Alain Delisle

Biomechanical differences between expert and novice workers in a manual material handling task.

The objective was to verify whether the methods were safer and more efficient when used by expert handlers than by novice handlers. Altogether, 15 expert and 15 novice handlers were recruited. Their task was to transfer four boxes from a conveyor to a hand trolley. Different characteristics of the load and lifting heights were modified to achieve a larger variety of methods by the participants. The results show that the net moments at the L5/S1 joint were not significantly different (p > 0.05) for the two groups. However, compared with the novices, the experts bent their lumbar region less (experts 54° (SD 11°); novices 66° (SD 15°)) but bent their knees more (experts approx. 72° (SD approx. 30°); novices approx. 53° (SD approx. 33°), which brought them closer to the box. The handlers posture therefore seems to be a major aspect that should be paid specific attention, mainly when there is maximum back loading. Statement of Relevance: The findings of this research will be useful for improving manual material handling training programmes. Most biomechanical research is based on novice workers and adding information about the approach used by expert handlers in performing their tasks will help provide new avenues for reducing the risk of injury caused by this demanding physical task.

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.

Relative importance of expertise, lifting height and weight lifted on posture and lumbar external loading during a transfer task in manual material handling

The objective of this study was to measure the effect size of three important factors in manual material handling, namely expertise, lifting height and weight lifted. The effect of expertise was evaluated by contrasting 15 expert and 15 novice handlers, the effect of the weight lifted with a 15-kg box and a 23-kg box and the effect of lifting height with two different box heights: ground level and a 32 cm height. The task consisted of transferring a series of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables (moments) while expertise had low impact. On the other hand, expertise showed a significant effect of posture variables on the lumbar spine and knees. All three factors are important, but for a reduction of external back loading, the focus should be on the lifting height and weight lifted. Practitioner Summary The objective was to measure the effect size of three important factors in a transfer of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables but expertise was a major determinant in back posture.

Physical exposure of sign language interpreters: baseline measures and reliability analysis

Measurement of physical exposure to musculoskeletal disorder risk factors must generally be performed directly in the field to assess the effectiveness of ergonomic interventions. To perform such an evaluation, the reliability of physical exposure measures under similar field conditions must be known. The objectives of this study were to estimate the reliability of physical exposure measures performed in the field and to establish the baseline values of physical exposure in sign language interpreters (SLI) before the implementation of an intervention. The electromyography (EMG) of the trapezius muscles as well as the wrist motions of the dominant arm were measured using goniometry on nine SLI on four different days. Several exposure parameters, proposed in the literature, were computed and the generalizability theory was used as a framework to assess reliability. Overall, SLI showed a relatively low level of trapezius muscle activity, but with little time at rest, and highly dynamic wrist motions. Electromyography exposure parameters showed poor to moderate reliability, while goniometry parameter reliability was moderate to excellent. For EMG parameters, performing repeated measurements on different days was more effective in increasing reliability than extending the duration of the measurement over one day. For goniometry, repeating measurements on different days was also effective in improving reliability, although good reliability could be obtained with a single sufficiently long measurement period.

Reliability of different thresholds for defining muscular rest of the trapezius muscles in computer office workers

This study aimed at documenting the reliability of different thresholds used for defining the muscular rest of the trapezius muscles of 27 computer office workers, using surface electromyography (EMG) signals collected in the field. Measurement strategies for increasing the reliability of the results were also explored. Ten different thresholds to define muscular rest were compared: 1) five normalised (individualised) thresholds; 2) three absolute thresholds (in μV); 3) two absolute but individualised thresholds. The reliability was assessed using both a 15-min standardised computer task and 45 min of regular computer work. The main findings were: 1) overall, in a repeated measures study design, muscular rest variables were more reliable with the use of absolute thresholds when compared to normalised and individualised thresholds; 2) excellent reliability (index of dependability >0.75) can be reached when averaging the scores over 2 days; 3) using a standardised task instead of regular work does not necessarily lead to more reliable results.

Effect of a desk attachment board on posture and muscle activity in women during computer work.

Working at a computer is part of a large number of jobs and has been associated with upper extremity musculoskeletal disorders and back pain. The study evaluated the effects of a board attachment on upper extremity and back. The findings are mixed in that the board may have a positive effect in preventing back pain, but may be detrimental to upper extremities. Effect of a desk attachment board on upper extremity and trunk posture, and muscle activity was assessed in women video display terminal users. Participants completed a standard 20-min computer task under two conditions: 1) using a standard desk; 2) using a desk attachment board designed to support the forearms. Bilateral electromyography of the trapezius, multifidus and longissimus muscles and the right anterior deltoid and forearm extensor muscles was recorded. 3-D trunk and upper extremity posture was monitored. Participants were tested before and after 2 weeks of familiarisation with the board in their workplace. Perceived tension and discomfort were recorded before and after use of the board. Use of the board tended to increase muscle activity in the right trapezius and forearm extensor and to decrease muscle activity in the back. Perceived tension in the low back decreased slightly with the board. The board may be useful in reducing tension in the low back during computer work, but may adversely affect the upper extremities.

Sex differences in lifting strategies during a repetitive palletizing task

Forty-five manual material handlers (15 females, 15 expert males and 15 novice males) performed series of box transfers under conditions similar to those of large distribution centers. The objective of the study was to verify whether sex differences in joint motions and in back loading variables (L5/S1 moments) exist during multiple box transfers. The task consisted in transferring 24 15-kg boxes from one pallet to another (4 layers of boxes; 6 boxes/layer: 3 in the front row, 3 in the back) at a self-determined pace and then at an imposed pace of 9 lifts/min. Full-body 3D kinematic data were collected as well as external foot forces. A dynamic 3D linked segment model was used to estimate the net moments at L5/S1. The results show that the peak L5/S1 moment during lifting for females was significantly lower than for males, but once normalized to body size the difference disappeared. In general, the female workers were very close to the posture adopted by the novice males at the instant of the peak resultant moment. However, females were closer to the box than the male workers. One major sex difference was seen when lifting from the ground, with the use of interjoint coordination analyses. Female workers showed a sequential motion initiated by the knees, followed by the hip and the back, while expert males showed a more synchronized motion. The lifting strategy of females likely stretches lumbar spine passive tissues, which in turn put them at greater risk of back injuries. As observed in our previous studies, these differences between expert males, novice males and females are especially notable when the box is lifted from the ground.

Sudden loading perturbation to determine the reflex response of different back muscles: A reliability study

In this study we estimate the reliability of reflex response variables to identify the main sources of variability and to estimate appropriate measurement strategies to obtain more reliable measures. Back muscle surface electromyography (EMG) was recorded in healthy males during anteriorly‐directed sudden loading perturbations applied to the trunk. Measures of EMG reflex latency and amplitude were obtained. The generalizability theory was used as a framework to estimate the magnitude of the different variance components and the reliability of the measures corresponding to various simulations of different measurement strategies. Reliability of the different variables was poor to moderate (intraclass correlation coefficient range 0–0.62). Averaging scores across homologous muscles and several trials were strategies to achieve more acceptable reliability. The reflex response of back muscles is inherently variable, and a large measurement effort is necessary to obtain reliable and, consequently, valid and responsive estimations of this neuromuscular function. Muscle Nerve, 2011

Evaluation of forearm support provided by the Workplace Board on perceived tension, comfort and productivity in pregnant and non-pregnant computer users.

The aim of the study was to evaluate the effect of forearm support provided by the Workplace Board on perceived tension, comfort and productivity among pregnant and non-pregnant female computer workers. Ten pregnant and 18 non-pregnant women participated in the study. Participants completed three sets of tension/discomfort questionnaires at two week intervals. The first set was completed prior to any workstation intervention; the second set was completed after two weeks working with an ergonomically adjusted workstation; the third set was completed after two weeks working with the Workplace Board integrated into the office workstation. With the Workplace Board, decreased perceived tension was reported in the left shoulder, wrist and low back in non-pregnant women only. The Board was generally liked by all participants, and increased comfort and productivity in all areas, with the exception of a negative effect on productivity of general office tasks. The board is suitable for integration in most office workstations and for most users, but has no special benefits for pregnant women.

Perception and biomechanics data in a manual handling task: a comparative study.

This paper explores the use of subjective perception tasks and its correlations with biomechanical data in the evaluation of manual material handling. Three main dimensions were considered for perception: physical regroups sensations issued from a specific body area; operative regroups feelings related to the execution of the task; and performance regroups feelings that involve a judgement on the execution or reflect overall sensations. The following questions were then explored. To what extent are perception data related to biomechanics data? Do both approaches lead to similar conclusions or interpretations when effect of practice, format and off-centre were tested? How can they complement one another? The task consisted of transferring 50 series of three 15 kg loads in order to verify the impact of free practice, format (box/cylinder) and load centre of gravity position. Eleven subjects rated perception on a CR-10 scale (Borg 1982) after each series. The session was completed with an interview on perception. The net resulting moment was systematically found to be the best correlated with data perception. While all physical and performance items corresponded in various ways to biomechanics data, perceptions associated with operative dimension appeared to be less related with biomechanical data. As regards the impact of practice, format and off-centre, both approaches would lead to the same conclusions, except for the effect of the off-centre. Verbal data add rational information about how or why perception can or cannot be reflected in biomechanics data. How both approaches can be matched more closely in manual handling is discussed.