How Ergonomists Evaluate Risk: A Guide to Common Assessment Tools
In industrial environments, preventing musculoskeletal disorders (MSDs) and optimizing worker safety requires more than just good intentions—it demands precise evaluation tools. Ergonomists rely on a suite of scientifically validated methods to assess risk in manual materials handling (MMH) and static exertion tasks. This post explores the most widely used tools and how they integrate into modern ergonomic software like Work(s).
Understanding the Risk Landscape
Ergonomic risk assessment is a multidisciplinary process that draws from:
Biomechanics: Evaluating forces on the body, especially the spine and joints.
Psychophysics: Understanding perceived exertion and acceptable effort levels.
Physiology: Measuring metabolic and muscular fatigue.
Epidemiology: Applying population-based data to predict injury risk.
Key Assessment Tools for MMH and Static Exertion
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The Dempsey Liberty Mutual Metabolic Equation Worksheet is a tool that estimates the metabolic energy costs for lifting, lowering, and depalletizing tasks, using specific parameters such as body weight, box dimensions, task frequency, load, and duration.
Using defined height ranges and box sizes, the tool categorizes lift/lower tasks as either “low” or “center,” and considers task combinations such as lifting and carrying. Additionally, it accounts for task frequency by determining the average efforts per minute across the task’s daily duration.
The Dempsey Liberty Mutual Metabolic Equation Worksheet outputs the total metabolic costs for each individual task (kcal) both per minute and per day. Additionally, it outputs the maximum acceptable load (kg) that can be used for each task.
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The AFF Method, developed by La Delfa & Potvin (2017), maps out strength capabilities of the arm across different postures and exertion directions. It uses a 3D force field model to:
Predict maximum voluntary exertion based on arm position.
Identify postures with reduced strength (e.g., overhead or extended reach).
Support task redesign by recommending safer arm positions.
This method is particularly useful for evaluating tasks involving reaching, lifting, or tool use above shoulder height, and is integrated into ergonomic software for visual analysis.
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HandPak™ is a specialized biomechanical modeling tool developed by Potvin Biomechanics Inc. It focuses on upper limb exertion, particularly:
Grip and push/pull forces
Joint torque and muscle strength
Postural analysis for arms and hands
HandPak™ allows users to input task parameters (e.g., force direction, hand position, duration) and outputs biomechanical loads and risk levels. It's widely used in industries where hand-intensive tasks are common, such as assembly lines, tool use, and packaging.
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The MAE Equation (Potvin, 2012) estimates the sustainable effort level for static exertion tasks. It’s based on psychophysical data and muscle fatigue models, and helps determine:
How long a worker can hold a posture or apply force without fatigue.
Whether a task requires rest breaks or redesign.
The acceptable force level for prolonged exertion.
This tool is especially useful for jobs involving static holding, such as welding, overhead assembly, or tool stabilization.
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The RCRA (Gibson & Potvin, 2016) calculates the required recovery time between exertions to prevent cumulative muscle fatigue. It considers:
Task duration and intensity
Muscle group involved
Recovery time needed to maintain safe performance
RCRA is vital for repetitive or high-intensity tasks, helping ergonomists design work-rest cycles and reduce fatigue-related injuries. It’s often used in conjunction with MAE and AFF methods for a full fatigue profile.
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The "Liberty Mutual Manual Materials Handling Population Percentiles" are based on the Liberty Mutual Manual Materials Handling Equations published by Potvin, et al., 2021. The manual material handling analysis tool provides both the male and female population percentages capable of performing manual material handling tasks without perceived overexertion. The user is strongly advised to use the female population percentiles for design purposes (see “Interpreting Results”). The results can be used to perform ergonomic assessments of lifting, lowering, pushing, pulling, and carrying tasks with the primary goal of supporting ergonomic design interventions.
Liberty Mutual Manual Material Handling Equation provides maximum allowable weight limits and population percent capable.
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The Peak compression and LiFFT Method Worksheet utilizes Potvin's (1997) equations to estimate lumbar compression forces during physical tasks by inputting parameters such as horizontal reach, vertical height, and load mass. This tool is designed to assess ergonomic risks associated with lifting tasks by estimating feasible trunk angles, derived from Potvin’s equations, which are used to calculate the average lumbar compression forces generated in task execution.
The worksheet also applies modified Jager (2018) Dortumund Limits to evaluate the acceptability of lumbar compression forces, adjusting based on worker age. For cumulative risk, it uses Gallagher et al.'s (2017) method to calculate lumbar cumulative damage for individual tasks and provides a total cumulative damage score across multiple tasks. This comprehensive evaluation aids in determining the potential for overexertion and cumulative strain injuries, following Gallagher et al.'s recommended limit of 0.03.
The LiFFT Method Worksheet provides a compression force (N) for each task in addition to a peak Compression (N), and also provides the cumulative damage as a ratio both per task and overall.
Integrating Tools with Software: The Role of Work(s)™
Many of these tools mentioned above are integrated into the Work(s)™ ergonomics software. This is a task analysis software that helps quantify the risk for work-related musculoskeletal disorders using the most valid science-based ergonomic assessment tools. Work(s)™ allows users to:
Simulate tasks and all possible postures
Apply multiple ergonomic models simultaneously
Visualize risk zones
Generate reports for compliance and redesign
The data output provides an indication of risk level as defined by the Demand/ Capacity ratio (DRC) based on the threshold limit value for each of the relevant variables in the posture/ task (i.e. Arm, hand/ wrist/ forearm, lumbar, neck etc.).
Why These Tools Matter
Using validated ergonomic tools ensures that risk assessments are:
Evidence-based: Grounded in scientific research.
Consistent: Repeatable across different evaluators and settings.
Actionable: Leading to clear recommendations for redesign or intervention.
It helps ergonomists use data to support the recommendations to make changes to your work environment.