Pierre Desjardins

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.

Destabilizing and stabilizing forces to assess equilibrium during everyday activities

Postural stability is essential to functional activities. This paper presents a new model of dynamic stability which takes into account both the equilibrium associated with the body position over the base of support (destabilizing force) and the effort the subject needs to produce to keep his/her centre of mass inside the base of support (stabilizing force). The ratio between these two forces (destabilizing over stabilizing) is calculated to provide an overall index of stability for an individual. Preliminary results from data collected during walking at preferred and maximal safe speed in four older adults (aged from 64 to 84yr) showed that both forces are lower for subjects with reduced maximal gait speed. In addition, the stabilizing force increases by 2-3 times from preferred to maximal speed, while the destabilizing force barely changes with gait speed. Overall, the model through the index of stability attributes lower dynamic stability to subjects with lower maximal gait speed. These preliminary results call for larger-scale studies to pursue the development and validation of the model and its application to different functional tasks.

Biomechanical assessment of sitting pivot transfer tasks using a newly developed instrumented transfer system among long-term wheelchair users

This paper describes the technical characteristics of a transfer assessment system, along with details on three-dimensional (3D) upper extremity (U/E) kinematics required to compute U/E joint forces and moments using inverse dynamics during a displacement of the body in a sitting position from an initial surface to a target one (sitting pivot transfer (SPT)). This system includes five instrumented surfaces designed to measure position (center of pressure (COP)), magnitude and direction of the tri-axial force components underneath the feet, hands (leading and trailing) and buttocks (initial and target seats) during SPTs. Linearity, COP position and natural frequency tests were performed to confirm the accuracy of the transfer assessment system outcomes. Preliminary data of one person with spinal cord injury performing SPTs toward a target seat of same height (50 cm) and additional ones toward a raised target seat (60 cm) are presented. The transfer assessment system was found to be safe, versatile in terms of height- and width-adjustment ranges, portable within a laboratory environment, easy for experienced rehabilitation scientists to use, and allowed for valid quantification of reaction forces during SPTs as confirmed by the overall accuracy test results. Combined with the 3D U/E kinematic and anthropometric parameters, the transfer assessment system outcomes allowed for the quantification of U/E joint forces and moments. Preliminary results highlight the kinematic and kinetic specificities of the leading and trailing shoulders and elbows during SPTs. The impact of modifying target seat heights on the kinematic and kinetic outcomes during SPTs is explored. The transfer assessment framework proposed is useful for research and offers a wide spectrum of possibilities for acquiring new biomechanical knowledge on SPTs that may strengthen clinical practice guidelines, targeting the preservation of U/E integrity following SCI.

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.

Strategies of load tilts and shoulders positioning in asymmetrical lifting. A concomitant evaluation of the reference systems of axes

OBJECTIVES To evaluate two handling factors on asymmetry of posture and efforts and to evaluate different reference systems of axes on the characterization of asymmetry.DESIGN. Ten novice workers performed three tasks evaluated for the effects of load tilting (tilts vs no tilts) and shoulders positioning (non-parallel vs parallel to the ground). Specific comparisons were made using three referential systems.BACKGROUND. Box handgrips/tilting and body posture are factors differentiating expert and novice workers which present a potential for reducing asymmetries. The literature also suggests that different conclusions may be reached about asymmetry with different systems of axes.METHODS. Net 3D moments at L5/S1 were obtained from two force platforms, four video cameras and inverse dynamic analyses using three reference systems of axes (trunk, pelvis, and joint).RESULTS. Tilting the load presented clear advantages of reducing the duration and trajectory of efforts, better positioning the load and reducing knee flexion, peak trunk extensors and mechanical work on the load. The maintenance of the shoulders parallel reduced asymmetries of posture and efforts but the results were affected by the type of reference system of axes chosen.CONCLUSIONS. Box tilting and shoulders positioning should be considered in the reduction of risk factors. More research is needed to define referential systems and characterize asymmetry. RelevanceBox tilting and shoulders positioning appear to be two fundamental elements to take into account in the formation programs based on prevention of risk factors. The importance of asymmetry among the risk factors emphasizes the need for better characterization of this risk.

More comments on the programming language Pascal

SummaryA. N. Habermann recently published some “Critical comments on the programming language Pascal”. His reproaches are principally that numerous constructs are ill-defined, that there is “confusion” amongst ranges, types and structures, and that the goto statement should have been abolished. The present reply successively deals with points that are clearly refutable, those which are debatable and those which constitute valid criticism. Its principal aim is to encourage the reader to form his own opinion.

Biomechanical differences between best and worst performances in repeated free asymmetrical lifts

Abstract The purpose of the study was to evaluate, in 10 novice workers, the effect of the free practice of asymmetrical lifts with 3 different 15-kg containers. Practice effects were evaluated during one session (trials 1, 25 and 49), and also one month later (trial 50); containers effects were evaluated for homogeneity (2 boxes: homogeneous vs. heterogeneous) and for format (homogeneous box and cylinder). Each subject performed 150 lifts (50 practice trials and 3 containers per trial) but only 12 lifts were first analyzed. The data were obtained from 4 video cameras and a large force plate. A 3D dynamic rigid body model was used to evaluate low-back kinetics and kinematics. There were not any significant differences between the practice trials nor the boxes but there was high variability of performance within and between subjects. The main objective was then to evaluate the variability of these trials by contrasting the worst and best trials using three safety criteria: mechanical work, back efforts and asymmetry. The best strategies of reduced mechanical work (mean difference: 31%) was mainly associated with reduced knee flexion at take-off; strategies of reduced low-back moments at take-off (27%) were associated with smaller knee flexion and asymmetrical trunk efforts but larger feetspacings; finally, strategies of reduced back asymmetry at take-off (155%) were associated with reduced asymmetry of posture i.e. a better parallelism between shoulders/pelvis/handgrips and shoulders more parallel to the ground. Conclusions based on analyses of single trials or even means may be misleading. Relevance to industry Analyses of handling strategies including best and worst performances on the basis of specific safety criteria may help to better understand risk factors and suggest potential solutions for safer handling.

Development and application of predictive equations of maximal static moments generated by the trunk musculature

OBJECTIVE: To develop and apply regression equations that predict the maximal static moments generated by the extensor, axial rotator and lateral flexor muscles of the trunk as a function of forward bending angles and asymmetrical positions of the torso in human subjects. DESIGN: Using experimental data, polynomial predictive equations were developed. The equations were applied in an algorithm to estimate the maximal moment generated by individual subjects. METHODS: Three groups of male subjects participated in the evaluation of the effect of forward bending and trunk asymmetry on the static moment of the extensor, right axial rotator, and right lateral flexor muscles of the trunk, respectively. Two other groups of male subjects were evaluated to test the validity of the algorithm. During the dynamometric tests, the three-dimensional position of the trunk was determined using cinematography techniques. RESULTS: For the extensor muscles, a curvilinear increase of the maximal moment was observed with forward bending, with a slight decrease of the moment on each side of the neutral position. For the right axial rotator muscles, both forward bending and left asymmetrical position of the trunk increased the maximal moments generated by these muscles. The right lateral flexors were stronger in the standing and mid-flexion positions than in other positions. First-order, second-order and interaction terms appear in the regression equations to account for the effect of forward bending and asymmetrical positions. The regression equation for the extensors and right axial rotator muscles have R(2) coefficients (range 0.56-0.76) higher than those of the lateral flexor regression equations (range 0.09-0.12). The algorithm results in an average error of 13% in predicting the strength of extensor and axial rotator muscles of individual subjects. CONCLUSION: The results have shown that strength of the extensor and axial rotator muscles is affected by forward bending and the asymmetry of the trunk, while strength of the lateral flexor muscles is only dependent on the forward bending position. The regression equations and the corresponding algorithm are applicable for the range of positions used in the study.

Load Acceleration and Footstep Strategies in Asymmetrical Lifting and Lowering.

Accelerated execution effects for lifting and lowering a 12-kg box using two footstep strategies associated with experienced workers were studied. Eight healthy male participants performed a normal and an accelerated execution of a lifting task and a lowering task, using a minimal feet displacement strategy (oblique-step) and a strategy with a step (crossed-step). It was hypothesized that the accelerated executions, as compared to the normal executions, would have a different effect on L5/S1 resultant moment, body posture, and other kinematic variables. A tridimensional dynamic rigid body model was used to compute L5/S1 resultant moments. Results showed that the accelerated condition did not reduce body asymmetry of posture, but it reduced the length of the path of the global center of gravity and the duration of the supporting phase of the box, and it did not significantly affect L5/S1 maximal resultant moments for lifting but increased them for lowering. These results indicate that the net work production for accelerated strategies might be smaller, which may represent an economy of energy. Furthermore, the results showed that the use of an accelerated strategy for lowering should be avoided.

A method to evaluate contractures effects during the gait of children with Duchenne dystrophy.

Joint contractures are the second major impairment affecting the locomotor system of children with Duchenne muscular dystrophy (DMD). While the negative influence of joint contractures has been documented, the passive moments produced by joint contractures could benefit the gait of patients with muscle weakness. We describe a biomechanical model that quantifies the mechanical contribution of ankle and hip flexion contractures to the gait of DMD children. Kinematic and kinetic parameters were measured under the same experimental conditions during the gait and passive resistance assessment of two subjects: one healthy child as a control, and one child with DMD. The child with DMD had a plantar flexion contracture and a greater ankle stiffness coefficient than the control child. During gait, the contribution of the ankle passive moment to the net moment was more important for the child with DMD than for the control child. At the hip, passive joint moments and passive moment contribution were more important for the control child but this was not related to the presence of hip flexion contrac-ture. These preliminary results suggest the model might be used to evaluate contractures effect on a larger cohort of subjects.