilaterally). There were high dynamic peak values for Deltoideus and Trapezius muscles, but long periods of static effort for the low back musculature. Both types of muscle activity are damaging for the musculoskeletal system, and this was confirmed by employees' subjective complaints regarding the back and shoulders.

There were clear differences between the three tasks analysed. Peak values for the first task (mounting the support bar for the bonnet) varied between 60 and 85% of the MVC bilaterally for the shoulder muscles. A static load of 15 to 40% MVC was exerted on the low back musculature. For assembly of the windscreen wipers, effort of the shoulder musculature was relatively low (50% MVC) but there were peaks in activity for the Erector Spinae muscles (80–90% MVC). There were relatively few peak efforts while opening the bonnet during adjustment of the lights in the third task, with some above 70% MVC in the back muscles.

For each of the tasks analysed, recommendations were given to the company to reduce the risks of back and shoulder complaints, addressing the arrangement of the work area and how the tasks are carried out. We will monitor the applications of our recommendations in future research.