The psychophysical lifting capacities of Chinese subjects

The psychophysical lifting capacity (MAWL) of twelve subjects was determined in this study. The subjects were all young Chinese males who performed lifting tasks in three lifting ranges (floor to knuckle, floor to shoulder, and knuckle to shoulder) and four lifting frequencies (one-time maximum, 1 lift/min, 4 lifts/min, and 6 lifts/min). The oxygen uptake (1/min) and heart rate (beats/min) were recorded while subjects were lifting. Upon completion of each lifting task, the subjects were required to rate their perceived exertion levels. The statistical analyses results indicated the following. Chinese subjects have smaller body size and MAWLs compared with past studies using the US population. The MAWLs decreased with an increase in lifting frequencies. The decrements of MAWL due to lifting frequencies were in agreement with the results of past studies. However, there were larger decreases due to lifting ranges. The MAWLs of the floor to knuckle height lift were the largest, followed by the MAWLs of the floor to shoulder height lift, and the MAWLs of the knuckle to shoulder height lift. The measured physiological responses were considered similar to those obtained in past studies. Subjects' perceived stress levels increased with the lifting frequency and the upper extremities received the most stress for the total range of lifting tasks. The comparisons of the Chinese MAWLs with the NIOSH lifting guidelines for limits (AL and MPL) indicated that the vertical discounting factor in the guidelines should be modified before the NIOSH limits can be applied to non-Western populations.     

Review paper A comparative study of methods for establishing load handling capabilities

There has been much effort in recent years to quantify manual handling capabilities. Four main techniques have been used to this end; biomechanical modelling; the measurement of intra-abdominal pressure; psychophysics; and metabolic/physiological criteria. The aim of this study was to compare quantitatively the data produced from the first three techniques. The comparisons were limited to bimanual, sagittal plane lifting, which of all manual handling activities has been studied the most comprehensively, except that pushing and pulling data were compared from the psychophysics and intra-abdominal pressure (‘force limits’) databases. It was found that the data from ‘force limits’ proposed weights for bimanual lifting in the sagittal plane are lower than those reported to be psychophysically acceptable except for lifting close to and around the shoulder. The closest agreement between the databases was for lifting from an origin above knuckle height. The ‘force limits’ data were found to propose weights of lift which are at a minimum when lifting with a freestyle posture from the floor whereas the psychophysical technique proposes weights which are at a maximum when lifting from the floor. The psychophysical data were found to generate compressive forces at L5/S1 according to a static sagittal plane biomechanical model about 10% in excess of the NIOSH action limit (NIOSH 1981) when lifting from the floor, although over other lifting ranges the compressive forces were less than the NIOSH action limit. Lifting the (force limits) weights generated compressive forces which were on average 55% less than the AL (range 45 to 60%) when lifting in an erect posture. The data for pushing according to the psychophysical and ‘force limits’ database showed good agreement, but for pulling the ‘force limits’ weights were considerably greater than those selected psych ophysically. The implications of these findings are discussed.     

A cross-validation of the NIOSH limits for manual lifting

The psychophysical, biomechanical, and physiological criteria used in establishing the NIOSH limits for manual lifting were cross-validated against the data published by different researchers in the subject literature. Assessment of the 1991 NIOSH lifting equation indicated that: (1) NIOSH-based limits are significantly different from the psychophysical limits in the (i) low and high frequencies of lift, and (ii) small and large horizontal distances; (2) NIOSH limits are highly correlated with the data of Snook and Ciriello (1991) in the low frequency range, with the Recommended Weight Limit (RWL) protecting about 85% of the female population and 95% of the male population; (3) the 3·4 kN limit for compression on the lumbosacral joint cannot protect the majority of the worker population on the basis of damage load concept; and (4) energy expenditure limits used in development of the RWL index can be sustained by 57 to 99% of worker population when compared to the physiological limits based on previous fatigue studies. Results of the cross-validation for psychophysical criterion confirmed the validity of assumptions made in the 1991 NIOSH revised lifting equation. However, the results of cross-validation for the biomechanical and physiological criteria were not in total agreement with the 1991 NIOSH model     

An ergonomic evaluation of handle height and load in maximal and submaximal cart pushing

Awareness of the hazards of repetitive lifting has brought about significant changes in the design of industrial jobs. Pushing and pulling tasks have become increasingly common as the result of the introduction of a variety of carts and other materials-handling assistance devices. In order to predict the peak performance of workers in these tasks, and the biomechanical stresses that can result from them, the exertions involved in cart pushing were studied. Four subjects of various strengths pushed carts with loads from 45 to 450 kg at several heights. Peak push forces reached 500 N for male subjects and 200 N for female subjects. Strong subjects moved a 45 kg cart at velocities of 1.1 m s(-1) and a 450 kg cart at velocities of 0.8 m s(-1). Weaker subjects moved the carts at velocities of 0.5 and 0.4 m s(-1) respectively. Calculated static compression forces at the L5/S1 spinal disc were consistently above the NIOSH Action Limit of 3400 N for strong subjects when the cart load reached 225 kg.     

THE STRENGTH OF THE LIFTING ACTION IN MAN

The maximum isometric force exertable on a horizontal bar situated in a frontal plane was found to vary, for eight young male subjects and for the different lifting conditions observed, from 15 to 120 kg. Increase of foot placement distance from 30 cm to 50 cm effected the largest reduction in lifting force. The reduction produced by increase of grasp height from 12½ cm to 50 cm was very much less, and the effect of type of grasp (overhand or underhand) and type of lifting action (using or not using knee extension) was generally small. It appeared that the magnitude of the lifting force was largely determined by the magnitude of the force moment which the body mass could exert to counterbalance the reaction of the lifting force upon the body. The nature of the muscular activity associated with this stabilization of the body during lifting operations is discussed.     

Comparison of prediction models for the compression force on the lumbosacral disc

The main objective of this research was to compare three representative methods of predicting the compressive forces on the lumbosacral disc: LP-based model, double LP-based model, and EMG-assisted model. Two subjects simulated lifting tasks that are frequently performed in the refractories industry of Korea, in which vertical and lateral distances, and weight of load were varied. To calculate the L5/ SI compressive forces, EMG signals from six trunk muscles were measured, and postural data and locations of load were recorded using the Motion Analysis System. The EMG-assisted model was shown to reflect well all three factors considered here, whereas the compressive forces from the two LP-based models were only significantly affected by weight of load. In addition, low lifting index (LI) values were observed for relatively high L5/S1 compressive forces from the EMG-assisted model, suggesting that the 1991 NIOSH lifting equations may not fully evaluate the risk of dynamic asymmetric lifting tasks.     

Manual materials handling: a survey of risks,and the selection and training of workers in Singapore

A survey of 83 plants employing safety officers indicated that in 66% of them, workers were involved in manual lifting. The average number of manual lifting tasks was three per plant. Only about 6% of the total workforce were involved in manual lifting. Using the NIOSH Lifting Guidelines, the survey revealed that 30% of the lifting tasks were above the Maximum Permissible Limit, 50% were between the Maximum Permissible Limit and the Action Level, and only 20% were below the Action Level. A majority of the plants used a self-selection method to match a worker to the physical demands of a given manual lifting task. Training of workers in manual materials handling was provided by 76% of the plants. The principal methods of training included a poster campaign and demonstration of lifting techniques. The basic handling skills were taught in 90% of the plants. The content of training was incomplete, however, and further improvement was necessary.     

Accuracy of measurements for the revised NIOSH lifting equation

Twenty-seven non-ergonomists who participated in a one-day training session on the use of the NIOSH lifting equation (NLE) were subsequently tested on a simulated lifting task eight weeks later to determine their accuracy in measuring the variables. Analysis of the results indicate that (1) inter-observer variability was small, especially for the most important factor (i.e. horizontal distance); (2) individuals can be trained to make measurements with sufficient accuracy to provide consistent recommended weight limit and lifting index values; and (3) measurement of the coupling and asymmetric variables were the least accurate.     

Empirical evaluation of training and a work analysis tool for participatory ergonomics

A controlled laboratory experiment was performed to test the effects of ergonomics training and the NIOSH lifting equation on the participatory redesign of a simulated manual material handling job. Before performing the job, 16 subjects were given ergonomics training and 16 were instructed on how to use the NIOSH lifting equation for manual lifting tasks. Compared to a control group, subjects who received the ergonomics instruction identified and eliminated more risk factors in the simulated job. While subjects who used the NIOSH lifting equation also identified more risk factors, they did not eliminate any more risk factors than the control group. No additive benefit was found using both the training and the lifting equation over either method alone. Ergonomics training led to better improvements than use of the lifting equation in terms of risk factors identified and eliminated. Implications for use of training and tools in participatory ergonomics approaches are discussed.

Relevance to industry

This study supports that ergonomics training should be a requisite for any participatory ergonomics approach. Given a fundamental level of ergonomics training, subjects demonstrated that they were better capable of identifying and eliminating risk factors in the job.     

A comparative study of methods for establishing load handling capabilities

There has been much effort in recent years to quantify manual handling capabilities. Four main techniques have been used to this end; biomechanical modelling; the measurement of intra-abdominal pressure; psychophysics; and metabolic/physiological criteria. The aim of this study was to compare quantitatively the data produced from the first three techniques. The comparisons were limited to bimanual, sagittal plane lifting, which of all manual handling activities has been studied the most comprehensively, except that pushing and pulling data were compared from the psychophysics and intra-abdominal pressure ('force limits') databases. It was found that the data from 'force limits' proposed weights for bimanual lifting in the sagittal plane which [corrected] are lower than those reported to be psychophysically acceptable except for lifting close to and around the shoulder. The closest agreement between the databases was for lifting from an origin above knuckle height. The 'force limits' data were found to propose weights of lift which are at a minimum when lifting with a freestyle posture from the floor whereas the psychophysical technique proposes weights which are at a maximum when lifting from the floor. The psychophysical data were found to generate compressive forces at L5/S1 according to a static sagittal plane biomechanical model about 10% in excess of the NIOSH action limit (NIOSH 1981) when lifting from the floor, although over other lifting ranges the compressive forces were less than the NIOSH action limit. Lifting the 'force limits' weights generated compressive forces which were on average 55% less than AL (range 45 to 60%) when lifting in an erect posture. The data for pushing according to the psychophysical and 'force limits' database showed good agreement, but for pulling the 'force limits' weights were considerably greater than those selected psychophysically. The implications of these findings are discussed.     

Spinal loads during individual and team lifting

The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, and (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting.     

Spinal loads during individual and team lifting

The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting.     

An evaluation of methods assessing the physical demands of manual lifting in scaffolding

Four methods assessing the physical demands of manual lifting were compared. The scaffolding job was evaluated and three distinct scaffolding tasks were ranked using: (1) the revised NIOSH lifting equation (NIOSH method), (2) lifting guidelines for the Dutch construction industry (Arbouw method), (3) rapid appraisal of the NIOSH lifting equation (practitioners' method), and (4) systematic observations. For the three first-mentioned methods the same dataset was used; observation took place in a different setting in the same company. At job level, all methods indicated that ergonomic interventions are required to protect scaffolders from an increased risk for low back pain. The NIOSH, Arbouw and practitioners' method resulted in a similar ranking order of tasks (transport>construction>dismantlement). In contrast, the observational method gave transport the lowest ranking. The underlying cause was probably that the observational method is more sensitive to durations of tasks and lifting within tasks than the three other methods.     

The effects of speed,frequency, and load on measured hand forces for a floor to knuckle lifting task

The purpose of this study was to describe and quantify measured hand forces during floor to knuckle lifting of various loads. Hand forces of five subjects were measured with a strain gauge apparatus for normal and fast speeds of lifting at 1,4, and 8 l/min. The pattern of hand force over time exhibited peaks in force in the shape of a spike for all fast lifts, indicating that subjects did not lift smoothly. For normal speed of lift, only one of the five subjects executed some lifts smoothly, indicating that it may be possible to lift smoothly, but most lifters probably do not. Peaks of horizontal and vertical components of hand force were tabulated by speed of lift, frequency, and load.     

Evaluating lifting tasks using subjective and biomechanical estimates of stress at the lower back

The objective of this study was to evaluate five different lifting tasks based on subjective and biomechanical estimates of stress at the lower back. Subjective estimates were obtained immediately after the subjects performed the lifting tasks. Rankings for different tasks were obtained according to the perceived level of stress at the lower back. A biomechanical model was used to predict the compressive force at the L5/S1 disc for the weight lifted considering link angles for the particular posture. The tasks were also ranked according to the compressive force loading at the L5/S1 disc. The weight lifted in these tasks for obtaining the subjective estimate of stress was the maximum acceptable weight of lift (MAWOL). This was determined separately for each subject using a psychophysical approach. Subjective estimates of stress were obtained for infrequent lifting, specifically for a single lift, as well as for lifting at a frequency of four lifts per min. The results showed that a lifting task acceptable from the biomechanical point of view may not be judged as a safe or acceptable task by the worker based on his subjective perception. This may result in a risk of the worker not performing the recommended task or not following the recommended method.     

The effects of speed, frequency, and load on measured hand forces for a floor to knuckle lifting task.

The purpose of this study was to describe and quantify measured hand forces during floor to knuckle lifting of various loads. Hand forces of five subjects were measured with a strain gauge apparatus for normal and fast speeds of lifting at 1, 4, and 8 l/min. The pattern of hand force over time exhibited peaks in force in the shape of a spike for all fast lifts, indicating that subjects did not lift smoothly. For normal speed of lift, only one of the five subjects executed some lifts smoothly, indicating that it may be possible to lift smoothly, but most lifters probably do not. Peaks of horizontal and vertical components of hand force were tabulated by speed of lift, frequency, and load.     

An expert cognitive approach to evaluate physical effort and injury risk in manual lifting—A brief report of a pilot study

Although the development of the National Institute for Occupational Safety and Health (NIOSH) equation was partly based on the expertise of committee members convened under the auspices of NIOSH, to our knowledge, there is no study reported in the published literature that examined the role of professional expertise in determining the relative contribution of different lifting task variables to effort exertion. In this study, we explored whether professional expertise can be relied on, through the use of a systematic procedure, to quantify the effects of lifting task parameters on perceived effort and risk of injury outcome measures. Three international experts participated in the research reported herein and evaluated the interactive effects of 6 lifting variables: (a) weight of load, (b) horizontal distance, (c) frequency of handling, (d) work duration, (e) twisting angle, and (f) height of lift. They predicted the lifting effort and the injury risk of a large number of lifting configurations. A linguistic approach was used to describe the lifting activities. Logistic regression analyses were employed to model effort as a function of various lifting task variables. The results showed that all 3 experts rated the weight of load as the most dominant variable and the height of lift as the least important variable. Furthermore, they differed slightly in ranking the relative importance of other variables. In general, the effect of weight of load on physical effort was, at a minimum, 2 times more important than other lifting task variables. The horizontal distance, work duration, frequency, and twisting angle variables were considered to be more important than the height of lift by 25% to 33%. Collectively, these findings indicate that the experts agreed on the most and least important variables. In between, the relative importance of other variables was dependent on the professional training of the expert. The results further demonstrated that an increase in perceived physical effort was associated with an increase in the perceived risk of injury in the moderate‐ to high‐range values. There was a large variance, however, at the lower levels of physical effort and perceived risk. Collectively, the aforementioned findings confirm the notion that low‐level effort exertion activities are not perceived by the experts as risky. As the level of exertion increases to moderate values, however, the experts start to be conscious about the risks involved in the manual lifting activities. Therefore, one should not treat these 2 variables as equal, because they may reflect 2 separate dimensions in the low range. In the moderate to high range, they have some common variance but they may still reflect some differences to a certain extent. © 2002 Wiley Periodicals, Inc.     

Does a back support have a positive biomechanical effect?

Back supports, or lifting belts, are widely used. Subjects, free of low back pain, lifted in a simulated task, meeting the 1993 NIOSH guidelines. The back support reduced the electromyographic signal in the dorsal muscles. The back support also reduced the height loss as measured by a stadiometer. In most subjects the support also gave a subjective impression of increased support and increased lifting capacity.