Effects of height and load weight on shoulder muscle work during overhead lifting task.

Few musculoskeletal models are available to assess shoulder deeper muscle demand during overhead lifting tasks. Our objective was to implement a musculoskeletal model to assess the effect of lifting height and load on shoulder muscle work. A musculoskeletal model scaled from 15 male subjects was used to calculate shoulder muscle work during six lifting tasks. Boxes containing three different loads (6, 12 and 18 kg) were lifted by the subjects from the waist to shoulder or eye level. After optimisation of the maximal isometric force of the model's muscles, the bio-fidelity of the model was improved by 19%. The latter was able to reproduce the subjects' lifting movements. Mechanical work of the rotator cuff muscles, upper trapezius and anterior deltoid was increased with lifting load and height augmentation. In conclusion, the use of a musculoskeletal model validated by electromyography enabled to evaluate the muscle demand of deep muscles during lifting tasks.

Normal birth weight variation and children's neuropsychological functioning: links between language, executive functioning, and theory of mind.

The effect of low birth weight on children's development has been documented for a range of neurocognitive outcomes. However, few previous studies have examined the effect of birth weight variability within the normal range on children's neuropsychological development. The current study examined birth weight variation amongst children weighing ≥2500 g in relation to their language, executive functioning (EF), and theory of mind (ToM), and specified a developmental pathway in which birth weight was hypothesized to be associated with children's EF and ToM through their intermediary language skills. The current study used a prospective community birth cohort of 468 children. Families were recruited when children were newborns and followed up every 18 months until children were age 4.5. Language was assessed at age 3 using a standardized measure of receptive vocabulary (PPVT), and EF and ToM were measured at age 4.5 using previously validated and developmentally appropriate tasks. After controlling for potential confounding variables (family income, parent education, gestational age), birth weight within the normal range was associated with language ability at age 3 (ß=.17; p=.012); and the effect of birth weight on both EF (z=2.09; p=.03) and ToM (z=2.07; p=.03) at age 4.5 operated indirectly through their language ability at age 3. Our findings indicate that the effects of birth weight on child neurocognition extend into the normal range of birth weight, and specific developmental mechanisms may link these skills over time.

Subject-specific regression equations to estimate lower spinal loads during symmetric and asymmetric static lifting.

Workplace safety assessment, personalized treatment design and back pain prevention programs require accurate subject-specific estimation of spinal loads. Since no noninvasive method can directly estimate spinal loads, easy-to-use regression equations that are constructed based on the results of complex musculoskeletal models appear as viable alternatives. Thus, we aim to develop subject-specific regression equations of L4-L5 and L5-S1 shear and compression forces during various symmetric/asymmetric tasks using a nonlinear personalized finite element musculoskeletal trunk model. Kinematics and electromyography (EMG) activities of 19 young healthy subjects were collected during 64 different symmetric/asymmetric tasks. To investigate the reliability and accuracy of the musculoskeletal model and regression equations, we compared estimated trunk muscle activities and L4-L5 intradiscal pressures (IDPs) respectively with our own electromyography data (EMGs) and reported in vivo pressure measurements. Although in general, six independent rotation components (three trunk T11 rotations and three pelvic S1 rotations) are required to determine kinematics along the spine, only two surrogate variables (trunk flexion and its asymmetric angles) satisfactorily predicted all six rotation components (R2 > 0.94). Regression equations, developed based on subject-specific inputs, predicted spinal loads in satisfactory agreement with IDP measurements (R2 = 0.85). Predicted muscle activities in the personalized musculoskeletal models were in moderate to weak agreements with our measured EMGs in 19 participants. Based on dominance analysis, trunk flexion and its asymmetry angle, hand-load weight, hand-load lever arm, and body weight were the most important variables while the effects of body height and sex on spinal loads remained small.

Validation of a low-cost inertial motion capture system for whole-body motion analysis.

While some low-cost inertial motion capture (IMC) systems are now commercially available, generally, they have not been evaluated against gold standard optical motion capture (OMC). The objective was to validate the low-cost Neuron IMC system with OMC. Whole-body kinematics were recorded on five healthy subjects during manual handling of boxes for about 32 min while wearing 17 magnetic and inertial measurement units with Optotrak clusters serving as a reference. The kinematical model was calibrated anatomically for OMC and with poses for IMC. Local coordinate systems were aligned with angular velocities to dissociate differences due to technology or kinematical model. Descriptive statistics including the root mean square error (RMSE), coefficient of multiple correlation (CMC) and limits of agreement (LoA) were applied to the joint angle curves. The average technological error yielded 5.8° and 4.9° for RMSE, 0.87 and 0.96 for CMC and 0.4 ± 8.6° and -0.3 ± 6.0° for LoA about the frontal and transverse axes respectively, whereas the longitudinal axis yielded 10.5° for RMSE, 0.78 for CMC and 3.3 ± 13.1° for LoA. Differences due to technology and to the model contributed similarly to the total difference between IMC and OMC. For many joints and axes, RMSE stayed under 5°, CMC over 0.9 and LoA under 10°, especially for the transverse axis and lower limb. The Neuron low-cost IMC system showed potential for tracking complex human movements of long duration in a normal laboratory environment with a certain error level that may be suitable for many applications involving large IMC distribution.

Motion-based prediction of external forces and moments and back loading during manual material handling tasks.

This paper evaluates a method for motion-based prediction of external forces and moments on manual material handling (MMH) tasks. From a set of hypothesized contact points between the subject and the environment (ground and load), external forces were calculated as the minimal forces at each contact point while ensuring the dynamics equilibrium. Ground reaction forces and moments (GRF&M) and load contact forces and moments (LCF&M) were computed from motion data alone. With an inverse dynamics method, the predicted data were then used to compute kinetic variables such as back loading. On a cohort of 65 subjects performing MMH tasks, the mean correlation coefficients between predicted and experimentally measured GRF for the vertical, antero-posterior and medio-lateral components were 0.91 (0.08), 0.95 (0.03) and 0.94 (0.08), respectively. The associated RMSE were 0.51 N/kg, 0.22 N/kg and 0.19 N/kg. The correlation coefficient between L5/S1 joint moments computed from predicted and measured data was 0.95 with a RMSE of 14 Nm for the flexion/extension component. In conclusion, this method allows the assessment of MMH tasks without force platforms, which increases the ecological aspect of the tasks studied and enables performance of dynamic analyses in real settings outside the laboratory.

A machine-learning method for classifying and analyzing foot placement: Application to manual material handling.

Foot placement strategy is an essential aspect in the study of movement involving full body displacement. To get beyond a qualitative analysis, this paper provides a foot placement classification and analysis method that can be used in sports, rehabilitation or ergonomics. The method is based on machine learning using a weighted k-nearest neighbors algorithm. The learning phase is performed by an observer who classifies a set of trials. The algorithm then automatically reproduces this classification on subsequent sets. The method also provides detailed analysis of foot placement strategy, such as estimating the average foot placements for each class or visualizing the variability of strategies. An example of applying the method to a manual material handling task demonstrates its usefulness. During the lifting phase, the foot placements were classified into four groups: front, contralateral foot behind, ipsilateral foot behind, and parallel. The accuracy of the classification, assessed with a holdout method, is about 97%. In this example, the classification method makes it possible to observe and analyze the handler's foot placement strategies with regards to the performed task.

The comparison of wavelet- and Fourier-based electromyographic indices of back muscle fatigue during dynamic contractions: validity and reliability results.

The purpose of this study was to compare the electromyographic (EMG) fatigue indices computed from short-time Fourier transform (STFT) and wavelet transform (WAV), by analyzing their criterion validity and test-retest reliability. The effect of averaging spectral estimates within and between repeated contractions (cycles) on EMG fatigue indices was also demonstrated. Thirty-one healthy subjects performed trunk flexion-extension cycles until exhaustion on a Biodex dynamometer. The load was determined theoretically as twice the L5-S1 moment produced by the trunk mass. To assess reliability, 10 subjects performed the same experimental protocol after a two-week interval. EMG signals were recorded bilaterally with 12 pairs of electrodes placed on the back muscles (at L4, L3, L1 and T10 levels), as well as on the gluteus maximus and biceps femoris. The endurance time and perceived muscle fatigue (Borg CR-10 scale) were used as fatigue criteria. EMG signals were processed using STFT and WAV to extract global (e.g, median frequency and instantaneous median frequency, respectively) or local (e.g., intensity contained in 8 frequency bands) information from the power spectrum. The slope values of these variables over time, obtained from regression analyses, were retained as EMG fatigue indices. EMG fatigue indices (STFT vs. WAV) were not significantly different within each muscle, had a variable association (Pearson's r range.: 0.06 to 0.68) with our fatigue criteria, and showed comparable reliability (Intra-class correlation range: 0.00 to 0.88), although they varied between muscles. The effect of averaging, within and between cycles, contributed to the strong association between EMG fatigue indices computed from STFT and WAV. As for EMG spectral indices of muscle fatigue, the conclusion is that both transforms carry essentially the same information.

Trunk response and stability in standing under sagittal-symmetric pull-push forces at different orientations, elevations and magnitudes.

To reduce lifting and associated low back injuries, manual material handling operations often involve pulling-pushing of carts at different weights, orientations, and heights. The loads on spine and risk of injury however need to be investigated. The aim of this study was to evaluate muscle forces, spinal loads and trunk stability in pull-push tasks in sagittal-symmetric, static upright standing posture. Three hand-held load magnitudes (80, 120 and 160 N) at four elevations (0, 20, 40 and 60 cm to the L5-S1) and 24 force directions covering all pull/push orientations were considered. For this purpose, a musculoskeletal finite element model with kinematics measured earlier were used. Results demonstrated that peak spinal forces occur under inclined pull (lift) at upper elevations but inclined push at the lowermost one. Minimal spinal loads, on the other hand, occurred at and around vertical pull directions. Overall, spinal forces closely followed variations in the net external moment of pull-push forces at the L5-S1. Local lumbar muscles were most active in pulls while global extensor muscles in lifts. The trunk stability margin decreased with load elevation except at and around horizontal push; it peaked under pulls and reached minimum at vertical lifts. It also increased with antagonist activity in muscles and intra-abdominal pressure. Results provide insight into the marked effects of variation in the load orientation and elevation on muscle forces, spinal loads and trunk stability and hence offer help in rehabilitation, performance enhancement training and design of safer workplaces.

Trunk musculoskeletal response in maximum voluntary exertions: A combined measurement-modeling investigation.

Maximum voluntary exertion (MVE) tasks quantify trunk strength and maximal muscle electromyography (EMG) activities with both clinical and biomechanical implications. The aims here are to evaluate the performance of an existing trunk musculoskeletal model, estimate maximum muscle stresses and spinal forces, and explore likely differences between males and females in maximum voluntary exertions. We, therefore, measured trunk strength and EMG activities of 19 healthy right-handed subjects (9 females and 10 males) in flexion, extension, lateral and axial directions. MVEs for all subjects were then simulated in a subject-specific trunk musculoskeletal model, and estimated muscle activities were compared with EMGs. Analysis of variance was used to compare measured moments and estimated spinal loads at the L5-S1 level between females and males. MVE moments in both sexes were greatest in extension (means of 236 Nm in males and 190 Nm in females) and least in left axial torque (97 Nm in males and 64 Nm in females). Being much greater in lateral and axial MVEs, coupled moments reached ∼50% of primary moments in average. Females exerted less moments in all directions reaching significance except in flexion. Muscle activity estimations were strongly correlated with measurements in flexion and extension (Pearson's r = 0.69 and 0.76), but the correlations were very weak in lateral and axial MVEs (Pearson's r = 0.27 and 0.13). Maximum muscle stress was in average 0.80 ±â€¯0.42 MPa but varied among muscles from 0.40 ±â€¯0.22  MPa in rectus abdominis to 0.99 ±â€¯0.29 MPa in external oblique. To estimate maximum muscle stresses and evaluate validity of a musculoskeletal model, MVEs in all directions with all coupled moments should be considered.

Effects of motion segment simulation and joint positioning on spinal loads in trunk musculoskeletal models.

Musculoskeletal models represent spinal motion segments by spherical joints/beams with linear/nonlinear properties placed at various locations. We investigated the fidelity of these simplified models (i.e., spherical joints with/without rotational springs and beams considering nonlinear/linear properties) in predicting kinematics of the ligamentous spine in comparison with a detailed finite element (FE) model while considering various anterior-posterior joint placements. Using the simplified models with different joint offsets in a subject-specific musculoskeletal model, we computed local spinal forces during forward flexion and compared results with intradiscal pressure measurements. In comparison to the detailed FE model, linearized beam and spherical joint models failed to reproduce kinematics whereas the nonlinear beam model with joint offsets at -2 to +4mm range (+: posterior) showed satisfactory performance. In the musculoskeletal models without a hand-load, removing rotational springs, linearizing passive properties and offsetting the joints posteriorly (by 4mm) increased compression (∼32%, 17% and 11%) and shear (∼63%, 26% and 15%) forces. Posterior shift in beam and spherical joints increased extensor muscle active forces but dropped their passive force components resulting in delayed flexion relaxation and lower antagonistic activity in abdominal muscles. Overall and in sagittally symmetric tasks, shear deformable beams with nonlinear properties performed best followed by the spherical joints with nonlinear rotational springs. Using linear rotational springs or beams is valid only in small flexion angles (<30°) and under small external loads. Joints should be placed at the mid-disc height within -2 to +4mm anterior-posterior range of the disc geometric center and passive properties (joint stiffnesses) should not be overlooked.

Evaluation of a hybrid system for three-dimensional measurement of trunk posture in motion.

Ambulatory assessment of trunk posture is important in improving our understanding of the risk of low back injury. Recently, small inertial sensors combining accelerometers, gyroscopes and magnetometers were developed and appear to be promising for measuring human movement. However, the validity of such sensors for assessing three-dimensional (3D) trunk posture in motion has not been documented. The purpose of this study was to evaluate a hybrid system (HS) composed of two inertial sensors for the 3D measurement of trunk posture. A secondary purpose was to explore the utility of adding another source of information, a potentiometer, to measure the relative rotation between both sensors in order to improve the validity of the system. The first sensor was placed over the sacrum and the second on the upper part of the thorax. Both sensors were linked by a flexible rod with a potentiometer. A complementary quaternion filter algorithm was used to estimate trunk orientation by taking advantage of the nine components of each sensor and the potentiometer. The HS's orientations were compared to those obtained from a 3D optoelectronic system. Validation of the HS was performed in three steps in which six subjects had to perform manual handling tasks in: (1) static postures; (2) dynamic motions of short duration (30s); and (3) dynamic motions of long duration (30min). The results showed that the root mean square (RMS) error of the HS was generally below 3 degrees for the flexion and lateral bending axes, and less than 6 degrees for the torsion axis, and that this error was lower for the short-duration tests compared to the long-duration one. The potentiometer proved to be an essential addition, particularly when the magnetometer signals were corrupted and only the gyroscope and accelerometer could be combined. It is concluded that the HS can be a useful tool for quantifying 3D trunk posture in motion.

Paramedics' working strategies while loading a stretcher into an ambulance.

For paramedics, loading a stretcher into an ambulance is an activity with a high risk of back injury and accidents. The objective of this study was to document strategies paramedics use at work while loading a powered stretcher into an ambulance. A total of 249 stretcher loading operations performed by 58 paramedics, and 51 semistructured post-intervention interviews were analyzed. Almost three quarters of loading operations required additional actions (e.g., raising the shoulders and additional lifting) to insert the stretcher into the cot fastener system in the ambulance. Some strategies that were necessary to complete the stretcher loading operation seemed to have negative impacts on the workers' health, such as repositioning the stretcher. This action wastes time and requires significant physical efforts, as it is usually done alone. This study suggests some potential solutions, related to equipment, training, workers and work organization, to reduce the risk of injury while loading stretchers.

Difference between male and female workers lifting the same relative load when palletizing boxes.

A few biomechanical studies have contrasted the work techniques of female and male workers during manual material handling (MMH). A recent study showed that female workers differed from males mostly in the strategy they used to lift 15-kg boxes from the ground, especially regarding task duration, knee and back postures and interjoint coordination. However, the lifting technique difference observed in females compared to males was perhaps due to a strength differences. The objective of this study was to test whether female workers would repeat the same lifting technique with a load adjusted to their overall strength (females: 10 kg; males: 15 kg), which can be considered a "relative load" since the overall back strength of females is 2/3 that of males. The task for the participants consisted in transferring boxes from one pallet to another. A dynamic 3D linked segment model was used to estimate the net moments at L5/S1, and different kinematic variables were considered. The results showed that the biomechanics of the lifting techniques used by males and females were similar in terms of task duration and cumulative loading, but different in terms of interjoint coordination pattern. The sequential interjoint coordination pattern previously seen in females with an absolute load (15 kg) was still present with the relative load, suggesting the influence of factors more intrinsically linked to sex. Considering that the female coordination pattern likely stretched posterior passive tissues when lifting boxes from the ground, potentially leading to higher risk of injury, the reason for this sex effect must be identified so that preventive interventions can be proposed.

Thinking and doing: the effects of dopamine and oxytocin genes and executive function on mothering behaviours.

Animal and human studies suggest that initial expression of maternal behaviour depends on oxytocin and dopamine systems. However, the mechanism by which these systems affect parenting behaviours and the timing of these effects are not well understood. This article explores the role of mothers' executive function in mediating the relation between oxytocin and dopamine gene variants and maternal responsiveness at 48 months post-partum. Participants (n = 157) were mothers recruited in the Maternal Adversity, Vulnerability and Neurodevelopment Study, which assesses longitudinally two cohorts of mothers and children in Canada. We examined single nucleotide polymorphisms (SNPs) related to the dopamine and oxytocin systems (DRD1 rs686, DRD1 rs265976, OXTR rs237885 and OXTR rs2254298), assessed mothers' decision-making at 48 months using the Cambridge Neurological Automated Testing Battery (CANTAB) and evaluated maternal responsiveness from videotaped interactions during the Etch-A-Sketch co-operation task. Mediation analyses showed that OXTR rs2254298 A-carriers had an indirect effect on positive parenting which was mediated by mothers' performance on decision-making task (estimate = 0.115, P < 0.005), while OXTR rs2254298 A-carriers had both direct and indirect effects on physically controlling parenting, also mediated through enhanced performance on decision-making (estimate = -0.059, P < 0.005). Dopamine SNPs were not associated with any measure of executive function or parenting (all P > 0.05). While oxytocin has previously been associated with only the early onset of maternal behaviour, we show that an OXTR polymorphism is involved in maternal behaviour at 48 months post-partum through mothers' executive function. This research highlights the importance of the oxytocin system to maternal parenting beyond infancy.

Reproducibility of a task description questionnaire for working pregnant women.

The objective of this study was to evaluate the reproducibility of a Task Description Questionnaire that was designed to investigate exposures to, and influential factors for, problematic tasks experienced by working pregnant women. The questionnaire comprised questions concerning 22 task components (covering working posture, manual material handling, work pace, prolonged postures and others), eight influential factors contributing to problematic tasks, discomfort (measured using a body map) and level of effort to perform the tasks. Reproducibility of the questionnaire was assessed by interviewing participants on two occasions one week apart for interviews at both 20 and 34 weeks of pregnancy. Eleven and 13 problematic tasks were reported by 21 working pregnant women at 20 and 34 weeks of pregnancy, respectively. These tasks were surveyed using the Task Description Questionnaire. Kappa statistics and correlation coefficients (supplemented by paired t-tests) were used to examine the reproducibility of responses to the questionnaire. The results showed that most of the variables were measured with very good or satisfactory reproducibility. The reproducibility of exposure to work posture was higher than that of exposure to manual material handling. There was no significant difference between test and retest means for the discomfort scores measured on the body map, except for the maximum discomfort score for the whole body in the 34 weeks survey. The study suggests that the questionnaire can be reliably used in the study of problematic tasks experienced by pregnant women. But an initial preview of the questions by the subjects and explanation of the questions given to the subjects by the interviewer may help to produce more reliable results.

Effects of variation in external pulling force magnitude, elevation, and orientation on trunk muscle forces, spinal loads and stability.

Nowadays in various daily, occupational and training activities, there are many occasions with forces supported in hands acting at various magnitudes, elevations, and orientations with substantial horizontal components. In this work, we aim to compute trunk muscle forces, stability, and spinal loads under pulling external forces applied at 3 elevations and 13 orientations. Under an identical upright standing posture and upper body weight, the trunk active-passive response is computed using a validated iterative finite element kinematics-driven model. Pulling forces of 80, 120, and 160N are resisted symmetrically in both hands held at 20, 40, and 60 cm elevations above the L5-S1 and oriented each in upward (-90°), inclined upward (-75°, -60°, -45°, -30°, and -15°), horizontal (0°), inclined downward (15°, 30°, 45°, 60°, and 75°) and finally downward in gravity direction (90°). In addition, in all analyses, an antagonist moment of 10Nm is applied in order to generate rather small antagonist coactivity and intra-abdominal pressures of 8-12 kPa are considered when abdominal muscles are active under upward pulling forces. Results demonstrated substantial differences in muscular response, spinal loads, and stability margin as the pulling force elevation, orientation, and magnitude altered. Compression and shear forces at lower lumbar levels peaked under forces at higher elevations acting with downward inclinations. Minimum spinal forces were computed at all elevations under pulling forces in the upward direction. Trunk stability was also maximum under these latter forces pulling upward. These findings have important consequences in rehabilitation, training, and design of safer occupational activities.

Subject-specific biomechanics of trunk: musculoskeletal scaling, internal loads and intradiscal pressure estimation.

Development of a subject-specific computational musculoskeletal trunk model (accounting for age, sex, body weight and body height), estimation of muscle forces and internal loads as well as subsequent validation by comparison with measured intradiscal pressure in various lifting tasks are novel, important and challenging. The objective of the present study is twofold. First, it aims to update and personalize the passive and active structures in an existing musculoskeletal kinematics-driven finite element model. The scaling scheme used an existing imaging database and biomechanical principles to adjust muscle geometries/cross-sectional-areas and passive joint geometry/properties in accordance with subjects' sex, age, body weight and body height. Second, using predictions of a detailed passive finite element model of the ligamentous lumbar spine, a novel nonlinear regression equation was proposed that relates the intradiscal pressure (IDP) at the L4-L5 disc to its compression force and intersegmental flexion rotation. Predicted IDPs and muscle activities of the personalized models under various tasks are found in good-to-excellent agreement with reported measurements. Results indicate the importance of personal parameters when computing muscle forces and spinal loads especially at larger trunk flexion angles as minor changes in individual parameters yielded up to 30 % differences in spinal forces. For more accurate subject-specific estimation of spinal loads and muscle activities, such a comprehensive trunk model should be used that accounts for subject's personalized features on active musculature and passive spinal structure.

Effects of sex, age, body height and body weight on spinal loads: Sensitivity analyses in a subject-specific trunk musculoskeletal model.

Subject-specific parameters influence spinal loads and the risk of back disorders but their relative effects are not well understood. The objective of this study is to investigate the effects of changes in age (35-60 years), sex (male, female), body height (BH: 150-190cm) and body weight (BW: 50-120kg) on spinal loads in a full-factorial simulation using a personalized (spine kinematics, geometry, musculature and passive properties) kinematics driven musculoskeletal trunk finite element model. Segmental weight distribution (magnitude and location along the trunk) was estimated by a novel technique to accurately represent obesity. Five symmetric sagittal loading conditions were considered, and main effect plots and analyses of variance were employed to identify influential parameters. In all 5 tasks simulated, BW (98.9% in compression and 96.1% in shear) had the greatest effect on spinal loads at the L4-L5 and L5-S1 levels followed by sex (0.7% in compression and 2.1% in shear), BH (0.4% in compression and 1.5% in shear) and finally age (<5.4%). At identical BH and BW, spinal loads in females were slightly greater than those in males by ~4.7% in compression and ~8.7% in shear. In tasks with no loads in hands, BW-normalized spinal loads further increased with BW highlighting the exponential increase in spinal loads with BW that indicates the greater risk of back disorders especially in obese individuals. Uneven distribution of weight in obese subjects, with more BW placed at the lower trunk, further (though slightly <7.5%) increased spinal loads.

Superficial shoulder muscle co-activations during lifting tasks: Influence of lifting height, weight and phase.

This study aimed to assess the level of co-activation of the superficial shoulder muscles during lifting movement. Boxes containing three different loads (6, 12, and 18 kg) were lifted by fourteen subjects from the waist to shoulder or eye level. The 3D kinematics and electromyograms of the three deltoids, latissimus dorsi and pectoralis major were recorded. A musculoskeletal model was used to determine direction of the moment arm of these muscles. Finally an index of muscle co-activation named the muscle focus was used to evaluate the effects of lifting height, weight lifted and phase (pulling, lifting and dropping phases) on superficial shoulder muscle coactivation. The muscle focus was lower (more co-contraction) during the dropping phase compared to the two other phases (-13%, p<0.001). This was explained by greater muscle activations and by a change in the direction of the muscle moment arm as a function of glenohumeral joint position. Consequently, the function of the shoulder superficial muscles varied with respect to the glenohumeral joint position. To increase the superficial muscle coactivation during the dropping phase may be a solution to increase glenohumeral joint stiffness.

Knee movement strategies differentiate expert from novice workers in asymmetrical manual materials handling.

One way to generate hypotheses on appropriate handling principles is to compare the strategies of highly skilled workers (experts) with those of novice workers (novices). This study was conducted to determine whether experts differ from novices in their handling strategies and to determine the effects of these differences in joint motions and net reaction moments at the trunk (L5/S1) and knees. Six experts and five novices were compared transferring two loads (12 and 22 kg) from four different initial low positions to a standardized low position on a trolley. The external forces were obtained from two AMTI force platforms, and two 16 mm Locam cameras coupled with two mirrors were used to obtain the three-dimensional kinematic data. An inverse dynamic analysis was performed on each segment and the net reaction moments were evaluated at L5/S1 and the knees; trunk posture, knee flexion angles and feet spacings were also calculated. Results showed that both groups minimized trunk asymmetries of posture and efforts; however, the experts were characterized by reduced knee flexion (31 degrees vs 53 degrees) and total knee excursion (20 degrees vs 38 degrees), increased flexion moments (72 vs 58 Nm) but reduced extension moments (4 vs 26 Nm) and valgus moments (30 vs 54 Nm). The experts' strategies appear safer overall because they reduce trunk asymmetry, asymmetrical efforts on the knee and stress on the femoropatellar joint. Moreover, these strategies present a potential for reduced mechanical energy expenditure due to lower knee excursions and increased knee stability due to reduced knee flexion.