Quantitative postural load assessment for whole body manual tasks based on perceived discomfort

Many Korean workers are exposed to repetitive manual tasks or prolonged poor working postures that are closely related to back pain or symptoms of musculoskeletal disorders. Workers engage in tasks that require not only handling of heavy materials, but also assuming prolonged or repetitive non-neutral work postures. Poor work postures that have been frequently observed in the workplaces of shipbuilding shops, manufacturing plants, automobile assembly lines and farms often require prolonged squatting, repetitive arm raising and wrist flexion and simultaneous trunk flexion and lateral bending. In most manufacturing industries, workers have to assume improper work postures repetitively, several hundreds of times per day depending on daily production rate. A series of psychophysical laboratory experiments were conducted to evaluate the postural load at various joints. A postural load assessment system was then developed based on a macro-postural classification scheme. The classification scheme was constructed based on perceived discomfort for various joint motions as well as previous research outcomes. On the basis of the perceived discomfort, postural stress levels for the postures at individual joints were also defined by a ratio scale to the standing neutral posture. Laboratory experiments simulating automobile assembly tasks were carried out to investigate the relationship between body-joint and whole-body discomfort. Results showed a linear relationship between the two types of discomfort, with the shoulder and low back postures being the dominant factor in determining the whole body postural stresses. The proposed method was implemented into a computer software program in order to automate the procedure of analysing postural load and to enhance usability and practical applicability.     

LUBA: an assessment technique for postural loading on the upper body based on joint motion discomfort and maximum holding time.

This paper presents a technique for postural loading on the upper body assessment (LUBA). The proposed method is based on the new experimental data for composite index of perceived discomfort (ratio values) for a set of joint motions, including the hand, arm, neck and back, and the corresponding maximum holding times in static postures. Twenty male subjects participated in the experiment designed to measure perceived joint discomforts. The free modulus technique of the magnitude estimation method was employed to obtain subjects' discomforts for varying joint motions. The developed postural classification scheme was based on the angular deviation levels from the neutral position for each joint motion. These were divided into groups with the same degree of discomforts based on the statistical analysis. Each group was assigned a numerical discomfort score relative to the perceived discomfort value of elbow flexion, which exhibited the lowest level among all joint motions investigated in this study, and, therefore, was set as a reference point. The criteria for evaluating stresses of working postures were proposed based on the four distinct action categories, in order to enable practitioners to apply appropriate corrective actions. The proposed scheme can be used for evaluating and redesigning static working postures in industry.     

A method for analytically generating three-dimensional isocomfort workspace based on perceived discomfort

The purpose of this study was to develop a new method for analytically generating three-dimensional isocomfort workspace for the upper extremities using the robot kinematics. Subjective perceived discomfort scores in varying postures for manipulating four types of controls were used. Fifteen healthy male subjects participated in the experiment. The subjects were asked to hold the given postures manipulating controls for 60 s in the seated position, and to rate their perceived discomfort during the following rest of 60 s using the magnitude estimation. Postures of the upper extremities set by shoulder and elbow motions, types of controls, and left right hand were selected as experimental variables, in which the L32 orthogonal array was adopted. The results showed that shoulder flexion and adduction-abduction, elbow flexion, and types of controls significantly affected perceived discomfort for postures operating controls, but hand used for operating controls did not. Depending upon the types of controls, four regression models predicting perceived discomfort were presented. Using the models, a sweeping algorithm to generate three-dimensional isocomfort workspace was developed, in which the robot kinematics was employed to describe the translational relationships between the upper arm and the lower arm/hand. It is expected that the isocomfort workspace can be used as a valuable design guideline when ergonomically designing three-dimensional workplaces.     

Evaluation of driver''s discomfort and postural change using dynamic body pressure distribution

The main objective of this study is the application of body pressure distribution measurements for the prediction of the driver's posture and its change. This requires quantitative analyses of dynamic body pressure distribution, which is the change of body pressure distribution with time. To investigate the relationship between dynamic body pressure data with driver's posture, 16 male subjects performed a simulated driving task for 45 min in a seating buck. During driving, the body posture and body-seat interface pressure were measured continuously, and the discomfort ratings were surveyed at the prescribed interval. For the statistical analyses, driving period, stature group, and lumbar support prominence were selected as independent variables, whereas subjective ratings of driver discomfort, driving posture, and body pressure values were selected as dependent variables. In this study, newly defined dynamic body pressure distribution variables were proposed, and the relationship between these pressure variables with subjective discomfort ratings were analyzed. The close correlations between the body pressure change variables and subjective discomfort ratings supported the possibility of using dynamic pressure data as a tool for the assessment of driver discomfort.

Relevance to industry

Since dynamic body pressure distribution data provide quantitative and objective indices in measuring driver's postural changes and discomfort while driving, the proposed method can be used for more effective automobile seat design and its evaluation.     

Predictors of perceived effort in the shoulder during load transfer tasks

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r(2) = 0.50) and muscle activity model-based muscle force predictions (MFPs): r(2) = 0.42, electromyography (EMG): r(2) = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r(2) = 0.74; MFP: r(2) = 0.67, EMG: r(2) = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.     

Predictors of perceived effort in the shoulder during load transfer tasks

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r² = 0.50) and muscle activity model-based muscle force predictions (MFPs): r² = 0.42, electromyography (EMG): r² = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r² = 0.74; MFP: r² = 0.67, EMG: r² = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.     

Effect of repetitive elbow holding tasks with low‐level muscular exertion on perceived muscle discomfort

This study aimed to investigate the effect of task repetition on perceived muscle discomfort (PMD). The experimental trial consisted of fatigue and recovery periods. In the fatigue period, an elbow‐holding task was performed with an external load of 10%, 20%, and 30% of the premeasured maximum voluntary contraction (MVC) for 7.0, 4.5, and 3.0 min, respectively. In the recovery period, the participants released the load and rested until they felt no PMD. The trial was repeated six times sequentially (Trials 1–6). PMD at the end of the fatigue period increased with task repetition at all %MVCs. The interclass correlation coefficients between Trial 1 and the other trials were lower than those for the combinations of Trials 2–6, irrespective of %MVC. In the recovery period, the absolute sensitivity of PMD in the range of 0–10 (or the speed of recovery) of Trial 1 was faster than that of Trials 2–6. The findings of this study could lead to the development of work regimes that limit the risk of PMD in workers. Possible changes include allowing for a longer recovery time if the task requires a relatively large flexion angle, such as those performed in narrow spaces.     

Impacts of frequency and posture on body mass index in manual handling tasks

Workspace design plays an important role in ensuring workers' safety and welfare. The issue is more pressing in the manufacturing industry, where many workers must remain in a standing position, assume awkward postures, and perform repetitive tasks for extended periods of time. In this research, an electromyographic measurement technique was used to measure activity of four back muscles: the trapezius p. descendens, the deltoideus p. scapularis, the infraspinatus, and the latissimus dorsi. The results showed a statistically significant (p < 0.05) impact of left versus right side of the body, the degree of rotation to the right side of the body (0, 30, and 60 degrees rotation to the mid‐sagittal plane), body mass index level (normal weight [<25 kg/m²] vs. overweight [?25 kg/m²]), and tasks (high vs. low frequency). In this study, the role that workers and workspace variation play is clearly associated with an increase in the amplitude of electromyography at the targeted back muscles. © 2009 Wiley Periodicals, Inc.     

Psychophysical models for manual lifting tasks

This paper provides two models for males and females to assess the psychophysical maximum acceptable weight of lift. The weight guidelines generated by the models are a function of lifting frequency, height of lift, sagittal or asymmetrical lifting, task duration, container size in the sagittal plane, presence or absence of container couplings, and percentage of the working population. The developed models were generated from a knowledge base available in the published literature. A computer program was written in BASIC to assist the user in determining the safe load that could be handled by a specified working population. Model validation showed that the models developed predict the maximum acceptable weight of lift with a reasonable degree of accuracy. A comparison between the National Institute for Occupational Safety and Health recommendations and those made on the basis of the models developed in this paper is also presented.     

A knowledge-based system for assessment of human physiological abilities in manual lifting tasks

The purpose of this paper is to develop and test an expert system for assessment of human physiological abilities in manual lifting tasks. The expert system was implemented on an IBM PC-XT personal computer. An example on how to utilize the expert system in designing manual lifting tasks from a physiological standpoint is given.     

Impact of ballistic body armour and load carriage on walking patterns and perceived comfort

This study investigated the impact of weight magnitude and distribution of body armour and carrying loads on military personnel's walking patterns and comfort perceptions. Spatio-temporal parameters of walking, plantar pressure and contact area were measured while seven healthy male right-handed military students wore seven different garments of varying weight (0.06, 9, 18 and 27 kg) and load distribution (balanced and unbalanced, on the front and back torso). Higher weight increased the foot contact time with the floor. In particular, weight placement on the non-dominant side of the front torso resulted in the greatest stance phase and double support. Increased plantar pressure and contact area observed during heavier loads entail increased impact forces, which can cause overuse injuries and foot blisters. Participants reported increasingly disagreeable pressure and strain in the shoulder, neck and lower back during heavier weight conditions and unnatural walking while wearing unbalanced weight distributed loads. This study shows the potentially synergistic impact of wearing body armour vest with differential loads on body movement and comfort perception.

Practitioner Summary: This study found that soldiers should balance loads, avoiding load placement on the non-dominant side front torso, thus minimising mobility restriction and potential injury risk. Implications for armour vest design modifications can also be found in the results.     

An evaluation of classification algorithms for manual material handling tasks based on data obtained using wearable technologies

With recent progress in wearable measurement systems, physical exposures can be feasibly assessed at high precision in the workplace. Such systems, however, generally lack contextual information for a given job (e.g. task type, duration). To extract such information, we explored three classification algorithms to classify manual material handling (MMH) tasks during a simulated job in a laboratory, using several combinations of outputs from commercially available inertial motion capture and in-shoe pressure measurement systems. A total of 10 participants completed three replications of four cycles of a simulated job. Precision and recall values of ≥ ～90% and 80%, respectively, and errors in estimated task duration of < ～14%, could be achieved across the MMH task examined. Classification performance, however, varied between classification algorithms, input data sets and task types. Overall, combining wearable technology with task classification could be an effective approach for field-based exposure assessment, though field-testing is needed to demonstrate the applicability of this method.

Practitioner Summary: Combining wearable technologies with task classification was explored to extract exposure context, specifically task type and duration. Results supported that task classification can facilitate the use of wearable technologies in field-based exposure assessment, specifically by aiding in task identification from within the rather large data sets obtained from these technologies.     

An endurance training programme for frequent manual carrying tasks

This study was conducted with the aim of testing whether the endurance time of new employees engaged in frequent industrial carrying tasks can be significantly increased through a short training programme (2.5 weeks). Endurance time was here defined as the maximum length of time during which an individual can continuously carry a 20 kg load over a 4 m distance 8 times/min. The results showed that (1) endurance time increased by almost 100% over an 8-session training programme; (2) for a fixed workload, endurance time increased without changing job demand perception; and (3) overall body fatigue may be the limiting factor in frequent carrying tasks.     

Optimum design of containers for manual material handling tasks

The container in manual material handling represents the point of interface between the worker and his task as well as with the surrounding environment. It is at this point that many of the well known handling hazards occur which manifest themselves in stresses and strains that are transmitted to the body via the musculo-skeletal system. If a substantial number of handling hazards is to be controlled or eliminated at the source, containers designed in accordance with principles of biomechanics and related recommendations provide a logical starting point. The container characteristics to be considered in the design process are weight (and its distribution), shape, stiffness, and availability of coupling devices. This paper presents several examples which outline and detail a number of problems associated with the design of containers involved in manual tasks. Application of basic mechanics, coupled with the use of optimization techniques, is presented as the approach for dealing with the hazards and problems of containers.     

Development of a safety index for manual lifting tasks

The concept of a safety index (SI) for assigning a worker to a particular manual lifting task is developed, and a simple formula for its calculation is presented. The proposed index is based upon the combined measure of acceptability of the biomechanical and physiological stress responses of the worker to a lifting task. Individual capacity norms, as opposed to the norms usually given based on population percentiles, are also defined. Numerical examples are given to illustrate the SI approach.     

An ergonomic intervention for manual load carrying on Indian farms

The farm inputs like seeds, fertilizers and produce are handled manually in most of the low-income and underdeveloped countries. Such practice of manual load carrying is one of the dominant contributors to back pain and associated physiological problems.In the present study, it was found that volume, quantum and distance determine the mode of load carrying. Bulky or voluminous commodities like harvested cereal crops and fodder are carried on the head. Heavy loads of paddy and vegetables filled in gunny bags, which limits the volume to bag size are carried on the back. Carrying the load on the head is by and large prevailing mode because it does not introduce eccentricity and postures remain balanced., In the present study, an attempt is made to develop a harness, which does not alter the head loading but prevents the loading of the spine. Three approaches considered were: Approach-I (harness H₁): load passing through shoulders only (one path only); Approach II (harness H₂): load passing through head and shoulder (through two paths only); Approach III (harness H₃): Load passing through head and shoulder (through two paths), but in this approach, load on the cervical spine was limited to 10 kg and rest of the load was transferred to shoulder with proper stability. The outcome of ergonomic studies showed that Approach-III is the most metaboliclly economical, compared to other developed approaches and could be adopted for Indian agricultural farm load carriage for various purposes.     

The effects of military body armour on trunk and hip kinematics during performance of manual handling tasks*

Musculoskeletal injuries are reported as burdening the military. An identified risk factor for injury is carrying heavy loads; however, soldiers are also required to wear their load as body armour. To investigate the effects of body armour on trunk and hip kinematics during military-specific manual handling tasks, 16 males completed 3 tasks while wearing each of 4 body armour conditions plus a control. Three-dimensional motion analysis captured and quantified all kinematic data. Average trunk flexion for the weightiest armour type was higher compared with control during the carry component of the ammunition box lift (p?<?0.001) and sandbag lift tasks (p?<?0.001). Trunk rotation ROM was lower for all armour types compared with control during the ammunition box place component (p?<?0.001). The altered kinematics with body armour occurred independent of armour design. In order to optimise armour design, manufacturers need to work with end-users to explore how armour configurations interact with range of personal and situational factors in operationally relevant environments.

Practitioner Summary: Musculoskeletal injuries are reported as burdening the military and may relate to body armour wear. Body armour increased trunk flexion and reduced trunk rotation during military-specific lifting and carrying tasks. The altered kinematics may contribute to injury risk, but more research is required.     

Placement of forearm surface EMG electrodes in the assessment of hand loading in manual tasks

Surface electromyography (EMG) is commonly used to study the loading of the forearm. Pro-supination movements cause surface electrodes to move in relation to the underlying muscles. We studied the effects of different electrode locations and forearm postures on the association between the EMG signals and external hand load in a laboratory experiment. Eleven subjects performed simulated work tasks with the forearm in neutral, pronated or supinated postures and with systematic variation of external load. The tasks included isometric gripping, pushing and pulling, and lifting and lowering weights. Surface EMG was recorded by six pairs of electrodes located on the forearm. The associations were studied using multiple regression models. EMG activity varied according to the forearm posture, location of electrodes and type of simulated task. Variation was lowest with a through-forearm setting of electrodes. This setting also showed the highest correlation between external loads and the EMG activity [coefficient of determination (R ²) = 0.25–0.66].

Practitioner Summary: Moving of surface electrodes in relation to the underlying muscles interferes with the assessment of loading in ergonomic settings. This laboratory experiment showed that a through-forearm location of electrodes seems to be an optimal option in the assessment of forearm loading.     

Comparison of manual control methods for space manipulator positioning tasks

The Russian Segment of the International Space Station will be equipped with a 10 m long anthropomorphic space manipulator with six degrees of freedom. The robot is called the European Robot Arm, or ERA. Its hand, or end-effector, contains a camera which will be used for fine positioning of the end-effector, in order to grasp an object. At the Delft University of Technology, a human-machine interface for manual control of the ERA is being developed. The control device is a Spaceball controller, which is used to control the spatial end-effector velocity. This article describes four control methods that transform the Spaceball movements into movements in the end-effector camera picture. The control methods are compared by means of human-machine experiments. The results are also useful in many other areas of spatial manipulation, like undersea robots, robots in the nuclear industry, and minimally invasive surgery.