The load on the back in different handling operations

Abstract

The study consisted of two parts. In part one the load on the back and muscle fatigue in bimanual symmetrical lifting from floor to table were studied in a lifting experiment of 1 hour' s duration. The following weights and lifting frequencies were used. 10% of max. lifting capacity (MLC) 6 and 15 times per min, 25% MLC 5 and 10 times per min and 50% MLC 3 and 6 times per min. The EMG mean amplitude from the back muscles (L3) showed that even light burdens (10 kg) cause considerable back-toads equivalent to 40–50% MVC. When lifting 50 kg burdens short lasting backloads near the MVC were present. Mean amplitudes and mean spectral frequencies of the EMG were in general increasing, respectively decreasing, as the lifting experiment progressed. Such changes in the EMG are normally interpreted as muscle fatigue caused by changes in the concentration of the chemical substances from the muscle. The EMG changes are most pronounced when lifting 50% MLC (6 times per min) and 10% MLC (15 times per min) and are to a higher extent dependent on the lifting frequency than on the weight or on the mechanical work performed on the burden. Further the RPE values from the back show the same pattern as the EMG. The V˙O₂ and HR, however, do not seem to discriminate as clearly between the different lifting tasks. In part two of the study the load on the back is studied by EMG during a number of manual handling operations applied when handling logs in the forest, i.e. frontal carrying, frontal carrying with a hook, frontal carrying with a pair of tongs, shoulder carrying and dragging with a pair of tongs. Three types of logs were used 1 m (30 kg), 3 m (30 kg), and 3 m (50 kg). All experiments were performed in the forest on two 5×3 m horizontal tracks standardized for the experiment, an easy and a difficult one. It was found that: normal manual handling operations in forestry work cause average backloads in young trained workers varying from 25% to 50% MVC, i.e. equivalent to 1400–2800 N extensor muscle tension of the back, assuming a 5 cm muscle lever arm. Backload levels equivalent to 75–100% MVC are present from a few per cent to 25% of the handling time in all the tasks studied. Asymmetrical loading of the back muscles is frequently seen most markedly in the lifting phases of the handling operations. Conclusion: the dragging method exposes the back to the smallest load on level smooth surface. Under difficult surface conditions, however, frontal carrying with hook and shoulder carrying seem to cause the smallest strain on the back. The backload measures obtained when lifting logs are considerably larger than the measures when lifting boxes of the same weight. Therefore, backload measures obtained in laboratory studies must be used with care when applied in actual working environments