Local muscular strain dependent on the direction of horizontal arm movements

Manual material handling still plays an important role in assembly work as well as in the service industry. By means of an ergonomic layout of working places, the effects of disadvantageous working conditions, especially static load resulting from unfavourable postures of the body as well as unnecessary physiological cost arising from body movements may be avoided. In this context knowledge about the optimum directions of horizontal arm movements is fundamental. Therefore by means of advanced methods of multi-channel electromyography (EMG) and applying computer assisted evaluation methods, physiological responses to one-handed horizontal material handling in a sitting position were recorded. In order to obtain myoelectric signals from the most important task-relevant muscles out of more than 40 muscle groups of the hand-arm-shoulder-system, preceding research was done also aiming at standardized lead positions for surface EMG. Groups of five young females each participated in a series of successive experiments, in which the direction of movements of the left arm was varied. The subjects worked without and with an external load imposed by weights of 0, 1, 2 and 4 kg. The movements between 20 degrees and 230 degrees--measured from the frontal plane of the subjects--included working from ahead and from behind. The lifting distance was 38 cm starting from different outward points within the reach to a fixed point near the body. Also in one test series the handling frequency was varied between 12, 24 and 48 lifts per minute. Assessment of the physiological cost by electromyographic activities (EA), especially the determination of the most stressful and least expensive directions of repetitive horizontal movements, was more effective when differentiating static and dynamic portions of muscular strain than common means of EA.