Revisited: Comparison of two techniques to establish maximum acceptable forces of dynamic pushing for male industrial workers

The purpose of this experiment was to replicate a previous psychophysical experiment [Ciriello, V.M., McGorry, R.W., Martin, S.E., Bezverkny, I.B., 1999b. Maximum acceptable forces of dynamic pushing: comparison of two techniques. Ergonomics 42, 32–39] which investigated maximum acceptable initial and sustained forces while performing a 7.6 m pushing task at a frequency of 1 min⁻¹ on a magnetic particle brake treadmill versus pushing on a high-inertia pushcart. Fourteen male industrial workers performed both a 40-min treadmill pushing task and a 2-h pushcart task, with a unique water loading system, in the context of a larger experiment. During pushing, the subjects were asked to select a workload they could sustain for 8 h without “straining themselves or without becoming unusually tired, weakened, overheated or out of breath.” The results revealed that similar to the previous study maximum acceptable sustained forces of pushing determined on the high inertia cart were significantly higher (21%) than the forces determined from the magnetic particle brake treadmill. These results were countered by an 18% decrease in maximum acceptable forces for the criterion magnetic particle brake treadmill task, perhaps due to secular changes in the industrial population. Based on the present findings, it is concluded that the existing pushing data [Snook, S.H., Ciriello, V.M., 1991. The design of manual tasks: revised tables of maximum acceptable weights and forces. Ergonomics 34, 1197–1213] still provides an accurate estimate of maximal acceptable forces for this pushing distance and frequency.

Relevance to industry

Jobs are often redesigned to eliminate lifting and to include carts for transporting loads. Our database on maximum acceptable forces of pushing on a magnetic particle braked treadmill has been used as a tool to design manual handling tasks. This article links the existing database with actual cart pushing.     

Psychophysical basis for maximum pushing and pulling forces: A review and recommendations

The objective of this paper was to perform a comprehensive review of psychophysically determined maximum acceptable pushing and pulling forces. Factors affecting pushing and pulling forces are identified and discussed. Recent studies show a significant decrease (compared to previous studies) in maximum acceptable forces for males but not for females when pushing and pulling on a treadmill. A comparison of pushing and pulling forces measured using a high inertia cart with those measured on a treadmill shows that the pushing and pulling forces using high inertia cart are higher for males but are about the same for females. It is concluded that the recommendations of Snook and Ciriello (1991) for pushing and pulling forces are still valid and provide reasonable recommendations for ergonomics practitioners. Regression equations as a function of handle height, frequency of exertion and pushing/pulling distance are provided to estimate maximum initial and sustained forces for pushing and pulling acceptable to 75% male and female workers.     

Psychophysical acceptability as a constraint in manual working capacity

Psychophysical theory and its use in establishing maximum acceptable workloads are briefly reviewed. A distinction is made between psychophysical criteria for static strength, and psychophysical criteria for dynamic strength. The experimental protocol and equipment developed during the series of Liberty Mutual studies is described. These studies used industrial workers in a controlled environment to develop maximum acceptable weights and forces for the basic manual handling tasks. Two recent experiments investigating task frequency and object size are also described. It is concluded that minor changes should be made in the tables of maximum acceptable weights and forces published by Snook [576].     

Maximum acceptable forces for repetitive wrist extension with a pinch grip

This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable forces for extension motions of the wrist performed with a pinch grip. Subjects grasped a handle with a pinch grip and moved it through a 1.57 rad (90°) extension wrist motion (similar to a light assembly operation). A psychophysical methodology was used in which the subject adjusted the resistance on the handle, and the experimenter manipulated or controlled all other variables. Twenty subjects performed the task at repetition rates of 15, 20 and 25 motions per minute. Subjects performed for 7 h per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work they performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results are presented and compared with maximum acceptable forces for other types of wrist motion investigated in previous studies. Maximum acceptable force for wrist extension with a pinch grip is smaller than any of the other motions investigated so far.

Relevance to industry

Cumulative trauma disorders of the upper extremities continue to be a problem for industrial workers who perform repetitive tasks. Although a number of physical risk factors have been identified, there are very few data available for establishing acceptable levels of these risk factors. This study attempted to collect such data.     

Dynamic pushing on three frictional surfaces: maximum acceptable forces,cardiopulmonary and calf muscle metabolic responses in healthy men

Pushing is an important materials handling activity in many occupations; however, pushing-related physiological investigations are still in infancy. The purpose was to evaluate maximum acceptable forces and physiological responses while pushing on: treadmill (TREAD); plywood floor (PLY); and Teflon floor (TEF). Acceptable forces, cardiopulmonary and calf muscle oxygenation and blood volume responses were collected simultaneously while 12 men (age 39 ± 13 years; height 178 ± 6 cm; and body mass 91.5 ± 16 kg) pushed for 2 h on each surface at their psychophysical workload. Participants selected higher forces on the PLY, resulting in higher pulmonary oxygen uptake compared to that of TEF (by ～9%) and TREAD (by ～18%). Pushing on the TEF demonstrated 50–56% lower blood volume changes and 1.5–1.8 times more oxygenation-force ratio than that for other surfaces. It is concluded that, to avoid a potential slip, participants were conservative in selecting acceptable forces to push on the slippery TEF. Part of this compensatory strategy on the TEF resulted in less muscle activity and, therefore, less demand for oxygen delivery to the calf muscle than for other surfaces. The present findings of significant force- and physiological-related differences in treadmill vs. high inertia pushcart clearly demonstrate that pushing experiments are essential to evaluate functional abilities of the workers.     

Psychophysical study of six hand movements.

This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist and hand conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable forces of six motions performed on separate days but within the context of the same experiment. The six motions were wrist flexion with a power grip, wrist extension with a power grip, wrist flexion with a pinch grip, wrist extension with a pinch grip, ulnar deviation with a power grip, and a handgrip task (with a power grip). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experimenter manipulated or controlled all other variables. Thirty-one subjects performed the six tasks at repetition rates of 15, 20 and 25 motions/min. Subjects performed the tasks for 7 h per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results revealed that maximum acceptable torques ranged from 11 to 19% of maximum isometric torque depending on frequency and motion. Maximum acceptable torques for the tasks that could be compared with previous studies showed the same patterns of response. However, the selected forces were substantially lower using the mixed protocol. A table of maximum acceptable torques and forces is presented for application in the field.     

Psychophysical study of six hand movements

This study represents a continuation of a series of psychophysical studies on repetitive motions of the wrist and hand conducted at the Liberty Mutual Research Center for Safety and Health. The purpose of the study was to quantify maximum acceptable forces of six motions performed on separate days but within the context of the same experiment. The six motions were wrist flexion with a power grip, wrist extension with a power grip, wrist flexion with a pinch grip, wrist extension with a pinch grip, ulnar deviation with a power grip, and a handgrip task (with a power grip). A psychophysical methodology was used in which the subject adjusted the resistance on the handle and the experimenter manipulated or controlled all other variables. Thirty-one subjects performed the six tasks at repetition rates of 15, 20 and 25 motions/min. Subjects performed the tasks for 7 h per day, 5 days per week, for 4 weeks. The subjects were instructed to work as if they were on an incentive basis, getting paid for the amount of work performed. Symptoms were recorded by the subjects during the last 5 min of each hour. The results revealed that maximum acceptable torques ranged from 11 to 19% of maximum isometric torque depending on frequency and motion. Maximum acceptable torques for the tasks that could be compared with previous studies showed the same patterns of response. However, the selected forces were substantially lower using the mixed protocol. A table of maximum acceptable torques and forces is presented for application in the field.     

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%.     

On the manual handling of wide-body carts used by cabin attendants in civil aircraft

A study was made of manual handling of wide-body carts used in civil aircraft. Under laboratory conditions, 11 females adjusted their pushing and pulling forces on fixed carts to the maximum amount they perceived as acceptable with repeated exertions. Subsequently their ability to push and pull the carts was tested with maximum exertions. The initial forces required to just get a fully loaded cart in motion were measured for different inclinations of the floor on which the cart stood. In a DC-9 aircraft, floor inclination and flying speed were measured while climbing to cruising altitude and during descent prior to landing. The maximum acceptable force for repetitive exertions was, on average, 68 N. The maximum force was, on average, 270 N. No significant differences were found between the pushing and pulling forces. The findings in the experiments caused the Swedish National Board of Occupational Safety and Health to reduce its recommended limit for repetitive push and pull in this task from 200 N to 100 N. As a result the handling of wide-body carts in the DC-9 should be delayed until at least 14-15 minutes after take-off to fulfil the new recommendation. On short flights, for which the DC-9 is used, this is not possible without reducing the level of service. A follow-up project on the development of an improved cart is under way, incorporating changes suggested in this paper and elsewhere.     

The effects of task duration on psychophysically-determined maximum acceptable weights and forces

The purpose of this experiment was to investigate maximum acceptable weights and forces when performing manual handling tasks continuously for four hours at frequencies of 4.3 min-1 or slower. Twelve female and ten male second shift industrial workers performed 18 varieties of lifting, lowering, pushing, pulling, and carrying. A psychophysical methodology was employed, whereby the subjects were asked to select a workload they could sustain for 8 h 'without straining themselves or without becoming unusually tired, weakened, overheated or out of breath'. Measurements of heart rate, oxygen consumption, dynamic and static strengths were also taken. The weights selected after 40 min were not significantly different from the weights selected after four hours. The average oxygen consumption for the fast tasks was 28% VO2 max, within physiological guidelines for eight hours. The results also revealed that the maximum acceptable weights for the combination task of lifting, carrying, and lowering were limited by the lifting and lowering components. It is concluded from the results of this study that the psychophysical methodology is appropriate for determining maximum acceptable weights for task frequencies of 4.3 min-1 or slower. It is also concluded that the maximum acceptable weight for a combination task is limited by the lowest acceptable weight of any of the components.     

Secular changes in psychophysically determined maximum acceptable weights and forces over 20 years for male industrial workers

The most frequent and expensive cause of compensable workplace injuries loss is manual material handling (MMH). In an attempt to minimise these losses, refinement of existing MMH guidelines is a component of redesigning high risk MMH jobs. In the development of the present MMH 1991 guidelines (Snook and Ciriello 1991), maximum acceptable weights (MAWs) and forces (MAFs) were derived from studies conducted in a 21 year time span before the above publication date. The question arises whether the present generation of workers have the same psychophysically determined weights and forces as those reflected in the guidelines. Therefore, the present study investigated whether secular changes had occurred in key MMH tasks in trials performed by present day local industrial workers. A total of 23 male industrial workers performed 20 variations of lifting, lowering, pushing, pulling and carrying tasks. A psychophysical methodology, identical to that of the authors' previous experiments, was used whereby the subjects were asked to select a workload they could sustain for 8 h 'without straining themselves or without becoming unusually tired, weakened, overheated or out of breath'. The results revealed that MAWs of lifting, lowering and carrying averaged 69% of the guideline values. MAFs of pushing and pulling showed less of a drop, averaging 82% and 94% respectively for initial and sustained forces. The results also indicated that the effects of the variables frequency, height, lifting vs. lowering, pushing vs. pulling were similar to earlier reported results, even though the absolute weights or forces were lower. It was concluded that consideration to change existing guidelines, reflecting this new psychophysical set point, may be appropriate if these significant performance decreases are confirmed in other locations, with greater subject numbers, and by other investigators.     

Muscular fatigue and maximum endurance time assessment for male and female industrial workers

A single arm pushing experiment was conducted in an electronic factory in Yantai, China to assess muscular fatigue using the theoretical models of muscular strength and maximum endurance time (MET) developed by Ma et al. (2009). Seventy seven workers, including 38 males and 39 females, participated in the study. The muscular strength of pushing was measured after the subject pushed a stick, with a force of 2.5 kgf, for 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 min. Fatigue rate was determined based on a regression approach. In addition to the theoretical model, the MET for such a pushing task was also determined using four empirical models in the literature. The results indicated that females were more resistant to muscular fatigue than males in the pushing task. The results of the muscular strength prediction show that the predictability of the muscular strength model is acceptable. The prediction errors for muscular strength for female subjects were significantly lower than those of the male subjects. The predicted MET using the theoretical model, with a group constant k, was highly correlated with those using the empirical models compared in the current study.     

Wheelchair pushing and turning: lumbar spine and shoulder loads and recommended limits

Abstract

The objective of this study was to determine how simulated manual wheelchair pushing influences biomechanical loading to the lumbar spine and shoulders. Sixty-two subjects performed simulated wheelchair pushing and turning in a laboratory. An electromyography-assisted biomechanical model was used to estimate spinal loads. Moments at the shoulder joint, external hand forces and net turning torque were also assessed. Multiple linear regression techniques were employed to develop biomechanically based wheelchair pushing guidelines relating resultant hand force or net torque to spinal load. Male subjects experienced significantly greater spinal loading (p < 0.01), and spine loads were also increased for wheelchair turning compared to straight wheelchair pushing (p < 0.001). Biomechanically determined maximum acceptable resultant hand forces were 17–18% lower than psychophysically determined limits. We conclude that manual wheelchair pushing and turning can pose biomechanical risk to the lumbar spine and shoulders. Psychophysically determined maximum acceptable push forces do not appear to be protective enough of this biomechanical risk.

Practitioner Summary: This laboratory study investigated biomechanical risk to the low back and shoulders during simulated wheelchair pushing. Manual wheelchair pushing posed biomechanical risk to the lumbar spine (in compression and A/P shear) and to the shoulders. Biomechanically determined wheelchair pushing thresholds are presented and are more protective than the closest psychophysically determined equivalents.     

Initial force and postural adaptations when pushing and pulling on floor surfaces with good and reduced resistance to slipping

The objective of the present study was to determine whether differences in the frictional properties of a floor surface may affect the kinematics and kinetics of pushing and pulling. Eight male participants were required to push and pull a four-wheeled trolley over two level surfaces, on which were mounted floor coverings with good (safety floor) and reduced (standard floor) frictional properties. A psychophysical approach was used to determine the initial maximum acceptable horizontal force required to move the trolley over a short distance (3 m). Three-dimensional (3D) hand and ground reaction forces and 3D postures were measured during initial force exertions. The results showed that psychophysically derived measures of initial horizontal force and horizontal components of hand forces did not differ significantly between floor surfaces. Despite the ability to exert similar forces, the measured maximum coefficient of friction varied according to floor surface. These changes reflected significant alterations in vertical and horizontal components of ground reaction and vertical hand forces, suggesting that participants had maximized the frictional properties available to them. Postures also changed as a consequence of floor surface, with significant changes occurring in knee flexion and trunk extension. This study has shown that handlers involved in the pushing and pulling of trolleys are capable of adjusting posture and the direction of hand and foot forces in order to compensate for reduced levels of floor friction. This has particular relevance when assessing the musculoskeletal loads imposed on the handler and the likely mechanisms of injury resulting from variations in floor conditions when workers undertake pushing and pulling tasks in the workplace.     

Cart pushing: The effects of magnitude and direction of the exerted push force, and of trunk inclination on low back loading

The primary objective of the present study was to quantify the relative effect of the magnitude and direction of the exerted push force and of trunk inclination on the mechanical load at the low back using a regression analysis for correlated data. In addition, we explored the effects of handle height and type of pushing activity (standing or walking) on the magnitude and direction of exerted forces, trunk inclination, and low back loading when pushing a four-wheeled cart on a treadmill. An experimental setup was designed in which nine participants pushed a four-wheeled cart on a treadmill. Kinematics and reaction forces on the hand were measured to calculate the net moment at the L5–S1 intervertebral disc. Results show that the magnitude and direction of the exerted push force and the trunk inclination significantly and independently affect low back load. It is concluded that for the ergonomic evaluation of pushing tasks, the inclination of the trunk should be considered, in addition to the magnitude and direction of exerted forces.

Relevance to industry

Pushing carts is a common activity for a considerable part of the workforce and has been associated with musculoskeletal complaints. This paper shows that not only the magnitude of exerted forces determines the low back load but also the direction of the exerted forces and the inclination of the trunk should be considered for ergonomic evaluation.     

Initial force and postural adaptations when pushing and pulling on floor surfaces with good and reduced resistance to slipping

The objective of the present study was to determine whether differences in the frictional properties of a floor surface may affect the kinematics and kinetics of pushing and pulling. Eight male participants were required to push and pull a four-wheeled trolley over two level surfaces, on which were mounted floor coverings with good (safety floor) and reduced (standard floor) frictional properties. A psychophysical approach was used to determine the initial maximum acceptable horizontal force required to move the trolley over a short distance (3 m). Three-dimensional (3D) hand and ground reaction forces and 3D postures were measured during initial force exertions. The results showed that psychophysically derived measures of initial horizontal force and horizontal components of hand forces did not differ significantly between floor surfaces. Despite the ability to exert similar forces, the measured maximum coefficient of friction varied according to floor surface. These changes reflected significant alterations in vertical and horizontal components of ground reaction and vertical hand forces, suggesting that participants had maximized the frictional properties available to them. Postures also changed as a consequence of floor surface, with significant changes occurring in knee flexion and trunk extension. This study has shown that handlers involved in the pushing and pulling of trolleys are capable of adjusting posture and the direction of hand and foot forces in order to compensate for reduced levels of floor friction. This has particular relevance when assessing the musculoskeletal loads imposed on the handler and the likely mechanisms of injury resulting from variations in floor conditions when workers undertake pushing and pulling tasks in the workplace.     

Effects of work experience on work methods during dynamic pushing and pulling

Pushing and pulling are potential risk factors for work-related low back disorders (WRLBDs). While several studies have evaluated differences in work methods related to work experience, such evidence for dynamic pushing and pulling is limited. Eight novices and eight experienced workers completed dynamic push/pull tasks using a cart weighted to 250% of individual body mass in two different configurations (preferred vs. elbow handle heights). Multiple measures [hand forces, torso kinematics and kinetics, and required coefficient of friction (RCOF)] were obtained to assess WRLBD and slip risks. Experienced workers generated higher medio-lateral hand forces, during both pulls and pushes, though with a more substantial difference during pushes (∼74%), and which involved the use of hand force components other than to move the cart in an anterior-posterior direction. Experienced workers also had lower peak torso kinematics in flexion/extension and lateral bending, and lower torso flexion/extension kinetics. The latter is suggestive of a lower risk for WRLBDs, though levels of exposures to WRLBD risk were low to moderate in both groups and were often relatively small and inconsistent across the task configurations. Group-level differences in RCOF were quite small, indicating a comparable slip risk between the two groups. Thus, it was considered inconclusive whether the work methods used by experienced workers during dynamic pushing and pulling are advantageous regarding WRLBD and slip risks.     

The design of manual handling tasks: revised tables of maximum acceptable weights and forces.

Four new manual handling experiments are reviewed. The experiment used male and female subjects to study lifting, lowering, pushing, pulling, and carrying tasks. Each experiment used a psychophysical methodology with measurements of oxygen consumption, heart rate, and anthropometric characteristics. Independent variables included task frequency, distance, height and duration; object size and handles; extended horizontal reach; and combination tasks. The results of the four experiments were integrated with the results of seven similar experiments published previously by this laboratory. The integrated data were used to revise maximum acceptable weights and forces originally published in 1978. The revised tables are presented and compared with the original tables.     

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.     

Mechanical loading of the low back and shoulders during pushing and pulling activities

The objective of this study was to quantify the mechanical load on the low back and shoulders during pushing and pulling in combination with three task constraints: the use of one or two hands, three cart weights, and two handle heights. The second objective was to explore the relation between the initial and sustained exerted forces and the mechanical load on the low back and shoulders. Detailed biomechanical models of the low back and shoulder joint were used to estimate mechanical loading. Using generalized estimating equations (GEE) the effects were quantified for exerted push/pull forces, net moments at the low back and shoulders, compressive and shear forces at the low back, and compressive forces at the glenohumeral joint. The results of this study appeared to be useful to estimate ergonomics consequences of interventions in the working constraints during pushing and pulling. Cart weight as well as handle height had a considerable effect on the mechanical load and it is recommended to maintain low cart weights and to push or pull at shoulder height. Initial and sustained exerted forces were not highly correlated with the mechanical load at the low back and shoulders within the studied range of the exerted forces.