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.     

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.     

Perception of posture of short duration in the spatial and temporal domains

Twenty normal male university students with a mean age of 21.4 years, body weight of 66.2 kg, and height of 170 cm, were asked to acquire nine postures, and in the last two they were asked to exert either pushing or pulling forces for periods ranging from 5 to 15 seconds. Their posture was recorded photographically, the duration of activities was measured using a stopwatch and the force exerted during pushing and pulling was recorded using a load cell and force monitor (ST-1). The subjects were asked to estimate their postures immediately after each activity using a three-dimensional mannequin and a line drawing on a paper according to instructions provided before. They were also asked to estimate the duration and force exerted using their judgement and record. After the completion of all activities they recorded all their estimations (except mannequin) again on the same day, a week later and four weeks later. The estimates were compared with actual values through Student's t-test Stooping and twisting were accurately estimated and recalled. Side bending, pushing, and pulling were consistently significantly different from actual (p < 0.05). Whereas the memory of posture estimates was stable for the period of study, the duration estimates deteriorated with passage of time. The force assessment during pushing activity was significantly different (p < 0.01) from actual but the pulling forces were estimated and recalled accurately.     

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.     

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.     

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.     

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.     

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.     

Gender differences in exerted forces and physiological load during pushing and pulling of wheeled cages by postal workers

The aim was to determine gender differences regarding exerted forces and physiological load during push/pull tasks simulating the daily working practice of postal workers. Eight female and four male workers handled four-wheeled cages under eight conditions corresponding to the cage weight (130, 250, 400, 550 kg) and the direction of force exertion (pushing, pulling). For each of the five dependent variables, average force, initial force, ending force, oxygen uptake and heart rate, two analyses of variance with repeated measurements were performed, i.e. with and without correction for the worker's body weight, body height and maximum capacity regarding the dependent variable. Exerted forces and physiological load were high for the cages weighing 400 and 550 kg. Gender differences were significant for all dependent variables (p=0.030-0.000). When the personal factors were included in the model, male workers exerted significantly higher average forces and ending forces than their females, while differences regarding initial forces and physiological load were not significant. However, none of the personal factors were significantly related to any of the dependent variables. It is concluded that gender differences in exerted forces were not caused by differences in anthropometry and maximum capacity, but due to application of different work methods by women in order to balance work demands and work ability.     

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.     

Ready steady push--a study of the role of arm posture in manual exertions

This study investigated arm posture and hand forces during bi-manual pushing. Nine male and eight female participants performed isometric exertions at two reach distances (0 and elbow-grip) and six different positions of the hand interface (handle), defined by the plane (longitudinal, lateral, horizontal) and orientation (0 degrees and 45 degrees). Electrogoniometer instruments were used to measure the displacements/postures of the wrist and elbow joints and the forearm, and force measuring strain gauges were used to measure the exerted hand forces (x-, y- and z-components). The results showed that ability to vary arm posture, particularly the forearm, is important during build up of force and that people tend to seek for a balance in the forces applied at the hands by exerting more in the vertical direction. Also, lateral plane handle positions permitted exertion of greater forces than longitudinal and horizontal plane positions.     

The influence of wood hardness and logging operation on coupling forces exerted by lumberjacks during wood harvesting

Petrol chain saws commonly used in forestry cause mechanical vibration, which may lead to the development of non-specific disorders in upper extremities of the chain saw operator, referred to as hand-arm vibration syndrome (HAVS). Progress of HAVS depends on the intensity of mechanical vibration transmitted throughout the body, which is directly proportional to coupling forces applied to the vibration tool. The aim of the study was to measure coupling forces exerted by lumberjacks on chain saws and find correlation between force magnitude, hardness of cut wood and kind of logging operation.Coupling forces exerted by workers with right and left hands were measured by means of hydro-electronic force meter. All measurements were done during harvesting wood in real work conditions.Maximal temporary forces exerted by woodcutters reached 275 N. The smallest average forces of 27 N were registered while limbing. During felling and cross-cutting chain saw operators exerted larger forces, reaching 50 N.The findings of this study suggest that coupling forces used by woodcutters during logging depend on wood hardness and kind of logging operation.

Relevance to industry

This study shows the relationship between coupling forces, wood hardness and technique of cut which are further expected to enhance our knowledge on the assessment of vibration exposure. Nowadays, understanding how changes in harvesting technique affect the magnitude of coupling force, should lead to improvements in ergonomic design of the tool and the workplace.     

Acclimatization to Severe Dry Heat by Brief Exposures to Humid Heat

Abstract

Maximal static strengths were determined for one-handed exertions in all directions in the fore and aft plane. Data from 12 males and 10 females (mean age 30·7 yrs, standard deviation (SD)=8·9 yrs, n=22) were obtained with handle heights of 1·0 and 1·75 m. Twelve of the subjects also performed two-handed exertions at the same handle heights. The ratio of mean strengths of females to that of males ranged from 0·50 to 0·83 (for absolute forces) and from 0·63 to 1·00 for forces normalized to body weight. The ratios of one-handed to two-handed strengths ranged from 0·64 to 1·04. Two-handed strengths commonly exceeded one-handed strengths at the lower handle height, but showed fewer significant strength differences (p<0·05) according to direction at l·75m. Both female/male and one-handed/two-handed strength ratios were found to be dependent on direction of exertion and handle height. The observed strength dependencies upon number of hands (one or two-handed), direction of exertion, handle height and sex are discussed. The strength data have implications for use in biomechanical models and task analysis.     

Gender differences in exerted forces and physiological load during pushing and pulling of wheeled cages by postal workers

The aim was to determine gender differences regarding exerted forces and physiological load during push/pull tasks simulating the daily working practice of postal workers. Eight female and four male workers handled four-wheeled cages under eight conditions corresponding to the cage weight (130, 250, 400, 550 kg) and the direction of force exertion (pushing, pulling). For each of the five dependent variables, average force, initial force, ending force, oxygen uptake and heart rate, two analyses of variance with repeated measurements were performed, i.e. with and without correction for the worker's body weight, body height and maximum capacity regarding the dependent variable. Exerted forces and physiological load were high for the cages weighing 400 and 550 kg. Gender differences were significant for all dependent variables (p = 0.030-0.000). When the personal factors were included in the model, male workers exerted significantly higher average forces and ending forces than their females, while differences regarding initial forces and physiological load were not significant. However, none of the personal factors were significantly related to any of the dependent variables. It is concluded that gender differences in exerted forces were not caused by differences in anthropometry and maximum capacity, but due to application of different work methods by women in order to balance work demands and work ability.     

Contributions and co-ordination of individual fingers in multiple finger prehension

The contributions and co-ordination of external ringer grip forces were examined during a lifting task with a precision grip using multiple fingers. The subjects ( n = 10) lifted a force transducer-equipped grip apparatus. Grip force from each of the five fingers was continuously measured under different object weight (200 g, 400 g, and 800 g) and surface structure (plastic and sandpaper) conditions. The effect of five-, four-, and three-finger grip modes was also examined. It was found that variation of object weight or surface friction resulted in change of the total grip force magnitude; the largest change in finger force, was that for the index finger, followed by the middle, ring, and little fingers. Percentage contribution of static grip force to the total grip force for the index, middle, ring, and little fingers was 420%, 27·4%, 17·6% and 12·9%, respectively. These values were fairly constant for all object weight conditions, as well as for all surface friction conditions, suggesting that all individual finger force adjustments for light loads less than 800 g are controlled comprehensively simply by using a single common scaling value. A higher surface friction provided faster lifting initiation and required lesser grip force exertion, indicating advantageous effect of a non-slippery surface over a slippery surface. The results indicate that nearly 40% force reduction can be obtained when a non-slippery surface is used. Variation in grip mode changed the total grip force, i.e., the fewer the number of fingers, the greater the total grip force. The percent value of static grip force for the index, middle, and ring fingers in the four-finger grip mode was 42·7%, 32·5%, and 24·7%, respectively, and that for the index and middle fingers in the three-finger grip mode was 43·0% and 56·9%, respectively. Therefore, the grip mode was found to influence the force contributions of the middle and ring fingers, but not of the index finger.     

Biomechanically determined hand force limits protecting the low back during occupational pushing and pulling tasks

Though biomechanically determined guidelines exist for lifting, existing recommendations for pushing and pulling were developed using a psychophysical approach. The current study aimed to establish objective hand force limits based on the results of a biomechanical assessment of the forces on the lumbar spine during occupational pushing and pulling activities. Sixty-two subjects performed pushing and pulling tasks in a laboratory setting. An electromyography-assisted biomechanical model estimated spinal loads, while hand force and turning torque were measured via hand transducers. Mixed modelling techniques correlated spinal load with hand force or torque throughout a wide range of exposures in order to develop biomechanically determined hand force and torque limits. Exertion type, exertion direction, handle height and their interactions significantly influenced dependent measures of spinal load, hand force and turning torque. The biomechanically determined guidelines presented herein are up to 30% lower than comparable psychophysically derived limits and particularly more protective for straight pushing.

Practitioner Summary: This study utilises a biomechanical model to develop objective biomechanically determined push/pull risk limits assessed via hand forces and turning torque. These limits can be up to 30% lower than existing psychophysically determined pushing and pulling recommendations. Practitioners should consider implementing these guidelines in both risk assessment and workplace design moving forward.     

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.     

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.     

A different approach for the ergonomic evaluation of pushing and pulling in practice

Recent epidemiological studies show that pushing and pulling increase the risks of shoulder complaints and not necessarily of low back complaints. Moreover, the magnitude of the exerted hand forces during pushing and pulling is poorly related to the magnitude of the mechanical loading of the low back and the shoulder. In light of that, this paper combines results of several studies to present an approach for evaluating not only the exerted hand forces, but also the low back and shoulder load during pushing and pulling in practice. The approach specifies, based on scientific evidence, that (1) in order to validly obtain exposure (frequency and duration) to pushing and pulling, 10 workers should be observed during eight periods of 30 min; (2) how the exerted hand forces and the load of the low back and shoulder can be estimated in practice based solemnly on the weight of the object, one-handed or two-handed pushing or pulling, and the height of the handle; and finally, (3) how these outcomes can be evaluated in combination with existing guidelines regarding exerted hand forces, compression forces on the low back and the moments at the shoulder. Two cases will be presented here to illustrate the application of the approach.

Relevance to industry

The presented approach is the first to offer practitioners a fairly simple method for the ergonomic evaluation of pushing and pulling carts and four-wheeled containers in practice, especially as regarding the shoulder load.     

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.