Liberty Mutual Manual Materials Handling Population Percentiles

Musculoskeletal pain “injuries” (and specifically back pain) does not occur ONLY because of the severity of the task parameters. Therefore tasks should not be evaluated based solely on population percentages. Other important considerations are:

  • Injuries - Any job that is producing injuries is a good candidate for redesign. This redesign should include improvements in task parameters AND organizational and psychosocial contributions to injury risk.

  • Bending - Any task that begins or ends with the hands below knuckle height presents some degree of risk. The deeper the bending motion, the greater is the physical stress on the low back. Frequent bending regardless of weight is not recommended.

  • Twisting - This motion puts uneven forces on the back thereby presenting additional physical stress. Most tasks in industry involve at least a minimum twist. The greater the twist, the more physically stressful the task. Because of the experimental design used with Liberty Mutual’s psychophysical research, twisting was incorporated into most lifts and lowers. For this reason twisting as a separate element is not included in the Liberty Mutual Manual Material Handling Equations, nor this online tool.

  • Reaching - The distance away from the body that a load is held greatly affects the forces on the back, shoulders, and arms. The farther the reach, the more physically stressful the task.

  • One-Handed Lifts - The online tool should not be used to evaluate one-handed tasks (including pushing/pulling). By nature, these tasks place uneven loads on the back and may present a greater physical stress than two-handed lifts.

  • Hand-Holds - A good grip on the object being handled allows the handler to handle MORE weight comfortably than with an uncomfortable or challenging hand position. Unfortunately, this does NOT mean they are necessarily at less risk for injury – especially if the handler has discretion to adjust the weight of the object being handled. A good grip makes the hands more comfortable but does not necessarily reduce risk. So even though the Liberty Mutual Manual Materials Handling Equations (and this online tool) incorporate the impact of hand coupling on the acceptability of the load, uncomfortable (“Poor” or “Fair” hand coupling) may actually reduce the risk of injury if it leads to reduced weights.

  • Do not use this online tool to determine whether male or female workers can perform certain jobs and then place workers accordingly.

  • Do use this online tool for designing manual handling jobs with physical requirements such that as many workers as possible can perform them without risk of injury. As described in the “Interpreting Results” section, jobs should be designed so that at least 75% of the female population can do the task without overexertion in order to reduce injury risk for ALL workers (males and females).

  • To effectively use this online tool requires training in ergonomics and task evaluation methods. Users should be trained in how to collect the hand distance, lifting distance, and task frequency measurements. Liberty Mutual Insurance workers compensation insured customers can call our Risk Control Services department for details (866-757-7324).

Liberty Mutual logoLiberty Medallion LogoThe illustrations, instructions, and principles contained in the material are general in scope and for marketing purposes. Our risk control services are advisory only. We assume no responsibility for: managing or controlling customer safety activities, implementing any recommended corrective measures, or identifying all potential hazards. The illustrations, instructions and principles contained in the material are based on published research and, to the best of our knowledge, current at the time of publication. No attempt has been made to interpret any referenced codes, standards, or regulations. Please refer to the appropriate code, standard, or regulation-making authority for interpretation or clarification.
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