What are the Snook and Ciriello tables?
The Snook and Ciriello tables are derived from psychophysical studies on manual handling. They provide weight and force references for specific lifting, lowering, carrying, pushing, and pulling conditions. Their interpretation relates the demands of a task to the population that could perform it according to the criteria of these studies.
These metrics are used in the assessment of manual handling of loads, especially when evaluating transport or pushing and pulling forces. They do not replace a complete job analysis. Their experimental origin and application conditions must be understood to prevent a single figure from becoming a universal limit detached from its context.
Task variables
The selection of reference points depends on the type of operation and variables such as hand height, distance, and frequency. The percentage of the population considered is also important. Before consulting a table, the activity must be described with sufficient precision, including differences between routes, products, and phases of the workday.
An occasional task is not the same as the same operation repeated continuously. Nor can a short, flat route be treated as identical to one with conditions that modify the effort required. The evaluation must verify which aspects are represented by the method and which require further assessment, without introducing extrapolations that the tool does not allow.
Initial force and sustained force
In pushing and pulling, a distinction is made between the force needed to initiate movement and the force required to maintain it. Both can be relevant. The weight of a cart and its contents does not directly indicate these forces, because wheels, the ground, maintenance, and movement conditions also play a role.
The measurement must replicate actual usage with an appropriate instrument and procedure. Units should be clearly identified, avoiding confusion between kilograms of mass, kilograms of force, or newtons. Any obstacles, turns, or changes in elevation must be specifically analyzed. A measurement taken only on the most favorable section does not necessarily represent the requirements of the entire route.
Percentage of population and interpretation
The percentage of the population is a parameter for interpreting the psychophysical model. It does not mean that every person included in that percentage is protected from any injury, nor does it allow for the individual identification of who can perform the task. The choice of criterion must be justified preventively and remain consistent with the population and activity being evaluated.
The differentiated data from the studies describe reference groups, not rules for selecting or excluding workers. The aim should be to adapt the work and reduce exposure. When an individual requires specific adjustments, the corresponding preventive and health procedures will be used, without replacing that assessment with an automatic classification derived from a table.
Further developments and choice of method
The LM-MMH equations represent a further development of these references and allow for the calculation of values within defined ranges. INSST’s NTP 1212 presents its application to pushing and pulling. The report must indicate whether it uses original tables, a specific version, or equations, because the procedures and assumptions must remain consistent.
Other tools, such as the NIOSH equation, may be relevant for lifting, depending on their conditions of application. There is no single method that fits all operations. It is advisable to choose the method that best represents the task and complete any gaps, avoiding simply using a calculator because it is the one commonly used in the organization.
Practical example
A warehouse evaluates a cart based solely on its maximum load capacity. However, the force required to initiate movement increases when some wheels are worn or when the cart is on a pavement joint. Measuring a cart on a smooth surface with a recently maintained cart produces a result that does not accurately reflect daily use.
The review characterizes equipment paths and condition, measures initial and sustained forces, and applies the appropriate method. Improvements include floor maintenance and repair, as well as reviewing load and frequency. The example shows why reducing the weight of goods by a few kilograms may be insufficient if the main resistances to movement remain.
Measures and monitoring
The measures can reduce force, distance, frequency, or unfavorable postural conditions. Mechanical aids, route design, grip height, and supply organization should all be considered. Biomechanics allows us to interpret how a change modifies the demands on the body and whether it shifts the problem to another phase of the activity.
After intervention, conditions should be checked and, where appropriate, comparable measurements should be repeated. It is advisable to record the instrument, procedure, units, and scenarios. Maintenance is especially important: a task that improves with new tires can deteriorate again if their condition is not maintained. The results should be integrated into planning and preventive maintenance.
Common mistakes
Common errors include equating weight and strength, ignoring initial effort, consulting a table for different conditions, or interpreting population percentages as an individual guarantee. It is also a mistake to use results outside the expected ranges or to mix equations and criteria from different versions without justifying the combination.
The useful evaluation describes what is actually done and explains its limitations. The tables provide references for decision-making, but they do not replace observation, measurement, and technical judgment. Their preventive value emerges when they help modify the task and verify that the improvement is maintained during the regular use of the equipment and routes.
