Effects of symmetry and load absorption of a falling load on 3D trunk muscular moments

Abstract

Some epidemiological data have suggested that many physical causes of low back pain such as bending and twisting were, in fact, sudden maximal efforts incidentally carried out at the moment of accident. Sudden loading conditions may be encountered in several circumstances, one of them being the recuperation of a falling load. Such conditions are more likely to occur in conditions of trunk asymmetry. The objective of this study was to determine spinal loadings associated with the reception of a falling box symmetrically and asymmetrically for two mechanisms of load absorption, one limited to the elbows and the other including full absorption with the elbows and lower limbs. It was hypothesized that asymmetrical receptions would be more strenuous for the spine; it was further hypothesized that the full absorption to decelerate the load might be more efficient in reducing the strain in the trunk muscular moments. Nine students in physical education with limited experience in manual materials handling were the subjects of the study. The tasks consisted of receiving a 6.6 kg load falling from a height of 50 cm above the flexed forearms when in a standing position. The subjects were tested with two AMTI force plates and two Locam cameras coupled with two mirrors; dynamic 3D multi-segment models were constructed and the net muscular moments as well as the angular velocities of the trunk relative to pelvis were determined about the three orthogonal axes of the trunk at L5/S1, in twisting, lateral bending, and flexion/extension. The dependent variables included maximal moments, maximal rates of loading for these moments, and the integration of these moments. Statistical analyses were performed to test the main effects of symmetry and absorption and their interaction. The results showed that asymmetrical conditions impose supplementary muscular exertions for trunk muscles, especially the trunk extensors and lateral flexors. Contrary to the proposed hypothesis, full absorption as used in the present study was a condition leading to considerably larger muscular exertions, especially for the loading rates. Thus it was concluded that the process of training for load absorption is essential to effectively decrease the risks of injuries. This factor would merit full consideration in future studies.