Cart Pushing Research Articles

Whole Body Control of Mobile Manipulators With Series Elastic Actuators for Cart Pushing Tasks.

Human-centered environments provide affordance for the use of two-handed mobile manipulators. Yet robots designed to function in and physically interact with such environments are not yet capable of meeting human users' requirements. This work proposes a whole body control framework of a two-handed mobile manipulator driven by series elastic actuators (SEAs) for cart pushing tasks. A whole body dynamic model for an integrated mobile platform and on-board arms is revealed, which takes into account the interaction forces with the cart. Then, the explicit force/position control of the mobile manipulator is performed. It enables the robot to interact dynamically with the environment while providing motion, i.e., the manipulators provide both output force control and motion control for pushing a cart. To cope with the highly nonlinear system dynamics and parameter variation of a SEA-driven mobile manipulator, this work proposes an adaptive robust controller based on a novel integral barrier Lyapunov function for cart pushing tasks by considering model uncertainty. The proposed controller enables the mobile manipulator to complete cart pushing tasks by regulating the position and output force of the mobile base and arms. The experimental results show the effectiveness of this approach in cart pushing tasks.

Effects of using passive back- and arm-support exoskeletons for cart pushing and pulling

Though studies have shown both BSEs and ASEs are effective in reducing physical demands for various manual material handling tasks, limited evidence is currently available on their effects in pushing and pulling tasks. We evaluated the impacts of using a passive back-support (BSE) and an arm-support exoskeleton (ASE) on trunk kinematics, and the back and arm musculature for pushing and pulling of a moderately loaded (100 kg) cart. Fourteen volunteers performed cart the tasks, each of whom was randomly assigned one of the exoskeletons (BSE or ASE). Wearing the BSE substantially reduced lumbar muscle activity during both pushing (up to ~39%) and pulling (up to ~35%) compared to not wearing EXOs, while wearing an ASE had no significant impacts. For the anterior deltoid muscle, neither BSE nor ASE had beneficial impacts. Findings from the current study help to understand the effects of BSEs and ASEs in pushing and pulling tasks.

PORTERAGE OPERATION IN RURAL MARKETS - A PIVOTAL LINK AND TRANSFORMATION IN AGRICULTURAL COMMODITIES TRANSPORTATION SYSTEM: CASE OF SOUTH-WESTERN NIGERIA

Due to poor infrastructure, unmotorable and heavily congested many urban and rural market spaces in most African societies, Nigeria inclusive, and consequent use of head porters in transporting market goods and wares, this paper seeks to explore porterage operation in rural markets with a view to establishing the pivotal linkage between porterage, market and agricultural commodities transportation system in south-western Nigeria. Specifically, the porterage, market and transportation linkage were conceptualized; socio-economic statuses of the porterage workers were profiled; types of agriculturally based porterage activities and porterage workers’ levels of involvement identified; reasons motivating and constraints limiting their involvement; and transformation in porterage operation in agricultural commodities transportation were discussed. Pre-tested and validated interview schedule was used to elicit information from 190 porterage workers randomly selected from across 10 agricultural commodity markets in the study area. Key informant interview was used to elicit qualitative information from their association leaders. Also, on-site photographs were captured to reinforced transformational innovation in rural porterage. Data collected were processed using statistical package for social sciences (SPSS) version 21. The paper established porterage as a sub-system in rural market transportation system. It also revealed that majority of porterage workers are young people (about 70%) under 40 years of age with a very low socio-economic status with no or little education. They earn an average income of ₦264,505.26 ($734.74) annually. They operate on an average of 6 days/week and about 21/2hrs/day without requiring any special skills and initial capital. They are mostly exposed to harsh weather condition and diverse physical risks. It noted the current transformational innovation the sub-system is undergoing with gradual migration from head carriage to wheelbarrow (34%), advancement into cart pushing (16%) and very few motorized tricycles. This is observed to be a strategic way of reducing stress and energy involve in the traditional porterage operation without losing the purpose of connecting the rural transportation system.

The Effects of Direction of Exertion, Path, and Load Placement in Nursing Cart Pushing and Pulling Tasks: An Electromyographical Study.

The purpose of this study was to explore the effects of direction of exertion (DOE) (pushing, pulling), path (walking in a straight line, turning left, walking uphill), and load placement (LP) (the 18 blocks were indicated by X, Y and Z axis; there were 3 levels on the X axis, 2 levels on the Y axis, and 3 levels on the Z axis) on muscle activity and ratings of perceived exertion in nursing cart pushing and pulling tasks. Ten participants who were female students and not experienced nurses were recruited to participate in the experiment. Each participant performed 108 experimental trials in the study, consisting of 2 directions of exertion (push and pull), 3 paths, and 18 load placements (indicated by X, Y and Z axes). A 23kg load was placed into one load placement. The dependent variables were electromyographic (EMG) data of four muscles collected bilaterally as follows: Left (L) and right (R) trapezius (TR), flexor digitorum superficialis (FDS), extensor digitorum (ED), and erector spinae (ES) and subjective ratings of perceived exertion (RPE). Split-split-plot ANOVA was conducted to analyze significant differences between DOE, path, and LP in the EMG and RPE data. Pulling cart tasks produced a significantly higher activation of the muscles (RTR:54.4%, LTR:50.3%, LFDS:57.0%, LED:63.4%, RES:40.7%, LES:36.7%) than pushing cart tasks (RTR:42.4%, LTR:35.1%, LFDS:32.3%, LED:55.1%, RES:33.3%, LES:32.1%). A significantly greater perceived exertion was found in pulling cart tasks than pushing cart tasks. Significantly higher activation of all muscles and perceived exertion were observed for walking uphill than walking in a straight line and turning left. Significantly lower muscle activity of all muscles and subject ratings were observed for the central position on the X axis, the bottom position on the Y axis, and the posterior position on the Z axis. These findings suggest that nursing staff should adopt forward pushing when moving a nursing cart, instead of backward pulling, and that uphill paths should be avoided in the design of work environments. In terms of distribution of the load in a nursing cart, heavier materials should be positioned at bottom of the cabinet, centered on the horizontal plane and close to the handle, to reduce the physical load of the nursing staff.

Kinematics of cart pushing and pulling under different loads and surface gradient conditions

Background: Manual material handling incidents are responsible for a large portion of lost work days annually. With the transition to e-commerce, cart pushing and pulling tasks have become more common. Objective: This research explored the effects of surface gradient and load on full body kinematics during cart pushing and pulling tasks. Methods: Ten participants were recruited to complete two sets of tasks. Participants performed cart pushing tasks on three surface gradients and three load masses and downhill cart pulling tasks on two surface gradients and three load masses while being recorded with an optical motion capture system. Full body, three-dimensional joint angles were calculated for each task, and peak angles of the major body joints were analyzed using general linear models to determine the effects of the dependent variables. Results: During the cart pushing tasks, increased load mass and surface gradient both caused a significant increase in the peak joint angles of most body joints with surface gradient having the larger effect. When cart pushing and pulling tasks were compared with 5° and 10° surface gradients and three load masses, cart pushing resulted in significantly higher joint angles. Conclusion: During manual material handling tasks involving a cart, surface gradients and load masses should be minimized.

Shoulder Joint Loading and Posture During Medicine Cart Pushing Task

Excessive physical loads and awkward shoulder postures during pushing and pulling are risk factors for shoulder pain. Pushing a medicine cart is a major component of a work shift for nurses and medical assistants in hospitals and other health care facilities. A laboratory experiment was conducted to examine the effects of common factors (e.g., lane congestion, cart load stability, floor surface friction) on shoulder joint moment and shoulder elevation angle of participants during cart pushing. Participants pushed a medicine cart on straight tracks and turning around right-angle corners. Peak shoulder joint moments reached 25.1 Nm, 20.3 Nm, and 26.8 Nm for initial, transition, and turning phases of the pushing tasks, indicating that shoulder joint loading while pushing a medical cart is comparable to levels previously reported from heavy manual activities encountered in industry (e.g., garbage collection). Also, except for user experience, all other main study factors, including congestion level, cart load stability, location of transition strip, shoulder tendency, surface friction, and handedness, significantly influenced shoulder joint moment and shoulder elevation angle. The findings provide a better understanding of shoulder exposures associated with medicine cart operations and may be helpful in designing and optimizing the physical environment where medicine carts are used.

Biomechanical loading of the shoulder complex and lumbosacral joints during dynamic cart pushing task

The primary objective of this study was to quantify the effect of dynamic cart pushing exertions on the biomechanical loading of shoulder and low back. Ten participants performed cart pushing tasks on flat (0°), 5°, and 10° ramped walkways at 20 kg, 30 kg, and 40 kg weight conditions. An optoelectronic motion capturing system configured with two force plates was used for the kinematic and ground reaction force data collection. The experimental data was modeled using AnyBody modeling system to compute three-dimensional peak reaction forces at the shoulder complex (sternoclavicular, acromioclavicular, and glenohumeral) and low back (lumbosacral) joints. The main effect of walkway gradient and cart weight, and gradient by weight interaction on the biomechanical loading of shoulder complex and low back joints was statistically significant (all p < 0.001). At the lumbosacral joint, negligible loading in the mediolateral direction was observed compared to the anterioposterior and compression directions. Among the shoulder complex joints, the peak reaction forces at the acromioclavicular and glenohumeral joints were comparable and much higher than the sternoclavicular joint. Increased shear loading of the lumbosacral joint, distraction loading of glenohumeral joint and inferosuperior loading of the acromioclavicular joint may contribute to the risk of work-related low back and shoulder musculoskeletal disorder with prolonged and repetitive use of carts.

Description and analysis of hand forces in medicine cart pushing tasks

The primary objectives of this study were to describe and analyze the hand force exertion patterns of experienced nursing home nurses and nursing students during dynamic medicine cart pushing tasks in Initial, Sustained, Turning, and Stopping motion phases. A 2 × 2 × 2 factorial experiment was conducted with 22 participants to estimate the effects of lane congestion, precision cart control, and floor surface on horizontal hand forces. Root mean squared (RMS) lane deviation patterns were also described to provide an indicator of cart handling difficulty across the different study conditions. Descriptive statistics revealed that nurses exerted greater mean hand force (10%) and made more (12%) lane deviation than students and that the highest two-hand forces of 147N were measured in the Turning phase on carpet. Strong correlations between work experience group, body mass, and BMI required that force data for nurses and students be collapsed in analytical models where no group differences existed. Predicted pushing forces on carpeted floor surface were significantly greater than on tile in Initial (14N), Sustained (14N) and Turning (18N), except in stopping where pulling forces were 37N lower. High lane congestion predicted significant peak force increases of 4N and 7N in Sustained and Turning, respectively, but decreased by 20N in Initial. High precision control led to significant decreases in two-hand forces that ranged from 4 to 20N across motion phases. Complex interactions among the experimental factors suggest that work environment (lane congestion and floor surface) and work demands (precision control) should be included in the evaluation of pushing tasks and considered prior to making renovations to nursing home environments.

Trunk muscle control in response to (un)expected turns in cart pushing

Before altering the travel direction in normal gait, anticipatory activation in trunk muscles is observed, followed by a top-down sequence of rotation of body segments. Turning while pushing a cart is a more challenging task for the trunk because of its low stiffness in pushing while walking and the interaction with the high inertia of the cart. 12 healthy subjects pushed a 200kg cart at shoulder and hip height while making turns (gradual, sharp and unexpected sharp). The normalized electromyogram amplitudes of left (right) lateral and anterior external oblique muscle, and right (left) internal oblique muscle were averaged to represent left (right) trunk rotator muscle activity. The baseline values of trunk rotator muscle activity before the turn and the peak values after the turn were determined. Additionally, peak values of hand forces, twisting moments and twisting motions were assessed. Before turning, higher trunk rotator muscle activity than in straight pushing without turning was only observed before making a turn in the gradual or sharp turn conditions. After the turn, clockwise twisting motion was associated with a clockwise twisting moment induced by the reaction forces at the left hand. Anticipatory activation was initially absent in the unexpected sharp turn, while bilateral trunk rotator muscle activity increased after the turn, indicating co-contraction. In the unexpected turn condition the combination of an uncontrolled twisting motion with delayed muscle activation may increase the potential risk of low-back injury.

Handle height and expectation of cart movement affect the control of trunk motion at movement onset in cart pushing

As unexpected sudden unloading of the trunk may cause low-back injury, the objective of the present study was to investigate whether handle height and the expectation of cart movement in pushing affect trunk control at movement onset. Eleven healthy male participants pushed a 200-kg cart with handles at shoulder and hip heights. The cart would suddenly move when externally released (externally triggered condition) or when static friction was overcome (self-initiated condition). Before self-initiated cart movement, trunk stiffness and muscle activity were significantly higher than before an externally triggered onset at comparable pushing force. Lower muscle activity and trunk stiffness at shoulder height compared with the hip height before the onset resulted in higher trunk inclination after the onset. In conclusion, higher preparatory activation of trunk muscles serves to increase trunk stiffness in anticipation of cart movement and may reduce the impact of the perturbation associated with the onset of cart movement. Statement of Relevance: Sudden cart movement in pushing causes an unexpected unloading perturbation to the trunk. This perturbation was shown to cause uncontrolled trunk movement, which may explain how pushing tasks can be associated with low-back injury. Effects of handle height and awareness of the subjects of the possible cart movement suggest directions for prevention.

Control of trunk motion following sudden stop perturbations during cart pushing

External perturbations during pushing tasks have been suggested to be a risk factor for low-back symptoms. An experiment was designed to investigate whether self-induced and externally induced sudden stops while pushing a high inertia cart influence trunk motions, and how flexor and extensor muscles counteract these perturbations. Twelve healthy male participants pushed a 200 kg cart at shoulder height and hip height. Pushing while walking was compared to situations in which participants had to stop the cart suddenly (self-induced stop) or in which the wheels of the cart were unexpectedly blocked (externally induced stop). For the perturbed conditions, the peak values and the maximum changes from the reference condition (pushing while walking) of the external moment at L5/S1, trunk inclination and electromyographic amplitudes of trunk muscles were determined. In the self-induced stop, a voluntary trunk extension occurred. Initial responses in both stops consisted of flexor and extensor muscle cocontraction. In self-induced stops this was followed by sustained extensor activity. In the externally induced stops, an external extension moment caused a decrease in trunk inclination. The opposite directions of the internal moment and trunk motion in the externally induced stop while pushing at shoulder height may indicate insufficient active control of trunk posture. Consequently, sudden blocking of the wheels in pushing at shoulder height may put the low back at risk of mechanical injury.

Gender comparison of psychophysical forces, cardiopulmonary, and muscle metabolic responses during a simulated cart pushing task

The purpose was to compare psychophysiological responses between healthy male and female workers during dynamic pushing. Using a psychophysical approach, 27 participants chose an acceptable force that they could push over a 7.6 m distance at a frequency of 1 push per min on a treadmill. On a separate day, cardiopulmonary (e.g., whole-body oxygen uptake, heart rate, ventilation volume) and muscle metabolic measurements (change in muscle blood volume [ΔtHb] and Tissue Oxygenation Index [TOI]) from the right and left gastrocnemius muscles were collected simultaneously while participants pushed the previously chosen acceptable force on the treadmill at a similar frequency and distance for 2 h. Results showed no significant difference between men and women for integrated force exerted on the instrumented treadmill handle and cardiopulmonary responses. In contrast, women demonstrated 45.7% lower ΔtHb but 3.6% higher TOI in the gastrocnemius region as compared to men, suggesting a lower hemoglobin concentration in women and high venous oxygen saturation during pushing. When ΔtHb and TOI were corrected for both body mass and pushing force, the disparity in gender was retained, implying an increased muscle oxygen saturation per force development in women than men during pushing. In the left gastrocnemius region, ΔtHb was 60% lower and TOI was 5.7% higher in women than men, suggesting an uneven muscle loading during pushing. Overall, the gender similarity in cardiopulmonary responses versus disparity in muscle metabolic responses suggest the importance of evaluating human performance during physical work at both whole-body and localized muscle levels.

Cart pushing capabilities for males and females: an update

Cart pushing capabilities for males and females: an update

Psychophysiological Responses in Women During Cart Pushing on Different Frictional Walkways

The aim of this study was to evaluate psychophysically determined acceptable forces, cardiopulmonary, and calf muscle metabolic responses in 15 workers while they pushed an instrumented cart on two walkways. In addition to the potential for increased musculoskeletal disorders in workers, pushing on various terrains is associated with occurrence of slips and falls at the workplace. Using a psychophysical approach, participants chose the maximum acceptable cart weight they could push without strain on walkways with coefficient of friction equaling 0.68 (plywood) and 0.26 (Teflon-coated.). Then, while participants pushed their psychophysically chosen cart weight for 2 hr on each walkway, horizontal and vertical forces applied on the cart handle and physiological responses were collected. Cardiopulmonary responses were measured using a telemetric metabolic cart. A tissue hemoglobin index (THI) and a tissue oxygenation index (TOI) from the right and left calf muscles were obtained using near-infrared spectroscopy. Participants generated higher horizontal forces (by 26%) on plywood than that on Teflon. Cardiopulmonary and TOI and THI responses were similar between walkways. However, greater ratios of absolute oxygen uptake per force (by 19%) and TOI per force (by 24%) on Teflon were demonstrated in the horizontal direction than on plywood. This increased muscle oxygenation-force ratio, coupled with increased oxygen uptake per force generated on Teflon, might suggest that pushing on the slippery surface results in higher metabolic demand. Findings from the present study will assist in revising previously established acceptable forces and in relating these forces to physiological responses with respect to pushing on different frictional walkways.

Cart Pushing Capabilities for Males and Females: An Update

The purpose of this experiment was to compare the differences in maximum acceptable forces of a 7.6 m pushing task at a frequency of 1 min-1 between a high-inertia pushcart and our criterion magnetic particle brake treadmill push task (Snook and Ciriello, 1991). Thirty female and 22 male industrial workers performed both a pushcart and a treadmil pushing task in the context of four larger experiments. The results revealed that maximum acceptable sustained forces of pushing on the pushcart were essentially identical to our magnetic particle brake treadmill data whereas the maximal acceptable initial forces were 7% to 15% higher than the criterion for males and females respectively. These findings may indicate that adjustments to our maximum acceptable sustained force data may not be necessary, however, maximum acceptable initial forces may present conservative estimates.

Comparison of psychophysiological responses in healthy men and women workers during cart pushing on two walkways of high and low coefficient of friction

The purpose of this study was to compare psychophysiological responses between healthy men and women workers during dynamic pushing on two frictional floor surfaces. First, using a psychophysical approach 27 participants chose a cart weight that they could push for 8 h “without strain or becoming unusually tired, weakened, overheated or out of breath” on Plywood (coefficient of friction = 0.68) and Teflon (coefficient of friction = 0.26) walkways. Second, cardiopulmonary and muscle metabolic measurements were collected simultaneously on two separate days while participants pushed the high-inertia cart over a 7.6 m distance at a frequency of 1 push/min at the psychophysically chosen workload for 2 h on each walkway. Muscle responses, Tissue Hemoglobin Index (THI) and Tissue Oxygenation Index (TOI), from the right and left gastrocnemius medialis, were obtained using near-infrared spectroscopy. Statistical analysis showed that cart weight chosen by both men and women on the Plywood walkway was 27% higher than that on the Teflon walkway ( P < 0.001), with women able to push 9% (on Teflon) and 27% (on Plywood) less than men ( P < 0.005). The resultant force exerted on the instrumented cart handle by both men and women on the Plywood walkway was 33% higher than that on the Teflon walkway ( P < 0.001), with women exerting 21% (on Teflon) and 35% (on Plywood) less force than men ( P < 0.001). The lower resultant forces exerted while pushing on the Teflon walkway produced lower oxygen uptake responses than that on the Plywood walkway, with women displaying 20% (on Teflon) and 22% (on Plywood) less than men. Women had 35% less hemoglobin concentration (i.e., THI) in calf muscles compared to men during cart pushing, but this gender difference in THI responses was nullified after adjustment for the resultant force. The net saturation of hemoglobin represented by TOI was not influenced by walkway, implying that an increase in oxygen extraction is matched by the increased blood flow to the calf muscles in both men and women irrespective of the walkway. However, the influence of the force of exertion on oxygen saturation in women was 31% more than men, suggesting the importance of investigating gender-specific physiological differences in skeletal muscles during dynamic pushing. Relevance to industry Using a combination of methodologies from psychophysics and physiology, the present study evaluated functional abilities of both men and women during cart pushing. Psychophysiological findings from the present study clearly demonstrate the need for further understanding of gender differences during a variety of repetitive manual materials-handling activities at 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.

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.

Differences between Male and Female Psychophysically Determined Maximum Acceptable Forces of a Cart Pushing Tasks

The purpose of this experiment was to compare the differences in maximum acceptable initial and sustained forces between males and females while performing a 7.6 m pushing task at a frequency of 1 min-1 on a high-inertia pushcart and to compare those differences with the gender differences of our criterion magnetic particle brake treadmill push task (Snook and Ciriello, 1991). Eleven female and eight male industrial workers performed two 2- hour pushcart tasks in the context of a larger experiment. The results revealed that maximum acceptable initial and sustained forces of pushing for women on the pushcart were 67% and 69% of the initial and sustained forces for men, comparable to our magnetic particle brake treadmill push task data. These findings may indicate that adjustments to our pushing data may not be necessary if replication of this experiment yields similar results.

Interface stability influences torso muscle recruitment and spinal load during pushing tasks

Handle or interface design can influence torso muscle recruitment and spinal load during pushing tasks. The objective of the study was to provide insight into the role of interface stability with regard to torso muscle recruitment and biomechanical loads on the spine. Fourteen subjects generated voluntary isometric trunk flexion force against a rigid interface and similar flexion exertions against an unstable interface, which simulated handle design in a cart pushing task. Normalized electromyographic (EMG) activity in the rectus abdominus, external oblique and internal oblique muscles increased with exertion effort. When using the unstable interface, EMG activity in the internal and external oblique muscle groups was greater than when using the rigid interface. Results agreed with trends from a biomechanical model implemented to predict the muscle activation necessary to generate isometric pushing forces and maintain spinal stability when using the two different interface designs. The co-contraction contributed to increased spinal load when using the unstable interface. It was concluded that handle or interface design and stability may influence spinal load and associated risk of musculoskeletal injury during manual materials tasks that involve pushing exertions.