Margin of safety for the human back: a probable consensus based on published studies.

Despite numerous efforts to control occurrences of work-related low back pain, it continues to be a significant problem. Since the causation of low back pain is under multifactorial control, it is suggested that the factor most vulnerable at any given time will determine the safety. Epidemiological, biomechanical, physiological, and psychophysical information presented in published literature has been analysed. The data available are related to injuries and the projected margin of safety. This paper therefore, is an integrative, inferential synthesis of the published work to discern the margin of safety. An attempt has been made to conclude, on the basis of objective evidence, an all encompassing criterion to ensure the safety of the back. The psychophysical approach appears to integrate biomechanical and physiological variables on a short term basis in the studies surveyed. However, it remains to be proven that the sensory conditioning due to prolonged and repetitive occupational tasks will remain responsive to cumulative load and fatigue failures.     

Manual lifting task risk evaluation using computer vision system

Low back injury has been a significant problem in industry. Studies indicate that inappropriate design of manual material handling (mmh) task is one of the major factors causing low back pain. Here, an automated system of evaluating the possible risk of low back injury in mmh task is developed. The system applies computer vision technique to identify the working posture, then incorporating biomechanical model and anthropometric data to calculate the low back compression force. By comparing with the specified standard limits, the system can indicate the risk level of the task. Some recommendations will then be provided. The system has been validated by comparing with the existing 2-D software, and it is found to be valid and effecient.     

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     

Manual material handling guidelines for the shoulder: Biomechanical support for the Liberty Mutual Tables as developed by Snook and Ciriello

Stover Snook and Vincent Ciriello laid the groundwork for psychophysical material handling guidelines in the 1970s. Since then, further research into psychophysical guidelines has been performed by numerous researchers. However, there still exists a gap between psychophysical and biomechanical guidelines. Snook and Ciriello's work eventually led to development of the Liberty Mutual Tables to reduce low-back pain episode in workers due to MMH tasks. Epidemiological evidence indicates pushing tasks may be more related to shoulder pain than low-back pain. A novel approach to protecting worker's shoulder complex by comparing the Liberty Mutual Table guidelines for pushing tasks to biomechanically derived pushing guidelines is presented. These biomechanically derived guidelines are based on muscle activation levels of the subscapularis muscle as determined using a biomechanical model of the shoulder complex. The subscapularis muscle may be a marker for subacromial impingement syndrome. In general, the psychophysical guidelines and the biomechanical guidelines achieve general agreement with respect to magnitude and shape. Differences between the two models range from 6 to 67%.     

Comprehensive manual handling limits for lowering,pushing, pulling and carrying activities

The objective of this study was to develop a set of mathematical models for manual lowering, pushing, pulling and carrying activities that would result in establishing load capacity limits to protect the lower back against occupational low-back disorders. In order to establish safe guidelines, a three-stage process was used. First, psychophysical data was used to generate the models' discounting factors and recommended load capacities. Second, biomechanical analysis was used to refine the recommended load capacities. Third, physiological criteria were used to validate the models' discounting factors. Both task and personal factors were considered in the models' development. When compared to the results from prior psychophysical research for these activities, the developed load capacity values are lower than previously established limits. The results of this study allowed the authors to validate the hypothesis proposed and tested by Karwowski (1983) that states that the combination of physiological and biomechanical stresses should lead to the overall measure of task acceptability or the psychophysical stress. This study also found that some of the discounting factors for the task frequency parameters recommended in the prior psychophysical research should not be used as several of the high frequency factors violated physiological limits.     

Association between spinal loads and the psychophysical determination of maximum acceptable force during pushing tasks

The objective of this study was to investigate potential associations between an individual's psychophysical maximum acceptable force (MAF) during pushing tasks and biomechanical tissue loads within the lumbar spine. Ten subjects (eight males, two females) pushed a cart with an unknown weight at one push every two minute for a distance of 3.9 m. Two independent variables were investigated, cart control and handle orientation while evaluating their association with the MAF. Dependent variables of hand force and tissue loads for each MAF determination and preceding push trial were assessed using a validated, electromyography-assisted biomechanical model that calculated spinal load distribution throughout the lumbar spine. Results showed no association between spinal loads and the MAF. Only hand forces were associated with the MAF. Therefore, MAFs may be dependent upon tactile sensations from the hands, not the loads on the spine and thus may be unrelated to risk of low back injury.

Practitioner Summary: Pushing tasks have become common in manual materials handling (MMH) and these tasks impose different tissue loads compared to lifting tasks. Industry has commonly used the psychophysical tables for job assent and decision of MMH tasks. However, due to the biomechanical complexity of pushing tasks, psychophysics may be misinterpreting risk.     

Estimating investment worthiness of an ergonomic intervention for preventing low back pain from a firm's perspective

A mathematical model was developed for estimating the net present value (NPV) of the cash flow resulting from an investment in an intervention to prevent occupational low back pain (LBP). It combines biomechanics, epidemiology, and finance to give an integrated tool for a firm to use to estimate the investment worthiness of an intervention based on a biomechanical analysis of working postures and hand loads. The model can be used by an ergonomist to estimate the investment worthiness of a proposed intervention. The analysis would begin with a biomechanical evaluation of the current job design and post-intervention job. Economic factors such as hourly labor cost, overhead, workers' compensation costs of LBP claims, and discount rate are combined with the biomechanical analysis to estimate the investment worthiness of the proposed intervention. While this model is limited to low back pain, the simulation framework could be applied to other musculoskeletal disorders. The model uses Monte Carlo simulation to compute the statistical distribution of NPV, and it uses a discrete event simulation paradigm based on four states: (1) working and no history of lost time due to LBP, (2) working and history of lost time due to LBP, (3) lost time due to LBP, and (4) leave job. Probabilities of transitions are based on an extensive review of the epidemiologic review of the low back pain literature. An example is presented.     

Previous history of LBP with work loss is related to lingering deficits in biomechanical,physiological, personal,psychosocial and motor control characteristics

A cross-sectional retrospective study was made of currently asymptomatic workers who perform physically demanding jobs. To further quantify the association between various biomechanical, physiological, personal psychosocial and motor control parameters that linger due to a history of low back disorders. Seventy-two workers were recruited from heavy industry, 26 of whom had a history of disabling low back disorders (LBDs) sufficient to miss work while the others did not. The strength of the study lies in the many detailed variables measured. Having a history of low back disorders was found to be associated with a larger waist girth, a greater potential for low back pain chronicity as predicted from psychosocial questionnaires, perturbed flexion to extension strength and endurance ratios, and widespread motor control deficits across a variety of tasks, some of which resulted in high back loads. In those workers who had missed work due to back disorders, the length of time since their last disabling episode was 261 weeks on average, suggesting that multiple deficits may remain for a period of time. Having a history of LBD is associated with changes in attitudes, in body composition, and in the way people move, load their backs and respond to a variety of motor and stability challenges.     

Comparing the revised NIOSH lifting equation to the psychophysical,biomechanical and physiological criteria used in its development

The purpose of this study was to perform a systematic and comprehensive comparison between the recommended weights of lift (RWL) from the revised NIOSH lifting equation, and the specific biomechanical, psychophysical and physiological criteria that were stated to be used in the equation's development. A composite acceptable load (CAL) table was developed for the 216 conditions presented in the female lifting table of Snook and Ciriello (1991). Each CAL value was calculated to correspond to the most conservative load of the three design criteria. The RWLs caused lumbar compression forces close to the biomechanical limit of 3400 N for lifts below knuckle height, but resulted in compression forces that are much lower at higher lifting heights. At moderate lifting frequencies, the average RWL would be acceptable to more than 95% of female workers according to the psychophysical criterion. At the highest frequencies, the RWL was found to be an average of 63% of the CAL values based on the integration of the psychophysical and physiological criteria. It is recommended that ergonomists acknowledge the very conservative nature of the revised NIOSH lifting equation when using it to evaluate occupational lifting tasks.     

Ergonomie guidelines for the prevention of low back pain at the workplace

The aim of this paper is to discuss the usefulness of common ergonomie guidelines for preventing low back pain at the workplace. Sixteen recent ergonomie, biomechanical and epidemiological books were reviewed to obtain common ergonomie guidelines for both static and dynamic work (e.g., sitting, lifting), and to obtain an overview of commonly-observed individual and work-related risk factors of low back pain. The results show that the aspects of work for which ergonomie guidelines are presented generally correspond to work-related risk factors as shown by epidemiological studies. However, in quantitative terms the guidelines show a great variety, possibly due to differences in criteria. In certain cases, it is not clear whether or not the guidelines are based on back load or back pain data, due to lack of references. It appears that many guidelines are based upon a combination of back load criteria and other criteria, although it is unknown how these criteria are combined. Hence, many guidelines do not apply specifically to low back pain.

With certain exceptions, most guidelines do not take into account individual factors, although epidemiological studies indicate that several factors such as age, strength, fitness, psychosocial factors, and history of back pain should be considered. However, because of the qualitative character of most current epidemiological studies, results cannot be readily implemented into quantitative ergonomie guidelines.

The above general results are discussed with examples. It is concluded that there is a great need for ergonomie guidelines that apply specifically to low back pain, and for quantitative epidemiological data on which these guidelines may be based.     

Significance of biomechanical and physiological variables during the determination of maximum acceptable weight of lift

The aim was to identify which biomechanical and physiological variables were associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL) in a psychophysical study. Fifteen male college students lifted a box of unknown weight at 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as heart rate, trunk positions, velocities and accelerations were measured during the lifting, as well as estimated spinal loading in terms of moments and spinal forces in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques identified variables associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that heart rate, predicted sagittal lift moment and low back disorder (LBD) risk index were associated with decreases in the weight prior to the next lift. Thus, historical measures of LBD risk (e.g. compression, shear force) were not associated with decreases in weight prior to the next lift. Additionally, the magnitudes of the predicted spinal forces and LBD risk were all very high at the MAWL when compared with literature sources of tolerance as well as observational studies on LBD risk. Our findings indicate that the psychophysical methodology may be useful for the decision to lower the weight of loads that may present extreme levels of risk of LBD; however, the psychophysical methodology does not seem to help in the decision to stop changing the weight at a safe load weight.     

Significance of biomechanical and physiological variables during the determination of maximum acceptable weight of lift.

The aim was to identify which biomechanical and physiological variables were associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL) in a psychophysical study. Fifteen male college students lifted a box of unknown weight at 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as heart rate, trunk positions, velocities and accelerations were measured during the lifting, as well as estimated spinal loading in terms of moments and spinal forces in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques identified variables associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that heart rate, predicted sagittal lift moment and low back disorder (LBD) risk index were associated with decreases in the weight prior to the next lift. Thus, historical measures of LBD risk (e.g. compression, shear force) were not associated with decreases in weight prior to the next lift. Additionally, the magnitudes of the predicted spinal forces and LBD risk were all very high at the MAWL when compared with literature sources of tolerance as well as observational studies on LBD risk. Our findings indicate that the psychophysical methodology may be useful for the decision to lower the weight of loads that may present extreme levels of risk of LBD; however, the psychophysical methodology does not seem to help in the decision to stop changing the weight at a safe load weight.     

Can postural modification reduce kinetic and kinematic loading during the bowing postures of Islamic prayer?

As stooped postures are known to increase kinematic and kinetic loading on the lumbar spine they can be problematic for people with low back pain and postural task modification is often recommended. For the Muslim with low back pain, the bowing postures during prayer can aggravate low back symptoms. The aims of this study were to describe lumbo-sacral and pelvic tilt kinematics and lumbo-sacral kinetics during the standard bowing postures of Islam and to compare these to kinematic and kinetic data gathered during a clinically recommended modified bowing posture. The study was a repeated measures within subject cross-over design with 33 healthy male Muslim participants. 3-D motion analysis data were gathered to calculate body joint angles during the two bowing postures. A 3-D biomechanical model was then used to calculate spinal loads. Paired t-test analyses showed that the use of the modified posture resulted in significantly less pelvic tilt range of motion and anterior shear force and compressive force L5/S1, at stages 1 and 5 of bowing. Although this study was conducted with healthy young Muslim males, the use of this modified bent knee posture is recommended for all Muslims with low back pain. Clinical trials are being considered to determine the clinical utility of this postural manoeuvre as an intervention.

Statement of Relevance:The presence of low back pain may hinder a Muslim's ability to use the traditional Islamic bowing posture. Muslims who have low back pain may benefit from adopting a modification to the traditional bowing posture,which has been found to reduce the loads and postural demands on the lower back.     

Can postural modification reduce kinetic and kinematic loading during the bowing postures of Islamic prayer?

As stooped postures are known to increase kinematic and kinetic loading on the lumbar spine they can be problematic for people with low back pain and postural task modification is often recommended. For the Muslim with low back pain, the bowing postures during prayer can aggravate low back symptoms. The aims of this study were to describe lumbo-sacral and pelvic tilt kinematics and lumbo-sacral kinetics during the standard bowing postures of Islam and to compare these to kinematic and kinetic data gathered during a clinically recommended modified bowing posture. The study was a repeated measures within subject cross-over design with 33 healthy male Muslim participants. 3-D motion analysis data were gathered to calculate body joint angles during the two bowing postures. A 3-D biomechanical model was then used to calculate spinal loads. Paired t-test analyses showed that the use of the modified posture resulted in significantly less pelvic tilt range of motion and anterior shear force and compressive force L5/S1, at stages 1 and 5 of bowing. Although this study was conducted with healthy young Muslim males, the use of this modified bent knee posture is recommended for all Muslims with low back pain. Clinical trials are being considered to determine the clinical utility of this postural manoeuvre as an intervention. STATEMENT OF RELEVANCE: The presence of low back pain may hinder a Muslim's ability to use the traditional Islamic bowing posture. Muslims who have low back pain may benefit from adopting a modification to the traditional bowing posture, which has been found to reduce the loads and postural demands on the lower back.     

Problems and solutions in manual materials handling: the state of the art

For several decades manual materials handling has been a topic of interest in many fields of research. The reason for this interest is the devastating cost of and human suffering caused by injuries associated with MMH. Prevention and control of these injuries has been a concern shared by many researchers. In order to control the nature and extent of these injuries, limits must be established for MMH, especially lifting. This paper summarizes the variables affecting the ability to handle materials and the three basic approaches to determining capacities and limits for MMH: the biomechanical, the physiological, and the psychophysical approaches. The paper presents the various models developed in each of these approaches, together with a discussion of progress made and difficulties encountered in deriving capacity recommendations from each. Finally, the paper presents the latest recommendation on how these approaches can be integrated into a single comprehensive model for establishing lifting limits.     

The impact of a sloped surface on low back pain during prolonged standing work: A biomechanical analysis

Background

Occupations that require prolonged periods of standing have been associated with increased reports of musculoskeletal disorders including low back pain. Previous work has utilized a prospective design of functionally inducing low back pain in previously asymptomatic individuals during a prolonged standing task. Increased trunk and gluteus medius muscle co-activation has been found in previously asymptomatic individuals who developed pain during standing compared with individuals who did not develop pain.

Purpose

The purpose of this study was to investigate the subjective and biomechanical responses of known pain developers and non-pain developers (previously determined during level standing) when exposed to the same prolonged standing task protocol completed while standing on a ±16° sloped surface.

Results

Overall low back pain scores were reduced by 59.4% for the pain development group, identified in level standing, when using the sloped surface. There was a marked decrease in the co-activation of the bilateral gluteus medius muscles in the known pain developers when standing on the sloped surface compared with level standing. However the non-pain developer group responded in the opposite direction by having an increase in the co-activation of these muscles, although they did not have a commensurate increase in low back pain. There were changes in both the postural and joint-loading variables examined. These changes were minimal and in most cases the sloped surface produced responses that bracketed the postures and loading magnitudes found in level standing depending on whether the participant was standing on the +16° or −16° surface.

Conclusions

The sloped surface resulted in decreased subjective low back pain during prolonged standing. There were also associated biomechanical changes resulting from using a sloped surface during prolonged standing. These positive findings were supported in an exit survey satisfaction rating with 87.5% indicating that they would use the sloped surface if they were in an occupational setting that required prolonged standing work.     

A comprehensive lifting model: beyond the NIOSH lifting equation

A comprehensive lifting model (CLM) for the evaluation and design of manual tasks was developed in two stages using 11 task, personal and environmental variables. In the first stage, the model was built using the psychophysical data. In the second stage, discounting factors of various variables were tested and adjusted using the physiological and biomechanical data. Two lifting indices are proposed to evaluate lifting tasks for a group of workers (relative lifting safety index or RLSI) and for an individual worker (personal lifting safety index or PLSI).     

Can back supports relieve the load on the lumbar spine for employees engaged in industrial operations?

Abstract

In recent years, there has been an increased use of back supports in US industry to reduce the frequency and concomitant costs of lower-back disorders. The obvious question is, ‘Can back supports relieve the load on the lumbar spine for employees engaged in industrial operations?’. This paper is directed towards answering this question because there have been mixed conclusions in the literature reporting on the efficacy of back supports. The literature concerning the biomechanical, physiological and psychophysical effects of back supports on the human spine has been reviewed as well as the use of back supports to control injury in the workplace. A'critical assessment of the findings reported by various investigators has been made together with a discussion of the mechanisms used by the trunk muscles to provide extrinsic stability to the spine. It is hypothesized that the extrinsic stability of the spine is manifested through more than one mechanism. These mechanisms may act simultaneously or sequentially to stabilize the trunk. Finally, the ergonomics of back supports as a corporate policy are discussed.     

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 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.