Psychophysically determined forces of dynamic pushing for female industrial workers: Comparison of two apparatuses

Using psychophysics, the maximum acceptable forces for pushing have been previously developed using a magnetic particle brake (MPB) treadmill at the Liberty Mutual Research Institute for Safety. The objective of this study was to investigate the reproducibility of maximum acceptable initial and sustained forces while performing a pushing task at a frequency of 1 min⁻¹ both on a MPB treadmill and on a high-inertia pushcart. This is important because our pushing guidelines are used extensively as a ergonomic redesign strategy and we would like the information to be as applicable as possible to cart pushing. On two separate days, nineteen female industrial workers performed a 40-min MPB treadmill pushing task and a 2-hr pushcart task, 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 demonstrated that maximum acceptable initial and sustained forces of pushing determined on the high inertia pushcart were 0.8% and 2.5% lower than the MPB treadmill. The results also show that the maximum acceptable sustained force of the MPB treadmill task was 0.5% higher than the maximum acceptable sustained force of Snook and Ciriello (1991). Overall, the findings confirm that the existing pushing data developed by the Liberty Mutual Research Institute for Safety still provides an accurate estimate of maximal acceptable forces for the selected combination of distance and frequency of push for female industrial workers.     

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.     

The effect of handle height and cart load on the initial hand forces in cart pushing and pulling

The objective of this study was to measure the three-dimensional hand forces people exert to initiate a cart push or pull for two cart loads: 73 and 181 kg, and three handle heights: knuckle, elbow, and shoulder heights. The cart used was equipped with 15.24 cm (6 in) diameter wheels. The floor was covered with carpet tiles. The laboratory-measured hand force exertions were compared to the minimum forces needed to push/pull the cart under the same conditions and to the psychophysical initial push/pull force limits. For pushing and pulling, the measured anterior-posterior hand forces were 2–2.4 times the minimum required forces. For the heavier cart load, lower forces were applied as handle height increased. Pull forces were 7% higher than push forces. The smallest vertical forces were measured at elbow height. Strength capability and gender did not have an effect on the applied forces. The mean strength percentile for the male sample was 64%, while the mean strength percentile for the female sample was 13% as determined from the Adjusted Torso Lift Strength Test and population strength data for this test. The comparison with the psychophysical limits indicated that the tasks were well within the maximum acceptable initial forces for males, but not for females.     

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.     

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.     

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.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a force-transducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45 degrees (SD 3.3 degrees) downward to near horizontal, while the pulling force direction changed from pulling upward by 14 degrees (SD 15.3 degrees) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

Effect of handle height on lower-back loading in cart pushing and pulling

This paper presents results of a study conducted to estimate lower back loadings in cart pushing and pulling. Experiments were conducted in the laboratory using a cart. Six subjects with different weights (ranging from 50 to 80 kg) were tested for three different pushing and pulling forces (98, 196 and 294 newtons), three different heights of exertion (660, 1090 and 1520 mm high) and two different moving speeds (1.8 and 3.6 km/h). It was found that, in general, pushing a cart results in lesser lower-back loading than pulling. Subject body weight affected the lower-back loadings more significantly in pulling (50% increase as body weight increased from 50 kg to 80 kg) than in pushing (25% increase). Handle height of 1090 mm was found to be better than other handle heights in pushing while 1520 mm handle height was better for pulling in reducing lower-back loadings.     

An evaluation of off-axis manual forces and upper extremity joint moments during unilateral pushing and pulling exertions

This study quantified changes in off-axis manual force production and upper extremity joint moments during sub-maximal one-handed push and pull tasks. Off-axis forces in the up/down and left/right directions were quantified in the presence or absence of constraints placed upon the direction of manual force application and/or arm posture. Resultant off-axis forces of 13.1% and 9.4% were produced for pulls and pushes, respectively. Off-axis forces during pulling were oriented downwards and to the right and were associated with a decreased should flexion moment when posture was constrained. Off-axis forces in the up/down direction were minimized with increased on-axis force level. Off-axis forces during pushing tended to be oriented to the left and were associated with increased elbow flexion moment when off-axis forces were allowed. By not accounting for these off-axis forces, we may not be accurately reflecting actionable muscle- and joint-level loading characteristics derived from biomechanically-based proactive ergonomics assessment approaches.

Practitioner Summary: Constrained arm postures and directions of manual force application influence the production of off-axis forces. As inaccurate estimation of true manual forces can markedly influence actionable outcomes of proactive ergonomic assessments, this study suggests that simplification of these estimates is insufficient and potentially misleading.     

Force direction and physical load in dynamic pushing and pulling

In pushing and pulling wheeled carts, the direction of force exertion may, beside the force magnitude, considerably affect musculoskeletal loading. This paper describes how force direction changes as handle height and force level change, and the effects this has on the loads on the shoulder and low back. Eight subjects pushed against or pulled on a stationary bar or movable cart at various handle heights and horizontal force levels while walking on a treadmill. The forces at the hands in the vertical and horizontal direction were measured by a forcetransducer. The forces, body movements and anthropometric data were used to calculate the net joint torques in the sagittal plane in the shoulder and the lumbosacral joint. The magnitudes and directions of forces did not differ between the cart and the bar pushing and pulling. Force direction was affected by the horizontal force level and handle height. As handle height and horizontal force level increased, the pushing force direction changed from 45° (SD 3.3°) downward to near horizontal, while the pulling force direction changed from pulling upward by 14° (SD 15.3°) to near horizontal. As a result, it was found that across conditions the changes in force exertion were frequently reflected in changes in shoulder torque and low back torque although of a much smaller magnitude. Therefore, an accurate evaluation of musculoskeletal loads in pushing and pulling requires, besides a knowledge of the force magnitude, knowledge of the direction of force exertion with respect to the body.     

Lower back muscle forces in pushing and pulling

In the investigation of lower back stress, the muscle forces of the erector spinae and the rectus abdominis are often calculated using the two-dimensional biomechanical model. These muscle forces are used to estimate the compressive forces at L5/S1 disc. This paper presents a study of the muscle forces predicted by a two-dimensional biomechanical model during pushing and pulling and myoelectric activity from the corresponding muscles. The goal was to investigate whether a simple two muscle torso model would reasonably estimate the muscle actions in pushing and pulling tasks. Six subjects participated in the experiment. EMG (rms) value was used as an indicator of muscle forces. The results show high correlation between the predicted muscle forces and the measured root-mean-square EMG values in trunk pushing and pulling (r2 = 0.93) and hand pushing and pulling (r2 = 0.96) in an erect posture with hips braced but low in hand pushing and pulling using a free posture (r2 = 0.37).     

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.     

Lower back muscle forces in pushing and pulling

Abstract

In the investigation of lower back stress, the muscle forces of the erector spinae and the rectus abdominis are often calculated using the two-dimensional biomechanical model. These muscle forces are used to estimate the compressive forces at L5/S1 disc This paper presents a study of the muscle forces predicted by a two-dimensional biomechanical model during pushing and pulling and myoelectric activity from the corresponding muscles. The goal was to investigate whether a simple two muscle torso model would reasonably estimate the muscle actions in pushing and pulling tasks. Six subjects participated in the experiment. EMG (rms) value was used as an indicator of muscle forces. The results show high correlation between the predicted muscle forces and the measured root-mean-square EMG values in trunk pushing and pulling (r²=0.93) and hand pushing and pulling (r²=0.96) in an erect posture with hips braced but low in hand pushing and pulling using a free posture (r²=0.37).     

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.     

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.     

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.     

Evaluation of ergonomic adjustments of catering carts to reduce external pushing forces

An existing standard catering cart was compared with two prototypes for pushbar and castor design. The first objective of this study was to find out which cart was accompanied with the lowest manually exerted external forces in pushing in a straight way and in pushing a 90 turn. The second objective was to explore effects of the pushbar and castor design of the carts. In the initial and ending phase, the prototypes were accompanied with higher exerted forces compared with the standard catering cart. In pushing straight. the reversed start position of the bigger castors of the prototypes hampered a fluent acceleration and caused higher initial forces. In decelerating, the lower rolling friction of the bigger castors required higher forces to stop the prototypes compared to the standard cart. During the sustained phase, the prototype carts were more favourable. Effects of pushbar and castor design were studied during a turn. The vertical pushbars of the prototypes resulted in lower time-integrated pushing forces. Providing an axis of rotation for turning activities by means of a fixed wheel was proven to be advantageous.     

One-handed dynamic pulling strength with special application to lawn mowers

A laboratory study was conducted to determine one-handed dynamic and static pulling strengths of 50 males and 49 females from 14 to 71 years of age. The dynamic strength for a 11m pull was measured to simulate the act of starting a lawn mower engine for four different starting-rope handle locations: on the engine, in the middle, on the right and on the left side of the frame. The last three were located at the back of the lawn mower at a vertical height of 63 cm from the floor and the handle on the engine was located at 42 cm. Static strength was measured for the handle in the middle. Ratings of perceived exertion were recorded for different body parts.

Peak and average dynamic pulling strengths were 55% and 34% of static pulling strengths. Dynamic pulling strengths were highly correlated with peak velocity (r = 0·84). Men in the age group 21-34 years had the highest strength and women in the age group 51-71 years the least strength. Dynamic pulling strengths for women were 62% of strengths for men. Women took 10% longer to pull, had a lower peak velocity (16%), reached peak force faster (17%) and took a longer time (6%) to reach peak velocity than men.

The starting handle located on the engine resulted in the maximum pulling strength and on the left side in the minimum strength. However, two out of three subjects preferred the handle either on the right side or in the middle. Fifty-five percent of the subjects indicated they would prefer a height higher than 63 cm.

Maximum stresses were perceived on the shoulder and upper arm with a mean rating between fairly light and somewhat hard. Graphs of cumulative frequency distributions of average and peak dynamic pulling strengths are presented to aid in the determining forces required to start a lawn mower engine in order to satisfy a desired percentage of the population.     

Modification of an EMG-assisted biomechanical model for pushing and pulling

Pushing and pulling tasks using carts and material handling devices have become more prevalent in occupational environments in an attempt to reduce the musculoskeletal risks associated with lifting. However, little change in low back disorder rates have been noted as tasks change from lifting to pushing and pulling indicating that we do not understand the mechanics of pushing and pulling well. Biomechanical assessments of pushing and pulling tasks using person-specific biologically assisted models offer a means to help understand how the spine is loaded under pushing and pulling conditions. However, critical components of these models must be adjusted so that they are sensitive to the different physiologic responses in the torso muscles expected during pushing and pulling compared to lifting tasks.The objective of this study was to modify an electromyography (EMG)-assisted biomechanical model designed to evaluate lifting tasks so that it can better represent the biomechanical forces expected during pushing and pulling tasks. Several key modifications were made. Based upon a literature review, changes in muscle cross-sectional area and muscle origins and insertions were made to better represent the geometry of the torso muscles. It was also necessary to adjust the length–force and velocity–force muscle relationships. Empirically derived length–force and velocity–force relationships were developed to independently represent the flexor and extensor musculature. These modifications were then systematically incorporated into the model.The model was exercised over several pushing and pulling conditions to assess the effect of these modifications on its ability to predict externally measured spinal moments. Results indicated that the alterations made to the preexisting EMG-assisted model resulted in acceptable model performance for pushing, pulling, and lifting activities.

Relevance to industry

The use of carts and material handling devices has become increasingly prevalent in industry, though little research has been done to examine the body's response. The modifications made to the biomechanical model would enable its use in the evaluation and design of material handling devices and pushing and pulling tasks.     

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.