Symmetric Lifting Research Articles

Dynamic biomechanical modelling of symmetric and asymmetric lifting tasks in restricted postures.

This article describes investigations of dynamic biomechanical stresses associated with lifting in stooping and kneeling postures. Twelve subjects volunteered to participate in two lifting experiments each having two levels of posture (stooped or kneeling), two levels of lifting height (350 or 700 mm), and three levels of weight (15, 20, or 25 kg). One study examined sagitally symmetric lifting, the other examined an asymmetric task. In each study, subjects lifted and lowered a box every 10 s for a period of 2 min in each treatment combination. Electromyography (EMG) of eight trunk muscles was collected during a specified lift. The EMG data, normalized to maximum extension and flexion exertions in each posture, was used to predict compression and shear forces at the L3 level of the lumbar spine. A comparison of symmetric and asymmetric lifting indicated that the average lumbar compression was greater in sagittal plane tasks; however, both anterior-posterior and lateral shear forces acting on the lumbar spine were increased with asymmetric lifts. Analysis of muscle recruitment indicated that the demands of lifting asymmetrically are shifted to ancillary muscles possessing smaller cross-sectional areas, which may be at greater risk of injury during manual materials handling (MMH) tasks. Model estimates indicated increased compression when kneeling, but increased shear forces when stooping. Increasing box weight and lifting height both significantly increased compressive and shear loading on the lumbar spine. A multivariate analysis of variance (MANOVA) indicated complex muscle recruitment schemes--each treatment combination elicited a unique pattern of muscle recruitment. The results of this investigation will help to evaluate safe loads for lifting in these restricted postures.

Quantification of velocity coupling during asymmetric lifting

The objective of this study was to quantify velocity coupling, which was operationally defined as simultaneous trunk motion in two or three planes of the body. Velocity coupling was measured as the percentage of time the velocity was above a given percentage of maximum voluntary velocity in two or three planes. Four typesvelocity coupling were quantified: sagittal/transverse (S/T), sagittal/frontal (S/F), transverse/frontal (T/F), and sagittal/transverse/frontal (S/T/F). These types of velocity coupling were quantified as a function of seven task asymmetries and two task weights. The results showed that the three types of two-way velocity coupling had different response patterns to increased taskasymmetries. The total percentage of two-way velocity coupling significantly increased as task asymmetry increased from 0 to 30 degrees. The percentage of S/T velocity coupling decreased as task weight increased. It was also found that twice as much velocity coupling occurred at lifting task asymmetries beyond 120 degrees as compared to symmetric lifts. The effects of velocity coupling on strength and spinal loading are discussed.

Utilization of Direct Estimation Method to Predict the Maximum Acceptable Weight of Symmetrical and Asymmetrical Lift

The objective of this research was to study the effectiveness and accuracy of using the direct estimation method to determine a worker's lifting capacity or MAWOL of symmetrical and asymmetrical lifting tasks. Sixteen lifting tasks involving four different lifting angles and lifting frequencies were studied. In the first part of the study, ten male subjects performed sixteen lifting tasks to predict the MAWOL for each task condition using the psychophysical method. In the second session of the study, the MAWOL for sixteen task conditions were estimated using the direct estimation method. The results of this study indicate that direct estimation is an accurate method to establish MAWOL quickly for a series of lifting tasks. Analysis of the MAWOLs determined by two different methods indicated no significant difference between these two values. However, a significant difference was found between the direct estimated MAWOLs and the psychophysical predicted MAWOLs at the frequency of 1 lift/min with 30° or 60° asymmetry. This finding indicates as the interval between lifts increases, subjects have difficulty in making accurate estimates of the stress of the specific task, therefor, the direct estimation method should be used with caution.

Spinal forces during symmetric lifting in four postures

Spinal forces during symmetric lifting in four postures

Magnitude of torsional and lateral bending moments at L5/S1 joint during symmetrical sagittal plane lifting

Magnitude of torsional and lateral bending moments at L5/S1 joint during symmetrical sagittal plane lifting

Some exact and numerical results for plane steady sheared flow of an incompressible inviscid fluid

Analytical and numerical solutions are presented for the steady flow of an inviscid fluid about symmetric lifting profiles at an angle of attack in a plane sheared onset flow for which conformal mapping plays a critical role. For uniform shear (i.e. the onset flow speed varies linearly with position) in two dimensions, the disturbance field is potential and hence a solution based on the conformal transformation technique may be constructed. The Moriya transformation, which employs a leading-term transformation coefficient that stretches and rotates the field at great distances from the foil (as distinct from other classical transformations which leave the far field unchanged) is used and, with a limited number of terms selected for the transformation, a simple elegant solution is obtained that may be easily evaluated at arbitrary points on the foil contour. An additional investigation is reported for the field solution — involving a locally similar but globally non-uniform sheared onset flow — about one of the foils for which a simple O-type grid is analytically generated from the mapping function. These data indicate that the uniform-shear solution overpredicts the lift and surface speed on the suction side of the foil relative to the more realistic onset flow: the numerical solution predicts surface speeds that generally lie between those for the uniform flow and the uniformly sheared flow solutions.

The reproducibility of the L5/S1 moment during sagitally symmetric lifting

In biomechanical studies of lifting, forces and moments acting on the lumbar spine are often analyzed. Since these mechanical quantities are influenced by the technique of lifting, reproducibility becomes an issue when determining the effect of changes in lifting parameters. The purpose of this study was to determine the reproducibility of the L5/S1 flexion-extension moment during lifting. Nine subjects performed the same lift ten times. The L5/S1 moment was compared between experiments and subjects and was predicted with a dynamic rigid link model. The profile for the L5/S1 moment and its magnitude was consistent for each lift performed by the same individual. Between individuals the moment magnitude changed, while the moment profile remained similar.

Prediction of the maximum acceptable weight of symmetrical and asymmetrical lift using direct estimation method

The objective of this research was to study the effectiveness and accuracy of using the direct estimation method to determine a worker's lifting capacity or MAWOL of symmetrical and asymmetrical lifting tasks. Sixteen lifting tasks involving four different lifting angles and lifting frequencies were studied. The influence of these independent variables on MA WOL was analysed. In the first session of the study, ten male subjects performed sixteen lifting tasks to predict the MA WOL for each task condition using the psychophysical method. In the second session of the study, the MAWOL for sixteen task conditions were estimated using the direct estimation method. The results of this study indicate that direct estimation is an accurate method to establish MAWOL quickly for a series of lifting tasks. Analysis of the MAWOLs determined by two different methods indicated no significant difference between these two values. However, a significant difference was found between the direct estimated MAWOL and the psycho...

Finite-Element Evaluation of Contact Loads on Facets of an L2-L3 Lumbar Segment in Complex Loads

In this study, the contact loads and regions on facets of a lumbar motion segment are quantitatively determined. The analysis accounts for both material and geometric nonlinearities and treats the facet articulation as a frictionless, nonlinear, moving contact problem. The loadings consist of axial torque and lateral bending acting alone or in combination with axial compression force. Complex loadings simulating symmetric and nonsymmetric liftings are also considered. Under identical magnitude of rotation, axial torque generates the largest contact forces, followed by extension, flexion, and lateral bending. Contact forces under loads simulating heavy symmetric lifting are as, or even larger than, those under large extension and axial torque loadings. The presence of axial and lateral rotations during forward flexion significantly increases the loads transmitted through the compression facet. The tension facet, in this case, became nearly unloaded. The large loads on the facets, predicted especially under nonsymmetric, combined flexion loadings, are likely to play a role in the observed disorders of the facet joints. Under various loadings, the transfer of forces from one facet to the adjacent one occurs through three distinct areas, namely, extension-type, flexion-type, and torsion-type contact areas. The relative magnitude of different components of the transmitted forces are also predicted to vary considerably, depending on these regions of contact.

Regression Modeling of Spinal Forces during Constrained Lifting Postures

Back injuries are the leading cause of lost work time in the U.S. coal mining industry. Miners frequently perform labor in constrained postures, namely stooped and kneeling, due to low seam heights. This work uses regression models coupled with a biomechanical model to predict forces and torques in the spine during lifting tasks performed in constrained postures. Twelve males performed tasks consisting of symmetric lifting, asymmetric lifting, symmetric lowering, and asymmetric lowering. Independent variables included posture (stooped or kneeling), height of lift, and weight of lift. Dependent variables included estimates of spinal compressive forces, spinal shear forces, and torques about the spine. Regression models were developed which predicted the spinal loads based upon the statistically significant independent variables and interactions. These regression models can be used to estimate spinal loads during actual lifting tasks, and these loads can be compared to known limits to determine the relative risk associated with a particular task. In this way, the models can be used to better design manual materials handling tasks that are performed in restricted postures.

A Muscular Endurance Training Program for Symmetrical and Asymmetrical Manual Lifting Tasks

The purpose of this study was to test whether the endurance limits of new employees engaged in symmetrical and asymmetrical manual lifting tasks can be significantly increased through a physical training program. Each subject participating in the training program performed a total of 16 sessions. A control group performed two sessions separated by a 4-week interval. The results of this study showed that endurance time increased by 248% for the symmetrical and 46% for the asymmetrical lifting tasks. Furthermore, the frequency of handling increased by 44% and 34% for these tasks, respectively. The control group showed no improvement in terms of endurance time and frequency of handling. The implementation of a physical training program as a tool to control overexertion injuries in industrial settings is outlined.

Predicting maximum acceptable weights of symmetrical and asymmetrical loads for symmetrical and asymmetrical lifting

A simple model is presented for predicting maximum acceptable weights of symmetrical and asymmetrical loads (also known as psychophysical lifting capacity or capability) for symmetrical and asymmetrical lifting. Using 18 adult males as subjects, data were collected for a variety of symmetrical and asymmetrical lifting tasks. These data were used in developing and verifying the model. The proposed model includes both the worker variables and the task variables. A procedure is described for using the model to predict maximum weights of lift acceptable to industrial workers for shifts of varied durations.

Strain in fibers of a lumbar disc. Analysis of the role of lifting in producing disc prolapse.

A three-dimensional nonlinear finite element model has been used to predict the strain in the disc fibers of a lumbar motion segment under various single and combined loads such as those representing symmetric and non-symmetric liftings. A progressive failure analysis also has been performed under loads representing lifting while bending to one side: assuming yield and ultimate fiber strains of 14 and 16%, respectively. Large tensile strains of about 10% in the disc fibers are predicted under the maximum loads simulating symmetric lifting. Addition of lateral bending and twisting significantly increases the maximum fiber strain to more than 20%, and hence augments the risk for disc rupture. The maximum fiber strain occurs in the innermost annulus layer at the posterolateral location. Loss of intradiscal pressure or volume has a marked diminishing effect on the magnitude of maximum fiber strain predicted under flexion loadings. Failure analysis indicates that rupture initiates in the fibers in the innermost layer at the posterolateral location. With a slight increase in the loads, rupture progresses radially to the adjacent outer layer. Further progress of rupture in the fibers toward the annulus outer periphery resulting in complete radial fissure and disc prolapse appears to require additional increase in the loads. In the presence of large intradiscal pressure, the generated partial or complete radial fissure is likely to result in annulus protrusion or disc herniation, respectively. The results of clinical, epidemiologic, and experimental studies support the failure mechanism predicted in the present study.

Maximum acceptable weights, heart rates and RPEs for one hour's repetitive asymmetric lifting.

A laboratory study was conducted to determine the effects of repetitive asymmetric lifting on psychophysically determined maximum acceptable weights and resulting heart rates and ratings of perceived exertion (RPE). Eight male college students lifted a box in the sagittal plane and at three different angles of asymmetry (30, 60 and 90°) from the floor to an 81 cm high table and from an 81 cm high table to a 152 cm high table at a rate of 3,6 and 9 lifts min−1 for a period of one hour. For each lifting task, static strength was measured at the origin of lift. The maximum acceptable weights and static strengths were significantly lower and heart rates and RPEs were significantly higher for asymmetric lifting than those for symmetric lifting in the sagittal plane (p 0·05) on the percentage dec...

Maximum acceptable lifting loads during seated and standing work positions

The psychophysical method was used to determine the maximal acceptable load that eight males (age 22–30 years) would lift in each of four different positions: (1) seated, two-handed, symmetrical lift from a table, to a position 38 cm forward of the edge, (2) a seated lift from a position at the subject's side, on to a table in front of the subject involving a 90 degree twist of the torso, (3) standing, two-handed, symmetrical lift from the table, to a position 38 cm forward of the edge, and (4) standing, vertical lift from 86 above the floor. Subsequent to a training period, subjects lifted a tray with slotted handles at the rate of 1 or 4 lifts/min. Each subject chose the weight of the tray which was acceptable to him by adding or removing flat pieces of lead over a 45 min period. The weight of the tray, heart rate, and the perceived exertion were measured at 15, 30 and 45 min. Oxygen consumption was measured during the last 5 min of the 45 min experiment. Statistical analysis revealed a significant frequency and position effect. An increase in frequency from 1 to 4 lifts/min resulted in a decrease of 1·6 to 2·1 kg in the maximum acceptable weight for the various tasks. On average, the maximum acceptable weight of lift for standing positions was 16% greater than for sitting positions. Oxygen consumption and heart rate were significantly higher for 4 lifts/min than for 1 lift/min; however, the rating of perceived exertion did not differ for any factor.

Quantitative trunk muscle electromyography during lifting at different speeds

This study was designed to investigate the effects of trunk motion under lifting conditions described by the Work Practices Guide for Manual Lifting (NIOSH, 1981). Eight male volunteers were used as subjects in this study. Three independent variables; lift style, load location and subjective lift velocity, were controlled under sagittally symmetric lifting conditions. Dependent variables consisted of trunk muscle electromyographic (EMG) activity, actual trunk velocity and load acceleration. There was no effect of lift style. However, as the trunk velocity increased, EMG activity increased within the lastissimus dorsi and rectus abdominus muscles but not within the erector spinae muscles. The erector spinae muscles, unlike the other muscles, was also unaffected by load location and load acceleration. These findings suggest ways in which lifting guides should be adjusted to account for the effects of dynamic motion.

Trunk force development during static and dynamic lifts.

Most lifting analyses have used static models to evaluate the loading of the trunk during the performance of work. Recent research has reported many differences in trunk muscle force capability when a wide range of trunk velocities are observed. This study focused upon these differences during slow trunk velocities, which would be expected during a lift. Forty-five subjects were tested for their ability to exert torque about their low back under static and dynamic sagittally symmetric lifting conditions. Trunk muscle electromyography was used as a measure of trunk loading. The results reveal that significantly greater loading occurs under slow dynamic conditions, as compared with static conditions. Hence, static models, which assume quasistatic motion, are not good indicators of dynamic trunk load. A regression model was developed that described trunk loading as a function of trunk angle and velocity. The correlation between torque and muscle electromyography in the erector spinae muscles was also found to increase significantly as trunk velocity increased. The implication of these findings upon lifting is discussed.

Kinetic analysis of manual lifting activities: Part II—Biomechanical analysis of task variables

This paper, the second of a series of two papers, presents the results of biomechanical analyses of task variables in manual lifting activities. The three-dimensional dynamic biomechanical model, presented in part I was used to analyze compressive and shear forces generated during symmetrical and asymmetrical lifting, lifting boxes with or without handles, and lifting loads in different size boxes (defined by the box dimension in the sagittal plane). The measured ground reaction forces were also analyzed for the effects of these task variables. The results indicated that even though low-weights are accepted for lifting when lifting loads asymmetrically or in bigger boxes or when handling boxes without handles, the spinal stresses generated are, in general, significantly higher than when lifting loads symmetrically or in compart boxes or when handling boxes with handles. At the maximum acceptable weights of lift, the compressive forces generated were observed to be at least 30% to 50% lower than the compressive failure limit of the spinal structure.

Maximum acceptable weights and maximum voluntary isometric strengths for asymmetric lifting.

A laboratory study was conducted to determine the effects of asymmetric lifting on psychophysically determined maximum acceptable weights and maximum voluntary isometric strengths. Thirteen male college students lifted three different boxes in the sagittal plane and at three different angles of asymmetry (30,60 and 90°) from floor to an 81-cm high table using a free-style lifting technique. For each lifting task, the maximum voluntary isometric strength was measured at the origin of lift. The maximum acceptable weights and the static strengths for asymmetric lifting were significantly lower than those for symmetric lifting in the sagittal plane for three box sizes (P<0·01). The decrease in maximum acceptable weight and static strength from the sagittal plane values increased with an increase in the angle of asymmetry (P < 0·01). Box size had no significant effect (P≥ 0·05) on the percentage decrease in maximum acceptable weight or voluntary isometric strength from the sagittal plane values. Correction factors of 7,15 and 22% for maximum acceptable weights and 12, 21 and 31% for static strength at 30, 60 and 90% of asymmetric lifting are recommended. Lastly, in the absence of epidemiological data, a comparison of maximum acceptable weight and static strength in the sagittal plane with the NIOSH guidelines for action and maximum permissible limits indicates that the guidelines may be conservative.

On Determination of Spinal Stresses Generated While Performing Symmetrical and Asymmetrical Manual Lifting Tasks

This paper describes the development, and subsequent use, of a three-dimensional kinetic biomechanical model for determining compressive and shear forces generated in the human spine during symmetric and asymmetric manual lifting tasks. Using the model, task variables such as couplings and box-size were investigated. Biomechanical analyses of data indicated that asymmetrical lifting, lifting larger boxes, or lifting boxes without handles is more stressful compared to symmetrical lifting, lifting compact boxes, or lifting boxes with handles.