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.     

Development of methodology in biomechanical simulation of manual lifting

The purpose of this paper is to introduce some techniques of biomechanical simulation through an example — simulation of manual lifting. The motivation for developing biomechanical simulation is discussed. Biomechanical simulation is defined and compared with the traditional biomechanical modeling. The formulation of biomechanical simulation of manual lifting is presented, and some practical techniques used to conduct a simulation of manual lifting are provided.     

Progressive fatigue effects on manual lifting factors

The purpose of this study was to evaluate the effect of progressive fatigue on factors that previously have been associated with increased risk of low back pain in various occupational settings, during a repetitive lifting task where a freestyle lifting technique was used. A laboratory experiment was conducted to evaluate electromyography amplitude, kinematic, and kinetic parameters of repetitive freestyle lifting during a 2‐hour lifting period. Subjective fatigue rating increased over time, indicating that the participant “felt” increasingly fatigued as the experiment progressed. Static composite strength decreased an average of 20% from the beginning to the end of the experiment. Effect of lifting posture (semi‐squat, semi‐stoop, and stoop) was observed on peak trunk flexion angle, trunk flexion angle at initiation of the lift, and knee angle at initiation of the lift indicating that, in freestyle lifting, participants assume quantitatively different lifting techniques. A significant effect of the time–posture interaction was observed on the dynamic leg lift floor to knuckle height strength, indicating that dynamic strength may change over time depending on lifting posture selected. © 2009 Wiley Periodicals, Inc.     

A psychophysical study of manual lifting in hot environments

A study was conducted to assess the lifting capabilities of acclimatized individuals in hot environments. Six male subjects were selected for the study. The subjects were heat acclimatized for ten days prior to participation in the experiment. A psychophysical experiment was then carried out using three temperature levels (22, 27, and 32°C Wet Bulb Globe Temperature (WBGT), three frequencies of lift (0.1, 3 and 6 lifts/min), and one height of lift (floor to knuckle height). The results indicated that the maximum acceptable weights of lift selected by the subjects at 27°C WBGT were not significantly different from the weights selected at 22°C WBGT. On the other hand, the maximum acceptable weights selected at 32°C WBGT were reduced by approximately 13 percent.     

A physiological study of manual lifting of loads in Indians

The employment of workers solely for lifting of loads is common in the developing countries. This task can be described in terms of its three principal variables, viz. the weight of the load, the height of the lift and the rate of lifting, but Jew attempts to quantitate the contributions of these variables in determining its strenuousness have been made.

Based on the observed range of variation in an industrial lifting operation, a total of 525 lifting experiments comprising combinations of three different weights of compact loads, lifts to three separate heights from the ground level and three different rates of lifting were carried out on 21 subjects selected from amongst the load lifters.

Comparison of the observed energy expenditures of these tasks with the maximum working capacities of the subjects showed that many of the tasks were unduly heavy. Regression equations depicting the relation between the energy expenditure of lifts of different heights with the other two variables are given. A chart linking these variables has also been prepared; this may be helpful in adjusting those lifting tasks which are continued for prolonged periods so that they are of ‘acceptable’ heaviness.     

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     

The role of general motion principles in asymmetrical lifting

This study has attempted to document how general motion alterations could change the loading on the lumbar spine during asymmetrical manual materials handling. Two general motion principles were explored: stability (modification of the width of the base of support) and fragmentation (insertion of a pause). It was hypothesized that during asymmetrical materials handling (1) a decrease in the width of the base of support may increase lumbar spine loading and (2) the insertion of a pause may reduce lumbar spine loading. Four male subjects performed asymmetrical tasks involving the reception and lifting of an 11.6 kg box. The magnitude of lumbar spine loading was estimated by computing 3D components of the net muscular moment, articular force and their respective loading rate at the L5/S1 joint. Comparisons were made between a wide and a narrow base of support task (stability) and between a continuous and a paused task (fragmentation). The results showed no difference for either moment, force or their loading rate between the experimental conditions, except for one component of the net moment in the fragmentation comparison. More research is needed to clarify the application of these two principles in asymmetrical materials handling.     

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.     

New procedure for assessing sequential manual lifting jobs using the revised NIOSH lifting equation

A sequential manual lifting job is defined as a job where workers rotate between a series of manual lifting rotation slots or elements at specified time intervals during the course of a work shift. The original NIOSH lifting equation lacked a method for assessing the physical demands of these types of jobs. This paper presents the sequential lifting index (SLI), a new conceptual method for assessing the physical demands for sequential manual lifting jobs. The new method is similar to the composite lifting index (CLI) method that was provided by NIOSH for assessing multi-task jobs. The SLI method expands upon the methods originally provided by NIOSH by providing a simple method for estimating the relative magnitude of physical stress for sequential manual lifting jobs. It should also be useful in assisting safety and health specialists to prioritize or rank hazardous jobs within a plant.     

Effect of ship motion on spinal loading during manual lifting

This study investigated the effects of ship motion on peak spinal loading during lifting. All measurements were done on a ship at sea. In 1-min trials, which were repeated over a wide range of sailing conditions, subjects lifted an 18 kg box five times. Ship motion, whole body kinematics, ground reaction forces and electromyography were measured and the effect of ship motion on peak spinal moments and compression forces was investigated. To investigate whether people time their lifts in order to reduce the effect of ship motion on back loading, trials were performed at a free and at a constrained (lifting every 10s) work pace. With increase of the (local) vertical ship acceleration, increased moments and compression forces were found. Furthermore, lifting at a free work pace did not result in smaller effects of ship motion on spinal moments and compression forces than working at a constrained work pace.     

Effect of ship motion on spinal loading during manual lifting

This study investigated the effects of ship motion on peak spinal loading during lifting. All measurements were done on a ship at sea. In 1-min trials, which were repeated over a wide range of sailing conditions, subjects lifted an 18 kg box five times. Ship motion, whole body kinematics, ground reaction forces and electromyography were measured and the effect of ship motion on peak spinal moments and compression forces was investigated. To investigate whether people time their lifts in order to reduce the effect of ship motion on back loading, trials were performed at a free and at a constrained (lifting every 10s) work pace. With increase of the (local) vertical ship acceleration, increased moments and compression forces were found. Furthermore, lifting at a free work pace did not result in smaller effects of ship motion on spinal moments and compression forces than working at a constrained work pace.     

Effect of lifting belts on trunk muscle activation during a suddenly applied load

The National Institute for Occupational Safety and Health suggests there is insufficient biomechanical or epidemiological evidence to recommend the use of back belts in industry. From a biomechanical perspective, previous work suggests that lifting belts stiffen the torso, particularly in the frontal and transverse planes. To determine whether lifting belts stiffen the torso and alter the trunk muscle response during a sudden loading event, we tested the hypotheses that (a) lifting belts alter peak muscle activity recorded with electromyography (EMG) during sudden loading and (b) lifting belts have a larger impact on trunk muscle response when sudden loads are applied asymmetric to the torso's midsagittal plane. A sudden load was delivered to 10 men and 10 women without history of low back disorder via a cable attached to a thoracic harness; motion was restricted to the lumbar spine. Results indicate that gender was not a significant factor in this study. The lifting belt reduced the peak normalized EMG of the erector spinae muscles on average by 3% during asymmetric loading, though peak normalized EMG was increased by 2% during symmetric loading. Lifting belts have been shown to slightly reduce peak erector spinae activity during asymmetric sudden loading events in a constrained paradigm; however, the effects of lifting belts are too small to provide effective protection of workers. Actual or potential applications include the assessment of lifting belts as protective devices in workers based on the effects of lifting belts on the trunk muscle activity.     

Ergonomic job design in frequent manual lifting tasks: A microcomputer-based model

The main objective of this paper is to generate ergonomic software that can be used in the design and evaluation of manual materials handling tasks so as to minimize the risk of injury. Specifically, this study is aimed at developing a microcomputer-based model for the design of frequent manual lifting tasks based ont he concept of job saverity index. The microcomputer-based software package is intended to be used by non-experts in the field of MMH. Possible engineering and administrative controls are implemented in the software in case if human lifting abilities are exceeded. The software was written in AutoLISP for the IBM personal computer. The knowledge base of the software is built upon a set of two models which were developed in the present study. The models were based on 2,736 observations.     

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.     

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.     

The relationship between isometric strength of Cantonese males and the US NIOSH guide for manual lifting

A laboratory study was undertaken to assess the relationship between the maximal voluntary isometric strength (MVIS) of a group of Hong Kong Cantonese males and recommended limiting values for lifting proposed in the Work Practices Guide for Manual Lifting (NIOSH, 1981). Forty-one male subjects were required to apply a 'free-style' vertical 'lift' force to a horizontal bar located 150 mm from the ground. Forces were applied with the horizontal location of the midpoint of the ankles at 200, 400, 600 and 800 mm from the bar. The results were compared with the Maximal Permissible Limit (MPL) and the Action Limit (AL) values proposed in the NIOSH guide. The results showed that the mean MVIS of the Hong Kong subjects lay between the MPL and AL when the load was 200 mm and 400 mm from the ankles, but was significantly below the AL at 600 mm. At 800 mm more than half of the subjects were unable to apply any measurable positive vertical force. The compressive force at the L5/S1 disc of four subjects was estimated using a static biomechanical model. The relationship between the disc pressure and the force exerted was largely in accordance with NIOSH guide values. The results are discussed in terms of differences in subject population and lifting technique. Comments on the applicability of the NIOSH standards internationally, are also presented.