What are push and pull?
These are forms of manual handling of loads in which human strength moves an object without necessarily supporting its full weight. They can occur when moving carts, containers, beds, transport cages, or equipment. Even if the object has wheels, the person still exerts effort that must be analyzed within the specific conditions of the task.
The assessment cannot be based solely on the weight of the load. Two carts of the same weight can require very different forces due to their wheels, maintenance, surface, or incline. The start of movement, turns, and the need to stop or control the object also play a role. The entire operation must be observed from beginning to end.
The Snook and Ciriello tables and the LM-MMH equations allow for comparing forces and task conditions with psychophysical references. It is necessary to differentiate between initial force, sustained force, and the weight of the displaced assembly.
Initial force and sustained force
Initial force is the force required to set the load in motion. Sustained force is the force applied to maintain its movement. These two must be distinguished because they respond to different conditions and may require specific measurement. A brief maneuver with a high peak force is not adequately described by the average effort over the entire range of motion.
The measurement conditions must reflect actual work: load, wheel orientation, surface, path, and grip. The instrument and procedure are selected based on technical criteria. Comparisons with reference values require compatible units and conditions; a mass expressed in kilograms should not be confused with a force expressed in newtons.
Equipment and route factors
The diameter, material, alignment, and condition of the wheels all affect the effort required. A damaged or blocked wheel can increase the force needed and alter the trajectory. The position of the handles should allow for a proper grip and posture for the users. The size of the load can also limit visibility and force users into awkward positions.
The route includes inclines, joints, thresholds, curves, doorways, and maneuvering space. An assessment conducted only on a smooth section may overlook the most demanding aspect. The physical workload should also consider the frequency of movement, the distance, and its combination with lifting or other tasks.
Task evaluation
The assessment identifies who performs the operation, with what variations, and for how long. It should consider both typical conditions and foreseeable periods of higher demand, without treating an extreme exception as if it describes the entire workday. Observations are supplemented with measurements when necessary to apply the selected method.
Reference methods and tools have scopes and assumptions that must be respected. The result does not equate to a universal authorization to move any load below a certain weight. The diversity of the workforce, environmental conditions, and other risks associated with the movement must be considered in the interpretation.
Preventive measures
The priority is to avoid or reduce the need for effort through process changes, appropriate assistance, and route design. It may be necessary to review the size of the units being transported, the warehouse layout, or the delivery method. The solution should be analyzed in conjunction with other risks, such as collisions, impacts, or loss of control of motorized equipment.
Maintaining wheels and flooring is important, but it must be integrated into a system that detects and removes defective equipment when necessary. Training explains how to use the available aids; training cannot compensate for an unsuitable cart or a slope that requires excessive effort. Ergonomic design should also encompass doors, access points, and delivery areas.
Practical example
In a laundromat, moving a cart is easy in the central aisle but difficult when passing through a doorway with a damaged threshold. The company had only assessed the task on the smooth section. Upon observing the entire route, they discovered that the greatest effort occurs when starting to pass through that point and when correcting the turn.
The intervention combines step repair, wheel inspection, and load redistribution. The operation is then tested using the usual carts and routes. Simply reducing the weight without addressing the underlying issue might only partially improve the task but leave the primary problem unresolved. This example illustrates why the unit of analysis must be the actual displacement.
Organization and recovery
The frequency and distribution of movements influence cumulative exposure. It’s important to avoid automatically translating productivity improvements into more operations without reviewing the demands. Breaks and recovery time at work can be part of the planning, along with appropriate task rotation.
It should not be assumed that working between two people always divides the effort evenly. Coordination, space, and maneuvering can create additional difficulties. When shared handling is planned, it should be evaluated as a specific task and a clear organizational structure established, without automatically replacing appropriate technical assistance.
Framework and monitoring
In Spain, Royal Decree 487/1997 includes pushing and pulling within the manual handling of loads, which can entail risks. Specific application requires evaluating the operation and selecting measures appropriate to the actual conditions. The INSST (National Institute for Safety and Health at Work) tools provide technical criteria that should be used within their scope.
The monitoring system checks forces, equipment status, the route, and the use of preventive measures. Difficulty alerts and recurring defects allow for the detection of deterioration or changes in the process. The absence of recorded injuries does not, in itself, demonstrate that the task is sufficiently controlled.
