A. Plamondon

Validation of two 3-D segment models to calculate the net reaction forces and moments at the L(5)/S(1) joint in lifting.

The purpose of this study was to examine the validity and sensitivity of two three-dimensional segment models to estimate the net reaction forces and moments at the L(5)/S(1) joint. The two dynamic three-dimensional multisegment models, applied to lifting activities, were a lower body model and an upper body model. Three healthy male subjects participated in this study. The asymmetrical task consisted of lifting a load of 9.6 kg in a normal speed condition and in a fast condition. Results from the two models in terms of joint reaction forces and joint reaction moments at L(5)/S(1) were compared and were then used to validate them. The correlation between the reaction moments from both models were generally above 0.95 and the root mean square (RMS) differences were generally below 10 Nm but could reach 38 Nm. Similar trends were observed in the sensitivity analysis. A proportion of the error was attributable to errors in the segment accelerations because of an increase in the RMS differences between the models with an increase in lifting speed. The use of the lower body model seemed to present some advantages over the upper body model because of the nature of the task analysed which did not require large accelerations from the lower part of the body.

Sensitivity analysis of segment models to estimate the net reaction moments at the L5/S1 joint in lifting.

This paper deals with the mathematics of inverse dynamic analysis for calculating the net reaction moments; the sensitivity of the net reaction moments to nine experimental values is quantified and two dynamic models are compared. The results indicate that, among all terms, the external forces in the case of a lower body model and the segment masses in the case of an upper body model contributed the most to differences in the sensitivity of the lumbosacral reaction moment about the transverse axis. About the other axes there was no clear difference between the two body models.

Moments at the L5/S1 joint during asymmetrical lifting: effects of different load trajectories and initial load positions

The purpose of this study was to examine the reaction moments at L(5)/S(1) about the three orthogonal orthopaedic axes on the trunk when performing asymmetrical lifting tasks under six experimental conditions. The first three conditions were three lifting modes referring to different load trajectories (diagonal, extension then twisting, twisting then extension). The next three conditions were three lifts referring to different initial load positions (90 degrees, 45 degrees, 0 degrees ). Ten male volunteers (mean age 23.4 years) participated in the study. The mass of the load was 11.6 kg and two shelves were used: a low shelf of 22 cm from which the load was removed and a high shelf of 80 cm onto which the load was laid down following the manoeuvre. A dynamic 3D multisegment model was used to compute spinal loadings. The maximum reaction moments generated at the joint for the three load trajectories and for the three initial load positions were approximately of the same magnitude. On the other hand, significant differences were observed in the trunk orientation when the maximal moment was generated. Symmetrical trunk positions appeared to be more advantageous because of the reduction in the capability of maximal extension force in asymmetric trunk posture. A possible alternative for avoiding excessive stress on the back in asymmetric trunk posture may be to keep the shoulders parallel to the ground. RELEVANCE: Three-dimensional studies of the resulting moments at L(5)/S(1) when performing lifting activities are not common, particularly when the tasks include muscular exertions in trunk flexion combined with either twisting or lateral bending. This type of analysis is particularly important since asymmetries have been identified as risk factors for low back disorders. The knowledge of reaction moments in relation to postures are essential to understand the mechanism of low back pain.

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

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.

Assessment of a three-dimensional robotic model for biomechanical-data acquisition of human movement

The use of a kinematic robotic model has not been implemented in the biomechanical-data acquisition protocol, as it has in workplace analysis, ergonomics and design. The purpose of this paper was to assess the use of a kinematic model to retrieve frames of human movements from data obtained at a low sampling frequency. From experimental trials with an original sampling frequency of 60 Hz, the data were sampled again at two lower frequencies, 5 Hz and 10 Hz. The model was then used to reconstitute the data to its original frequency (60 Hz). The results demonstrated that it was possible to retrieve a full 3-D human movement from a sampling rate lower than normal without sacrificing accuracy. It was observed from both reduced sampling frequencies that the error level was comparable to the usual accuracy of a DLT 3-D reconstruction technique. It was therefore concluded that the data retrieved from these two frequencies were very similar to the original data sampled at 60 Hz.